Technical Report NTB 90-49

Joint seismic, hydrogeological and geomechanical investigations of a fracture zone in the Grimsel Rock Laboatory, Switzerland

Summary

From 1987 to 1989 The United States Department of Energy (DOE) and the Swiss Cooperative for the Disposal of Nuclear Waste (Nagra) participated in an agreement to carry out experiments for understanding the effect of fractures in the storage and disposal of nuclear waste. As part of this joint work field and laboratory experiments were conducted at a controlled site in the Nagra underground Grimsel test site in Switzerland. The primary goal of the experiments in this fractured granite was to determine the fundamental nature of the propagation of seismic waves in fractured media, and to relate the seismological parameters to the hydrological parameters. The work is ultimately aimed at the characterization and monitoring of subsurface sites for the storage of nuclear waste. The seismic experiments utilized high frequency (1,000 to 10,000 Hz) signals in a crosshole configuration at scales of several tens of meters. Two-, three-, and four-sided tomographic images of the fractures and geologic structure were produced from over 60,000 ray paths through a 10 by 21 meter region bounded by to nearly horizontal boreholes and to tunnels. Intersecting this region was a dominant fracture zone which was the target of the investigations. In addition to these controlled seismic imaging experiments, laboratory work using core from this region was performed to establish the relation between fracture content, saturation, and seismic velocity and attenuation. In-situ geomechanical and hydrologic tests were carried out to determine the mechanical stiffness and conductivity of the fractures. The results indicate that both P-waves and S-waves can be used to map the location of fractures, both natural and induced from mining activities. In addition, it appears that the frequencies approaching several kilohertz, attenuation measurements are more useful than velocity measurements. At lower frequencies the opposite seems to be true. In addition, fractures that are open and hydrologically conductive are much more visible to seismic waves than non-conductive fractures.

Technical Report NTB 90-48

STRENG: A source-term model for vitrified high-level waste

Summary

A source term model STRENG has been devised to analyse the diffusive release of radionuclides from vitrified high level waste in a repository with cylindrical geometry. The model includes the dissolution of the glass waste matrix, the diffusive transport of the radionuclides through a bentonite clay buffer, the effects of solubility limits, as well as the radioactive decay and ingrowth.

The model requires as input the following parameters:

From the above data the model produces as output:

Technischer Bericht NTB 90-47

Wellenberg
Arbeitsprogramm
Teil 2 für Untersuchungen in den Sondierbohrungen SB 6 und SB 2

Summary

Im hier vorliegenden Arbeitsprogramm werden die geplanten Untersuchungen in den Sondierbohrungen SB2 und SB6 vorgestellt. Besonderes Augenmerk gilt der Darlegung des Untersuchungsumfanges; hinsichtlich der Methodik wird auf NTB 89-12 (Wellenberg Arbeitsprogramm Teil 1 für Untersuchungen von der Erdoberfläche aus und Sondierbohrungen SB1, 3 und 4) verwiesen.

Das erste Kapitel des Arbeitsprogramms behandelt die grundsätzlichen Aspekte des Untersuchungsvorhabens, in Kapitel 2 werden die Untersuchungsziele definiert.

In Kapitel 3 wird der geplante Untersuchungsumfang pro Sondierbohrung quantifiziert. Hierbei gewähren die Ablaufpläne (Arbeitsschritte) sowie Datenblätter einen informativen Überblick über die operationellen Arbeiten in den Sondierbohrungen SB2 und SB6.

Schliesslich folgen in den Kapiteln 5, 6 und 7 Erläuterungen zur Berichterstattung, Logistik, Terminplanung und Projektorganisation.

Für Planung, Vorbereitung und Ausführung der operationellen Arbeiten wurde Nagra-intern ein Projektteam zusammengestellt, welches auch das vorliegende Arbeitsprogramm erarbeitet hat.

Technischer Bericht NTB 90-46

Seismizität der Nordschweiz 1987 – 1989, und Auswertung der Erdbebenserien von Günsberg, Läufelfingen und Zeglingen

Summary

In the course of a neotectonic research program undertaken by Nagra, the Swiss Seismological Service, since 1983, has been monitoring in detail the seismicity of northern Switzerland. The present study, which comprises the period 1987 – 1989, constitutes a further interim report of the still ongoing research project.

Seismic activity in northern Switzerland is generally low (magnitudes < 4.2) and is concentrated mainly in two separate areas. The first area, in the northwest, represents the continuation below the Jura Mountains of the activity associated with the southern Rhinegraben and the faults bordering the Dinkelberg region, and includes some diffuse activity north of the Lake of Biel. The second area comprises a wide band of epicentres in northeastern Switzerland extending from the northwestern end of the Bodensee into the Canton of Zurich. Farther east there is an additional concentration of epicentres at the northern edge of the Alpstein. In the central part of northern Switzerland activity was restricted to only a few small events.

Contrary to what is observed in the Alps, where earthquakes are restricted to the upper crust, focal depths below northern Switzerland extend all the way down to the crust-mantle boundary. This observation is difficult to reconcile with traditional models of crustal rheology.

Based on high-precision relative locations of hypocenters in two microearthquake swarms below the Jura Mountains of Solothurn und Basel, it has been possible to demonstrate for the first time that both sinistral N-S trending faults and dextral WNW-ESE trending faults are active within the crystalline basement. Both mechanisms are compatible with the well established orientations of maximum crustal shortening and extension in NNW-SSE and WSW-ENE directions respectively. An analysis of all available focal mechanism solutions shows a remarkable lack of possible fault planes with a strike between 40 and 90 degrees from north.

The increased seismic station density in the central part of northern Switzerland has not only lowered the detection threshold for weak earthquakes and greatly improved the accuracy of their hypocentral locations, but has also given new insights into the rheological behaviour of the earth's crust and into the orientation and mechanisms of active faults in the crystalline basement. In the western and eastern parts, on the other hand, due to the smaller number of stations, accurate focal depth determinations and fault plane solutions are only possible for the stronger events.

Technical Report NTB 90-44

Stripa Project
Prediction of inflow into the D-holes at the Stripa mine

Summary

Groundwater flow through three-dimensional networks of discrete fractures was modelled to predict the flux into a set of parallel boreholes, as part of the Site Characterization and Validation Project conducted during Phase 3 of the Stripa Project. Influx was predicted from fracture statistics derived from geological, geophysical, and hydrological site characterization data. Individual fractures were treated as probabilistic (random) features, whereas the major fracture zones inferred from geophysics were treated as deterministically located zones of relatively high fracture intensity. The flow predictions were produced by generating multiple Monte Carlo realizations of the fracture population, and by solving the flow equation for each population using the finite element method. The predictions thus produced are presented in the form of probability distributions for flux. The most likely value for total influx to the boreholes was predicted to be 90 litres/hour, with a 90 % confidence interval extending from 30 to 5700 litres/hour.

Technical Report NTB 90-43

Stripa Project
Site characterization and validation borehole radar investigations, Stage III

Summary

The borehole radar investigation program Stage III of the SCV-site has comprised single hole reflection measurements with centre frequencies of 22 and 60 MHz. Single hole reflection measurement with both omnidirectional and directional antennas have been performed in the boreholes C1, C2, C3 and the D-holes (D1 – D6). Crosshole tomographic measurements as well as crosshole reflection measurements have been made between the boreholes C1 – C2, W1 – C1 and W1 – C2. The range obtained in the single hole reflection measurements was approximately 100 m for the lower frequency (22 MHz) and about 60 – 70 m for the centre frequency 60 MHz. In the crosshole measurements transmitter-receiver separations from 20 to 120 m have been used.

The Stage III radar investigations have essentially confirmed the three dimensional description of the structures at the SCV-site. The conceptual model of the site which was produced based on the Stage I data included three major zones (GA, GB, and GH), two minor zones (GC and GI) and a circular feature (RQ). The major features are considered to be the most significant at the site and are all observed in the Stage III boreholes close to their predicted locations. The circular feature RQ has also been found in two of the additional tomograms at the predicted location. RQ is seen as a ring shaped feature in the attenuation tomograms and as a single spot anomaly in the slowness tomogram.

The results indicate that the zones are not homogeneous but rather that they are highly irregular containing parts of considerably increased fracturing and parts where their contrast to the background rock is quite small. The zones appear to be approximately planar at least at the scale of the site. At a smaller scale the zones can appear quite irregular.

Technical Report NTB 90-42

The application of moment methods to the analysis of fluid electrical conductivity logs in boretoles

Summary

The present report describes new methods for the quantitative analysis of dynamic fluid electrical conductivity logs measured in boreholes under production or free outflow conditions. The methods form powerful complements and extensions to a testing procedure and best-fit simulation approach previously presented by Tsang et al. (1990). The testing and analysis procedures allow an exact location of all fractures flowing under the given head drawdown and intersected by the borehole (± 1 m in deep boreholes), as well as the calculation of the transmissivity (and head) of the individual flowing fractures.

The three methodologies presented in the present report start from the Classical Moment approach applied to the 1D advection-diffusion equation, which is then further developed to allow for the quantitative analysis of multiple interfering fracture outflows into the borehole.

After a general discussion of the testing procedure and the governing equation used for analysis (Section 1), the zero'th and first moment relationships of a single fracture outflow into a borehole during pumping are derived and related to the volumetric well bore flow rates below and above an outflow zone (Section 2). These parameters can be used directly to evaluate the volumetric flow rate and the fluid electrolyte concentration of all non-interfering fracture outflows.

Sections 3 and 4 present new approaches for the analysis of interfering peaks. The Partial Moment Method (Section 3) is based on integral quantities with a similar structure as the classical moments, but measured between fracture outflows and not across. The assumption made in the development of the fundamental equations is that of 1D advective-dispersive electrolyte transport along the borehole with constant velocity, v, and dispersion, k, within the short intervals between fractures. The method allows, on principle, for an independent determination of v and k within any interval of the logged section and for any time, t, during logging. This implies that the method can also be used when both the fracture fluid concentration and the fracture flow rate are changing with time. The major limitation of the Partial Moment Method is related to numerical instabilities at late logging times.

The Direct Integral Method (Section 4) is based on a mass balance approximation taking into account early time zero'th moment information and avoids the integral derivatives with their potential instabilities. The electrolyte transport is simplified with respect to the governing equation outlined above in the sense that only advective transport is considered at locations where the wellbore velocity is estimated.

In Section 5 the three methods are applied to synthetic test cases generated with a 1D advective-dispersive transport simulator (BORE). Two different types of test cases, with parameters similar to the field data measured in the Leuggern deep well of northern Switzerland, are considered: Test Case I with widely separated and only weakly interfering outflow peaks, Test Case II with closely spaced and strongly interfering fracture outflows. The application shows the different sensitivities of the analytical methods considered. The methods are ab1 e to reproduce the input with an error smaller than 50 % and for certain cases down to a difference of 1 or 2 %. It is shown that for the given parameters early time data give much more important information than late time data and that test duration can be reduced significantly (to 50 or 100 hours of logging, compared to 600 hours in the case of Leuggern) without losing relevant information.

In Section 6 the three methods are applied to real field data from the Leuggern deep borehole, measured in 1987. Besides precise localization of all flowing fractures intersected by the borehole, it can be shown that the Classical Moment Method and the Direct Integral Method form a valuable input and starting base for simulation and refinement with a simulator like BORE. By comparison with independently derived fracture flow rates from packer testing performed extensively in the Leuggern borehole, it can be shown that the Partial Moment Method is capable of reproducing the packer test results within a factor of 2 or 3 (greatest difference smaller than half an order of magnitude). These ranges lay within what is generally recognized as the uncertainty in packer testing.

Section 7 describes the implications for field experiment design and data analysis. The field experiment design should account for the needs of the analytical methods outlined. With respect to data analysis a comprehensive analysis procedure that takes into account conceptual uncertainty, data quality, and type of conductivity logging data (early or late time) is developed.

Technischer Bericht NTB 90-41

Sedimentäre Architektur der distalen Unteren Süsswassermolasse und ihre Beziehung zur Diagenese und der Petrophysik. Eigenschaften am Beispiel der Bohrungen Langenthal.

Summary

Sedimentological study at outcrop and quarry exposures of distal alluvial deposits in the upper part of the Lower Freshwater Molasse ("Aquitan") has revealed four principal facies. The same facies may also be recognised in boreholes (Langenthal, Weiach, Schafisheim) penetrating the continental succession. These facies make up distinct sediment bodies, or "architectural elements", which may be characterised by their different geometries, and by their petrographic and petrophysical properties: grain size and mineralogical composition; porosity P and permeability k.

Each architectural element may be assigned to a particular depositional setting: Meander Belt RG (MB); Crevasse Splays and Crevasse Channels DFR (CS), Levees and Distal Splays UW (LS), Overbank, Paleosols and Swamps UPS (OPS) and Lacustrine LAK (LAC). The individual architectural elements show strong heterogeneity, resulting both from primary sedimentological differences (particularly in hydrodynamic sorting) and from diagenetic modification (notably through variable cementation).

Meander belt sandstones in the distal Lower Freshwater Molasse reflect the occurrence of two types of mixed load channels. Those with a higher bed load component were dominant in the Lower Freshwater Molasse, producing laterally extensive and longitudinally continuous sandstone bodies 150-1500 m in width. Rarer channels with a higher suspended load component deposited complex amalgamated sandstone bodies 40-100 m in width, commonly in erosive contact with each other.

Architectural modelling permits consideration of the distal Lower Freshwater Molasse in terms of a sandy framework, comprising the channel sandstone bodies, each in contact with their associated crevasse splay and crevasse channel sandstones (median porosity 13.1% (RG) and 8.6% (DFR), median permeability 486md (RG) and 5md (DFR)), and a muddy-silty matrix consisting of interbedded floodplain sediments (practically without measurable porosity and with much lower permeability). The anomalously high total mud content of the sandstone units chiefly reflects the elevated mud content of the incorporated lithic rock fragments.

Comparison with the morphologies of modern alluvial systems suggests that channel sandstone bodies in the Lower Freshwater Molasse are likely to show greater longitudinal continuity, greater width and a higher degree of interconnectedness in an upstream direction (i.e. in proximal areas towards the west and towards the Alpine chain). Conversely, primary depositional porosities and permeabilities of the sandstone units are likely to decrease downstream towards more distal areas in the east and at the northern margin of the basin, although regional differences in tectonic style and diagenesis will also exert a major control on the petrophysical and hydrological properties in any given area.

Technical Report NTB 90-40

A physico-chemical characterisation technique for determining the pore-water chemistry in argillaceous rocks

Summary

A prerequisite for carrying out credible sorption studies is the definition of an aqueous phase composition which is in equilibrium with the solid phase. Experimental methods and data analysis procedures are described which enable an equilibrium water composition to be produced for argillaceous rocks which is not dependent on liquid to solid (L:S) ratios. Since a Valanginian marl formation is under consideration by Nagra as a potential host rock for the disposal of low and short-lived intermediate level radioactive waste in Switzerland, samples of this material were chosen for this investigation.

Aqueous phase and nickel ethylenediamine extraction experiments were carried out at different L:S ratios under controlled atmosphere conditions (P_CO2 = 10^-2 bar, O₂ ≤ 5 ppm). The results from these tests and petrographical examinations were combined to define the system in terms of the physico-chemical characteristics of the clay mineral component (CEC and cation occupancies) and the identities of highly soluble and solubility limited phases in the marl. The geochemical code PHREEQE was used in conjunction with the Gapon equations to calculate the pore water composition.

This work clearly showed that pore water chemistries obtained from aqueous extracts alone may lead to an arbitrary water chemistry in argillaceous rock systems, particularly with respect to ionic composition and ionic strength, which may have important consequences for radionuclide speciation and sorption studies.

Technical Report NTB 90-39

Grimsel Test Site
The Grimsel Migration Experiment:
A hydrogeochemical equilibrium test

Summary

Within the migration experiment at the Grimsel Test Site (GTS) various breakthrough tests, including non-sorbing as well as reactive tracers, are performed in a single fracture. During a special experiment, the water naturally flowing in the migration fracture (MI­water), was replaced by a groundwater of a somewhat different chemical composition (EM-water), which was pumped from borehole BOEM 85.012 located 200 m south of the migration site. The dipole flow field for this breakthrough experiment was established between borehole BOMI 86.004 (injection well) and borehole BOMI 87.006 (extraction well). The initial objective of this hydrogeochemical equilibrium test was to observe how the system responded to the injection of a "foreign" water and to determine the time required to reach new steady state conditions. A total of 22 water samples was taken from the extraction well for subsequent laboratory analyses. This test was part of the necessary preparation work required for future tracer migration experiments in which it might be necessary to inject a water with a different chemistry if a constant supply of natural migration water in the quantities required can not be guaranteed over the anticipated experimental time scales.

The results of the hydrogeochemical equilibration test showed clearly that anions such as F- and CI- reached steady state concentrations in the extracted water within ~ 50 hours whereas the major cations, Na⁺ and Ca²⁺ required ~ 150 hours. The three cations, Sr²⁺, K⁺ and Mg²⁺ showed evidence of being retarded more strongly. Sr²⁺ did not reach steady state until after ~ 250 hours and K⁺ and Mg²⁺ appeared to be still increasing slowly when the experiment was terminated after ~ 312 hours. The conclusion from this first part of the work was that EM-water should not be used as a source of injection water in any future migration experiment with sorbing tracers.

These data were further analysed and interpreted assuming that the protomylonite fracture material behaves as a weak cation exchange medium giving rise to composition changes in the extracted water in addition to those resulting from simple mixing processes.

From the concentration-time data for the extracted water, curves representing the "interaction" component of the "foreign" water with the fracture material could be deduced. These results, together with selectivity coefficients for the major ions with respect to Ca²⁺, and some simple assumptions concerning the fracture geometry, enabled a cation exchange capacity for the fracture material to be estimated. Using the estimated CEC value, in situ K_d values for Na⁺ and Sr²⁺ were predicted. The sorption characteristics of Na⁺ and Sr²⁺ are of direct relevance to the Nagra/PSI programme at the GTS since field migration experiments with radioisotopes of both of these tracers are being carried out during 1990/1991.

Technischer Bericht NTB 90-38

Sondierbohrung Siblingen
Bau- und Umweltaspekte, Bohrtechnik

Summary

In 1980, Nagra (the Swiss National Cooperative for the Disposal of Radioactive Waste) initiated a comprehensive geological investigation programme covering an area of around 1200 km² in Northern Switzerland. The aim of this programme was to provide the geological information required for assessing the suitability of the Palaeozoic basement as a potential host rock for a high-level waste repository.

The investigations consisted of a deep drilling programme, surface geophysical reconnaissance of rocks and structures in the region, a hydrogeological programme which studied water flow paths in the deep basement and, finally, a neotectonic programme aimed at identifying active crustal movements in the investigation region.

The aim of the deep boreholes was to investigate the geology of the crystalline basement and the overlying sedimentary formations. Within a wider, regional context, the boreholes were also intended to provide hydrodynamic and geochemical data as input for a mathematical model of the hydrogeological conditions prevailing between the northern slopes of the Alps in central and eastern Switzerland and the Black Forest Massif.

The Siblingen borehole was located 1.5 kilometres to the north-west of the village of Siblingen in the Canton of Schaffhausen, on the 'Siblinger Höhe' in the Schaffhausen Tabular Jura. It was the seventh and final borehole of Phase I of the deep drilling programme in Northern Switzerland and reached a final depth of 1522.0 m.

Following a licensing procedure which lasted several years and also involved the federal courts, preparatory work for constructing the drill site started on 16th May 1988; actual drilling began on 1st September 1988. Drilling was completed on 3rd April 1989 and was followed by a test phase which continued up to 30th April 1989.

The construction of the drill site ran according to schedule. The soil investigations which were carried out during the initial stages of site preparation provided input for the detailed planning of the excavation work; they also provided background information which allowed useful extensions to be made to the drainage network when the site was recultivated. This improved the cultivation potential of the ground and there was thus no negative impact on the productiveness of the soil.

Unfortunately, following completion of the drilling phase and during the dismantling work at the drill site, a tragic accident occurred. One of two engineers of the EKS under contract to remove the electricity supply received fatal injuries following a current surge. The reason for the accident was failure to observe electrotechnical safety regulations.

In order to protect groundwater resources, the working surfaces at the drill site were covered with a layer of asphalt and appropriate drainage systems were installed. The conductor pipe was tightly embedded in the bedrock and in the drill cellar. Together with the cantonal water conservation authorities, the efficiency of these measures was checked as part of a groundwater monitoring programme. No influence of the drilling operations on the existing groundwater deposits could be detected.

Despite the fact that the drill site was relatively far away from the village of Siblingen, the noise levels were measured at the closest single farmhouse and residential quarter. The results showed that at no time was anyone adversely affected by noise emissions from the site.

Disposal of all solid and liquid wastes was based on a concept which was developed in cooperation with, and licensed by, the building department of Canton Schaffhausen. This concept has proved itself in practice, and there have been no grounds for complaint at the disposal locations.

The increase in road traffic due to lorry transport was registered during the entire operational phase and showed a low level of less than 20 journeys per week. However, transportation during the phases of constructing and recultivating the drill site was not registered; this would have shown a much higher frequency, but over a shorter time span.

A streamlined scientific investigation programme was carried out in the Siblingen borehole, some use being made of alternative investigation techniques. It was agreed that the investigations would be restricted to questions which are directly related to disposal, and a new drilling concept with a greatly reduced hole diameter in the crystalline was used; this represented a synthesis of mining exploration technology and deep drilling techniques. In this way it was possible to observe the requirements as to depth and quality of information, scientific data output and minimisation of technical risk and, at the same time, to realise an economically optimised exploration project.

The borehole passed through Jurassic and Triassic sediments. The crystalline basement was encountered at a depth of 348.6 m and was drilled down to the final depth of 1522.0 m.

The entire length of the borehole was drilled using the wire line core drilling technique. For the first time in the deep drilling programme, an all-hydraulic core drilling rig (type Wirth B8A) with top-drive was used. This was operated electrically and was also equipped with a specially developed soundproofed drive unit for optional electric or diesel drive.

The sediments and the top section of the crystalline basement were drilled with 6 1/4 × 4" coring tools and a special 5 1/2" wire line coring stem. 96 mm × 57 mm bits with 3 1/2" wire line coring equipment were used from a depth of 490.9 m.

In addition to a conductor pipe and a 9 5/8" anchor casing which was installed down to a depth of 171.9 m, to facilitate drilling and investigations a 7" intermediate casing string was installed in the Muschelkalk at 338.0 m and a 5" end casing was installed in the upper section of the crystalline at 490.4 m. With the exception of the 5" casing, which was partly removed after the drilling phase, all the casings were cemented in up to ground level.

Even in the sediment zone, which was drilled using a conventional clay-water drilling fluid, various borehole sealing measures became necessary due to often complete loss of drilling fluid as a result of strong fracturing of the rock and the presence of sub-hydrostatic pressure conditions.

Over the 1173.4 m long crystalline section, a leucocratic biotite-cordierite-granite with heterogeneous tectonic stressing was encountered. This occurred alternately in the form of a fresh, massive granite and sections of strongly fractured granitic rock which had been hydrothermally overprinted and penetrated by shear zones.

The alternating structural and textural fabric of the rocks and the steeply-dipping fracture systems placed considerable demands on the selection and development of suitable coring tools.

The situation was also complicated by strongly sub-hydrostatic pressure conditions, combined with the use of clear water drilling fluid without additives. Complete losses of circulation and borehole instability meant that rehabilitation measures such as back cementation and plugging were often necessary.

As a result of the difficult drilling conditions, the tools could often not be used in their optimum parameter range. The strong fracturing of the rock also led to a reduction in terms of maximum core recovery as the average length of the continuous core sections was shorter.

In spite of all these complicating factors, the drilling of the borehole was highly successful both technically and economically and no significant complications or costly special work was involved. Compared with the previous boreholes in the deep drilling campaign, all aspects of performance showed a significant improvement, while the costs per drilled meter decreased considerably.

The scientific investigations in the overlying sediments were carried out as planned. The conditions encountered in the crystalline basement meant that the programme frequently had to be adapted to take account of unexpected factors; the hydraulic characterisation of the rock also required more effort than originally anticipated. However, the duration of the subsequent test phase was greatly reduced.

To summarise, all aspects of the scientific programme were realised with a considerable degree of success.

All the work carried out at Siblingen was monitored by a review commission consisting of representatives of the community of Siblingen, Canton Schaffhausen and the Federal Government.

In the report of this commission to the Federal Council, Nagra was accredited with having carried out the work correctly according to the legal regulations.

Technischer Bericht NTB 90-37

Zur Chemie von Kolloiden
Verfügbare Sorptionsmodelle und zur Frage der Kolloidhaftung

Summary

A safety analysis of a radioactive waste repository should consider the possibility of nuclide transport by colloids. This would involve describing the sorption properties of the colloids and their transport in porous and fissured media. This report deals with a few selected aspects of the chemistry of this complex subject.

Because the mechanisms of ion adsorption onto surfaces are material-specific, increased attention should be paid to identifying the material constitution of aquatic colloids. Suitable models already exist for describing reversible adsorption; these models describe sorption using mass action equations. The surface coordination model, developed for hydrous oxide surfaces, allows a uniform approach to be adopted for different classes of materials. This model is also predictive and has been applied successfully to natural systems.

From the point of view of nuclide transport by colloids, irreversible sorption represents the most unfavorable situation. There is virtually no information available on the extent of reversibility and on the desorption kinetics of important nuclide/colloid combinations. Experimental investigations are therefore necessary in this respect.

The only question considered in connection with colloid transport and its modelling is that of colloid sticking. Natural colloids, and the surfaces of the rock on which they may be collected, generally have negative surface charges so that colloid sticking will be difficult. The DLVO theory contains an approach for calculating the sticking factor from the surface potentials of the solid phases and the ionic strength of the water. However, it has been shown that this theory is inapplicable because of inherent shortcomings which lead to completely unrealistic predictions. The sticking probability of colloids should therefore be determined experimentally for systems which correspond as closely as possible to reality.

Technical Report NTB 90-36

On the problem of silicia solubility at high pH

Summary

The aqueous system Na₂O-H₂O-SiO₂ is considered to play an important role when strong alkaline pore waters of a cement based intermediate level radioactive waste repository intrude into the rock formations surrounding the near field. Under such con­ditions unknown quantities of silica may dissolve. Therefore the pH-dependence of the solubility of amorphous silica and quartz is investigated by a parameter variation study using the geochemical speciation code MINEQL/EIR. Published silica solubility data obtained in sodium hydroxide solutions at 25 and 90°C are compared with the results of four models which use different proposed values of the rather uncertain equilibrium constants. Of main interest is the question of whether, in a high pH region, the silica solubility can be explained with different monomeric species only, or to what extent additional polymeric silica species have to be considered as well. Furthermore, the influence of silica-sodium ion pairing under such extreme conditions is also studied.

The solubility of amorphous silica at 25°C is well understood up to a pH of about 10.5, where it is determined by the solubility product and the first dissociation constant of monomeric silicic acid. In more alkaline media the total solubility increases dramatically. This behaviour is less well understood due to uncertainties of the second deprotonation and polymerisation constants. Despite these uncertainties, it is shown that the most probable cause of the increased solubility of amorphous silica in the region between pH 10.5 and 11.3 is the formation of dimers, trimers and tetramers. Below a total silica concentration of 0.01 M and pH ≤ 10.0, however, polymerisation proves to be insignificant.

Besides low temperature studies using amorphous silica, the solubility of quartz has also been measured in NaOH solutions at 90°C. As is the case at lower temperatures, the reported values for the second dissociation constant at 90°C scatter widely. Although it is not possible to extract this value from the few analytical data, it can be shown that in a NaOH medium up to 0.1 M only mononuclear silica species are stable. Therefore it is concluded that the trend of monomers to form polymers decreases strongly with temperature. In addition, there are indications that, in strong NaOH solutions at elevated temperatures, silica-sodium ion pairing gains importance.

Technical Report NTB 90-35

Stripa Project
Annual Report 1989

Summary

The Stripa Project is organised as an autonomous project of the Nuclear Energy Agency of the OECD. Over the time period 1986 – 1991 (Phase 3 of the Project), an international cooperative programme of investigations is being carried out in an underground rock laboratory in Sweden. Building on experience gained in Phases 1 and 2, the following research will be carried out:

Technischer Bericht NTB 90-34

Sondierbohrung Siblingen
Untersuchungsbericht

Summary

In 1980, Nagra (the National Cooperative for the Disposal of Radioactive Waste) launched an extensive geological investigation programme covering an area of some 1'200 m² in Northern Switzerland. The aim of the programme was to collect the geological information necessary for assessing the suitability of the crystalline basement as a disposal medium for high-level radioactive waste. The wide range of investigations performed can be divided logically into a deep drilling programme, geophysical reconnaissance of the rock formations and structures in the region, a hydrogeological programme for investigation of water flow-paths deep in the bedrock and a neotectonic programme for identification of active crustal movements in the area.

The Siblingen borehole was the seventh in the deep drilling programme. The drill site (coordinates 680.090/286.693, 574.35 m a.s.l.) lies within the community of Siblingen in Canton Schaffhausen, approximately 1.5 km north-west of the Siblingen village boundary. The final depth of the borehole was 1'522 m.

The borehole penetrated through the sedimentary overburden and more than 1,000 m into the crystalline basement. A wide range of investigations was carried out in the borehole itself, as well as on drillcores and water samples. State-­of-the-art methods were used to draw a detailed picture of the geology of the basement and the deep groundwater regime.

Drilling began on 1^st September 1988 and continued, with regular interruptions for scientific investigations, until 2nd April 1989. During the whole of April 1989, further tests and investigations were carried out. Because of prevailing hydraulic conditions (numerous interconnected disturbed zones), it was decided not to install a multipacker system for long-term observations and, at the beginning of June, an additional fluid logging run with a newly developed heat pulse packer-flowmeter was performed. An estimate of the horizontal regional gradient using return flow of tracer-marked drilling fluid to the borehole, investigations of the influence of earth tides on water levels and several temperature/conductivity logs completed the programme. Periodical water measurements have been carried out since 17^th May 1989 and are still continuing. The planned investigation programme was carried out more or less in full, and with a considerable degree of success.

The drilling and investigation programmes were planned and managed by Nagra employees with the cooperation of external advisers. More than 40 technical colleges, contractors and service companies from Switzerland, Germany, France, Great Britain, the Netherlands, Canada and the USA, representing a total manpower of over 150 scientists and technicians, were involved in the investigations.

This report summarises the most significant results obtained during the course of the programme. The series of investigation reports for the deep boreholes (Böttstein, Weiach, Riniken, Leuggern, Schafisheim, Kaisten and Siblingen) is structured in such a way that cross comparisons can easily be made between the results and datasets for the different locations. However, the conclusions to be drawn from the data will be reserved for a final synthesis report. Detailed documentation of the individual experiments is provided in Nagra's Technical Report series (see NTB list at the end of this report).

Technical Report NTB 90-33

Poços de Caldas Report No. 15
Summary and implications for radioactive waste management

Summary

This report provides an overview of the Poços de Caldas natural analogue study, aspects of which have been described in detail in the previous 14 reports of this series.

The first part of this report provides the historical background to the project, describes the research programme developed at the Osamu Utsumi and Morro do Ferro sites and summarises the main findings of the geological, hydrologic and geochemical characterisation work.

This is followed by reviews of the four analogue sub-projects – testing geochemical models of trace element solubility and speciation, evaluation of models of redox front development and movement, examining the role of natural colloids as a vector for transport of trace elements and appraisal of a modelling approach to quantifying hydrothermal alteration and solute transport processes.

Finally, the direct implications of the project to radioactive waste management are discussed along with some valuable spin-off which was identified. Such a large multidisciplinary study proved a very valuable focus for forging contacts between specialists – from different disciplines in a manner closely analogous to that required for an integrated site assessment. The iterative development of the modellers' wish lists and the field/lab analytical programme was a key to the success of this Project and provides guidelines for site-specific performance assessment.

Technical Report NTB 90-32

Poços de Caldas Report No. 14
Geochemical modelling of water-rock interactions at the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil

Summary

Geochemical processes involving water-rock-gas interactions have been modelled using groundwater compositions, mineralogical data, ion plots and computations of speciation, non-thermodynamic mass balance and thermodynamic mass transfer for two natural analogue sites near Poços de Caldas, Brazil: the Osamu Utsumi mine and Morro do Ferro. The main rock type is an alkaline igneous complex composed of volcanic and sub-volcanic phonolites that have been hydrothermally altered and highly weathered. This altered rock mass grades from a laterite at the surface to a saprolite and finally to unweathered, hydrothermally altered bedrock at depth. The mine site contains high concentrations of uranium and Morro do Ferro contains high concentrations of thorium and rare-earths. The reaction models can reproduce the water chemistry and mineral occurrences and they were validated by predicting the masses of minerals precipitated and the pH of the final water. The model computations can also reproduce the pH and iron concentrations of the water samples during CO₂ degassing and iron(II) oxidation from exposure to air. The results from the geochemical reaction models reveal that the dominant processes are production of CO₂ in the soil zone through aerobic decay of organic matter, dissolution of fluorite, calcite, K-feldspar, albite and manganese oxides, oxidation of pyrite and sphalerite and precipitation of ferric oxides, silica and kaolinite. Recharge waters are undersaturated with respect to barite and discharging waters and deeper groundwaters are saturated to supersaturated with respect to barite, demonstrating a strong equilibrium solubility control. Strontium isotope data demonstrate that sources other than calcium-bearing minerals are required to account for the dissolved strontium in the groundwaters. These may include K-feldspar, smectite-chlorite mixed-layer clays and goyazite.

Technical Report NTB 90-31

Poços de Caldas Report No. 13
Near-field high-temperature transport: Evidence from the genesis of the Osamu Utsumi uranium mine, Poços de Caldas alkaline complex, Brazil

Summary

The chemical, isotopic and mineralogical alteration which occurred during primary uranium ore deposition at the breccia pipe-hosted Osamu Utsumi mine, Poços de Caldas, Brazil, was studied as a natural analogue for near-field radionuclide migration. Chemical and isotopic alteration models were combined with finite difference models of the convective cooling of caldera intrusives. The modelling indicates that the intense chemical, isotopic and mineralogical alteration of the Osamu Utsumi breccia pipe requires the circulation of > 10⁵ kg of boiling hydrothermal fluid > 220°C through each square centimetre cross-section of the pipe. The circulation can be driven by heat from a 6 km diameter intrusion extending to 10 km depth. Even with this large amount of circulation concentrated in the permeable breccia pipe, uranium solubilities must be 2 1/2 orders of magnitude greater than indicated in the most recent experiments (and more in line with previous estimates) to produce the primary uranium mineralization at the Osamu Utsumi mine.

The same models applied to a hypothetical high-temperature waste repository show that heat from radioactive decay will produce a hydrothermal circulation system remarkably similar to that studied at the natural analogue site at Poços de Caldas. The depth of fluid convection induced by the hypothetical repository would be 5 to 10 km, the maximum temperature ~ 300°C, the lifetime of the high-temperature phase a few 1000s of years and boiling would cause most of the alteration of the waste repository. The physical analysis emphasizes the importance of permeability on a 10 × 10 × 10 km scale in controlling the potential amount of circulation through the hypothetical repository.

Application of the chemical models successfully used to interpret mineralization and alteration at the Osamu Utsumi mine to the hypothetical waste repository shows that, even in a worst case scenario (waste emplaced in a permeable host rock with no measures taken to inhibit flow through the repository), the amount of hydrothermal alteration in the hypothetical repository will be ~ 0.1 % of that in the breccia pipe at the Osamu Utsumi mine. Assuming no barriers to uranium mobility, uranium precipitation above the hypothetical repository would be 0.05 ppm (rather than 50 ppm), hydrothermal alteration 0.03 wt.% (rather than 30 wt.%), etc.

The analysis indicates that mineralogical alteration is extremely sensitive to thermodynamic data. Prediction of mineralogical alteration (which may be necessary to predict the migration of radionuclides other than uranium, for example) probably cannot be based directly on even very carefully collected laboratory thermodynamic data. Mineralogical complexities of the system, as well as database uncertainties, will require calibration of the thermodynamic framework against mineralogical alteration observed in the laboratory or field by procedures briefly described.

Technical Report NTB 90-30

Poços de Caldas Report No. 12
Testing models of redox front migration and geochemistry at the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil

Summary

Redox fronts occur at a number of locations in repository systems and models have been established to describe their chemical evolution and spatial development. Such models can be tested against detailed observations of the well-developed redox fronts at the Osamu Utsumi mine.

Simple scoping calculations can explain the formation of redox fronts in very general terms but greatly simplify the processes known to be occurring at such fronts. Coupled transport / chemistry models can provide a better simulation of the fronts, but these are primarily interpretative models which have not yet displayed any convincing predictive abilities. They tend to be rather poor, in particular, in simulating trace element chemistry in either solution or solid phases.

Interpretative modelling of microbial activity, natural series profiles and trace element distributions gives strong indications of the reasons for the limitations of the chemical modelling. The role of microbial catalysis seems to be very significant in such systems, particularly affecting the redox chemistry of sulphur. Natural series measurements indicate very slow redox front movement at particular sites which could be due to precipitation processes limiting accessible porosity, a point not considered in any of the models. Finally, the trace element distributions strongly suggest immobilisation of many elements as co-precipitates or solid solutions in secondary iron minerals, again a process not considered by current models.

Technical Report NTB 90-29

Poços de Caldas Report No. 11
Testing of geochemical models in the Poços de Caldas analogue study

Summary

In order to test the geochemical models used in repository performance assessment, modelling groups were provided with selected major element analyses of Poços de Caldas groundwaters and asked to predict 'blind' the solubility, speciation and limiting solid for a number of trace elements. This report documents these predictions and compares them to field analyses. These tests illustrate particular strengths and weaknesses in current models/databases and allow recommendations for amendments/improvements to be made.

Technical Report NTB 90-28

Poços de Caldas Report No. 10
Microbiological analysis at the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil

Summary

The Poços de Caldas project is a wide-ranging natural analogue study focussed on a number of areas of concern in the performance assessment of the disposal of radioactive waste. Part of the work has involved characterising microbial populations and their influence in various processes. Core material and groundwaters have been sampled for microbiological content at various depths from boreholes at the Osamu Utsumi open pit uranium mine and Morro do Ferro Th/REE ore body. Microbes were found in all samples but numbers do not appear to be related to depth. Analyses of groundwaters gave higher numbers than with solid material and demonstrated the presence of sulphur cycle bacteria.

These observations have been compared with predictions of a model used in performance assessment to calculate the maximum biomass/microbial activity based on constraints set by available nutrients and energy. The main conclusions of this analysis are:

Technical Report NTB 90-27

Poços de Caldas Report No. 9
Chemical and physical characterisationof suspended particles andcolloids in waters from the OsamuUtsumi mine and Morro do Ferroanalogue study sites, Poços de Caldas, Brazil

Summary

Data are presented on suspended particles and colloids in groundwaters from the Osamu Utsumi mine and the Morro do Ferro analogue study sites. Cross-flow ultrafiltration with membranes of different pore sizes (450 nm to 1.5 nm) was used to prepare colloid concentrates and ultrafiltrates for analyses of major and trace elements and U- and Th-isotopic compositions. Additional characterisation of colloidal and particulate material was performed by ESCA, SEM and X-ray diffraction. The results obtained indicate the presence of low concentrations ( 450 nm) in the waters show the same trends as the colloids with respect to U, Th and REE associations, but elemental concentrations were typically higher by a factor of 1'000 or more. Calculated association ratios (in mI·g^-1) are in the order of 10⁴ to 10⁵ for U, 10⁵ to 10⁶ for the REEs and 10⁶ to 10⁷ for Th. In waters with low pH and high sulphate content, these ratios are considerably lower. Due to the low concentrations of suspended particles in groundwaters from the Osamu Utsumi U-mine (< 0.5 mg/l), these particles carry only a minor amount of U and REEs (< 10 % of the total concentration of the unfiltered groundwater), but a significant, usually predominant, fraction of Th (30 – 70 %). The suspended particle load in groundwaters from the Morro do Ferro environment is typically higher than in those from the mine by a factor of 5 to 10. This suggests that U and the REEs could be transported predominantly by particulate matter. However, there is chemical and mineralogical evidence that these particles, composed mainly of Fe(III) oxyhydroxides and humic-like compounds, have a low capacity for migration.

Technical Report NTB 90-26

Thermodynamique du gonflement de systèmes eau-bentonite, eau-métabentonite, eau-illite

Summary

The water vapour adsorption isotherms and enthalpies of immersion of 15 samples of natural clays (bentonites, metabentonites and illites) in water have been measured at 20°C. The aim of these measurements was firstly to study the influence of ultrasonic treatment of the solids, and secondly to observe the change in water adsorption at different stages in the illitisation process of montmorillonite.

As regards the ultrasonic treatment, we can assert that it has little influence on the different samples, with the exception of the metabentonite "B". This shows an important increase in its BET surface area and micropore volume, and in the swelling pressure at every degree of hydration. The other solids, on the contrary, show a small decrease in both the BET surface area and the swelling pressure at low water contents. At higher degrees of hydration, the swelling pressure increases after ultrasonic treatment, as pointed out by Kahr and Müller-Vonmoos. However, it is not possible to derive information valid for other degrees of hydration from measurements obtained for a specific water content.

We also observed that adsorption depends strongly on the K+ content of the clay. The higher the content, the weaker the adsorption, and a corresponding decrease is observed for the swelling pressure and the heat of immersion in water.

Finally, owing to the difficulty of finding a suitable mathematical fit to describe the enthalpy change with the amount of pre-adsorbed water, the problem of the entropy of the water molecules is only dealt with in a qualitative way.

Technical Report NTB 90-25

Natural radionuclide and stable element studies of rock samples from the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil.

Summary

This report describes a study of the distribution and behaviour of natural radionuclides and selected stable elements at the Poços de Caldas natural analogue study sites. At the Osamu Utsumi mine, the study was focussed upon investigation of the behaviour of natural decay series radionuclides and stable elements at the redox fronts which exist in the mine. Uranium nodules from the mine were also analyzed for natural decay series radionuclides in order to characterize their ages and growth rates, and for natural plutonium. At Morro do Ferro, the objective was to provide additional information on the geochemical behaviour of thorium, uranium and the light rare-earth elements and to relate this to the groundwater flow pattern in an attempt to evaluate the degree of mobilization of these species.

A summary review of those aspects of the geochemistry of natural decay series radionuclides relevant to the interpretation of radioactive disequilibrium is provided along with a detailed treatment of mathematical modelling of natural decay series disequilibria in rock-water interactions. The overall study comprised a number of discrete subprojects carried out in five different laboratories and each of these is described in detail. In addition to constituting a self-contained study, the results and conclusions of this work were also used as an input for modelling studies and other aspects of this integrated research programme.

The natural decay series studies at the Osamu Utsumi mine confirmed the generally greater mobility of U(VI) and Ra than of U(IV), Th and Pa in groundwater. The results further confirmed that, at the position of the drillcore studied, the long-term direction of groundwater flow had been downwards along the line of the main fissure system, rather than upwards as is presently observed.

Dissolution of uranium and other elements at the redox fronts followed by diffusive movement into both the oxidized and the reduced rock is identified as the mechanism giving rise to the observed concentration profiles about the redox fronts. Deposition of uranium, either as thin, dispersed coatings on other minerals or as discrete nodules, occurs in the reduced rock as a consequence of the reduction of U(VI) to U(IV). Deposition of uranium, almost certainly by uptake on iron oxides, is also identified as a significant retardation process in the oxidized rock Some of the uranium nodules are young (on a 10⁵ – 10⁶ year timescale) and exhibit growth rates of 1.8 – 2.6 cm in 10⁶ a, whereas others are old on this timescale and exhibit equilibrium within the natural decay series. The time required for growth of nodules in the reduced rock is estimated to be of the order of 10⁵ a at least, while a time of the order of 10⁴ – 10⁵ a is required for dissolution of micronodules stranded in the oxidized rock following the passage of the redox front. The natural ²³⁹Pu content of a nodule from the reduced rock was measured as 2.3 ± 0.7 × 10⁸ atoms per gram, consistent with a state of secular equilibrium between ²³⁸U and ²³⁹PU. The study thus reveals that uranium (plus daughters) and plutonium are chemically stable in the form of nodules in the reduced rock for a time of at least 10⁵ years.

One oft he redox fronts studied was concluded to have been effectively static (on a cm scale) for a period of at least 7 × 10⁵a, while the natural decay series data for the other fronts were consistent with rates of movement in the range 2 – 20 m in 10⁶a, in good general agreement with the estimated rate of regional erosion.

Redistribution of thorium was observed at the redox fronts, with preferential deposition on the reduced sides of the fronts. The degree of this redistribution of thorium is estimated to be at least two orders of magnitude less than the corresponding redistribution of uranium. Separation of uranium from thorium is observed as the redox fronts move downwards and the degree of separation increases with increased length of flow path of the groundwater.

Summary data are provided for a range of stable elements, including the rare-earths, which support the observations made above for uranium and indicate that the zone around the redox fronts contains generally elevated concentrations of most elements in conjunction with active dissolution at the front. This results in a variety of distributions of different elements about the redox fronts. The redox fronts thus represent an initial zone of retardation which would be a positive factor in far-field radionuclide migration considerations. However, a negative aspect of this situation is that the zone of enhanced concentrations moves in response to movement of the redox front and, given a sufficient distance of travel, could result in a breakthrough of high concentrations of radionuclides into the near-surface environment.

Technical Report NTB 90-24

Chemical and isotopic composition of groundwaters and their seasonal variability at the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil

Summary

Groundwaters and a few surface waters were collected over a period of 3 years from two natural analogue sites near Poços de Caldas, Brazil: the Osamu Utsumi uranium mine and the Morro do Ferro thorium/rare-earth deposit. These were analysed for major constituents, several trace elements, tritium, deuterium and ¹⁸O, to provide hydrochemical data for the modelling objectives as defined within the natural analogue study programme. The groundwaters are a K-Fe-SO₄ type, which classifies them as a highly unusual composition related to the weathering of a hydrothermally altered, mineralised complex of volcanic to sub-volcanic phonolites.

A subset of selected constituents (Fe(II), Fe(total), SO₄, pH, Eh, alkalinity, F and U) was monitored to gain detailed information on the seasonal variability. Seasonal patterns were only apparent from the very shallow groundwater data, but a trend of continuously increasing dissolved solids for the furthest down-gradient sampling point in deep groundwater indicates a growing plume of water affected by pyrite oxidation but without the residual acidity. Tritium and stable isotope measurements indicate that all groundwaters are of meteoric origin and are not affected significantly by evaporation or by water-rock interactions. Recharging groundwaters at both study sites demonstrate infiltration of water of less than about 35 years in age, whereas deep groundwaters are below 1 TU (tritium unit) but still contain detectable tritium in most cases. These deeper groundwaters may be interpreted as being 35 – 60 or more years in age, resulting mainly from an admixture of younger with older groundwaters and/or indicating the influence of subsurface-produced tritium.

Technical Report NTB 90-23

Poços de Caldas Report No. 5
Geomorphological and hydrogeological features of the Poços de Caldas caldera and the Osamu Utsumi mine and Morro do Ferro analogue study sites, Brazil

Summary

The Osamu Utsumi mine and Morro do Ferro study sites lie within the Poços de Caldas plateau which is roughly circular in outline with a diameter of 35 km and an area of approximately 800 m2 Its general altitude lies between 1300 and 1600 m. The plateau is the eroded form of a caldera which was initially intruded some 80 million years ago.

Geomorphologically, both sites occupy watershed areas adjacent to small streams in the centre of the plateau. The climate of the area has a marked wet season from November to April and is dry the rest of the year. The streams are ephemeral in their upper reaches, tending to dry up in the dry season as they are fed by a declining base flow. In the wet season they exhibit flash floods fed by high-intensity rainfall causing overland flow. The wet season also provides recharge to the groundwater. Natural slopes are steep and the original vegetation was thin forest cover which is now restricted to the valley bottoms; usable slopes have poor quality grass cover used for cattle grazing. The plateau is a stable feature and its surface has been eroding at an average rate of 12 m per million years over a period of 50 million years.

The mine geology is dominantly volcanic to subvolcanic phonolites that have been hydrothermally altered. Fracturing of the rock is extensive. Downward diffusion of oxygen in groundwaters during deep weathering has produced a distinct redox zone seen as a colour change from green/grey to brown/yellow. Morro do Ferro has a more weathered version (laterite/clay) of the same geology penetrated by magnetite breccias dykes. Whilst the area surrounding Morro do Ferro remains untouched, that around the mine has been seriously disturbed by mining activity which has penetrated the water table. The existing mine has modified groundwater flow patterns and disturbed the movement of oxidising and reducing waters.

Technical Report NTB 90-22

Pogos de Caldas Report No.4
Isotopic geochemical characterization of selected nepheline syenites and phonolites from the Po~os de Caldas alkaline complex, Minas Gerais, Brazil.

Summary

This paper presents and discusses the isotopic data from the hydrothermal studies of the Poços de Caldas Natural Analogue Project. The purpose of this study was to elucidate the mass transport of relevant elements and isotopes associated with hydrothermal mineralization and alteration at the Osamu Utsumi uranium mine, particularly as applicable to radwaste isolation concerns in the U.S. nuclear waste program.

Research efforts were focussed on studying the thermal, chemical and hydrologic nature of the palaeohydrothermal regime associated with a breccia pipe at the Osamu Utsumi mine, and related to the geochemical, geochronological and petrological characterization studies of unaltered regional nepheline syenite and phonolite.

The regional rocks studied have a vertically elongated δD, δ¹⁸O pattern, which possibly indicates meteoric water/rock interaction. Regression of Rb-Sr whole-rock data for the regional nepheline syenite and phonolite samples did not produce isochrons. An internal, mineral separate isochron regression from a nepheline syenite sample, considered representative of unaltered nepheline syenite of the Poços de Caldas plateau, yields an age of 78 Ma, and an initial 87Sr/86Sr ratio of 0.70511. The moderate initial ⁸⁷Sr/⁸⁶Sr ratios of the regional nepheline syenites are possibly indicative of a mantle source for the alkaline magmatism, with some incorporation of old, high Rb/Sr crustal material. The greater-than-mantle values of δ¹⁸O, if not due solely to surficial processes, also appear to require some assimilation of crustal material. Sm-Nd data for the regional rocks do not define any isochrons, although the nepheline syenite samples conform very well to a calculated model isochron for 78 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd of 0.512359. The regional phonolite samples lie markedly off this isochron. This open system behavior is probably due to the phonolite samples having different initial ¹⁴³Nd/¹⁴⁴Nd values. Even so, all regional samples lie within the "Mantle Array" trend. Their location within the ε_Nd-ε_Sr space indicates an asthenospheric Mid Ocean Ridge Basalt (MORB)-type source magma also contaminated by continental igneous and metamorphic rocks (e.g. the Precambrian schist surrounding the Poços de Caldas plateau).

The rocks studied at the Osamu Utsumi mine from the F4 drillcore have experienced varying degrees of hydrothermal mineralization and metasomatism, and deep weathering. The hydrothermally altered rocks have a quite pronounced δD shift, with only a slight δ¹⁸O shift.

The δD-δ¹⁸O trend of the hydrothermally altered F4 samples most likely reflects the variability of temperature, hydrologic flow and, therefore, water/rock interaction and isotopic exchange in the palaeohydrothermal regime.

Regression of Rb-Sr whole-rock data for subsamples from a nepheline syenite xenolith sample yields an age of 76 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70530. Due to the marked hydrothermal alteration and metasomatism of this sample, the Rb-Sr isotopic system is interpreted as being re-equilibrated and thus the regressed age is the age of the hydrothermal event. Using a (⁸⁷Sr/⁸⁶Sr)i versus 1/Sr mixing diagram, distinct trends are seen for hydrothermal alteration, mineralization and weathering.

Again, the nepheline syenite subsamples do not define a Sm-Nd isochron, but conform very well to a calculated model isochron for 78 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd of 0.512365. The Sm-Nd isotopic data also exhibit a likely disturbance by the hydrothermal, metasomatic alteration.

A lamproite dyke which crosscuts the hydrothermal alteration in the Osamu Utsumi mine gives an age of 76 Ma, which is essentially the same as the Rb-Sr age of the nepheline syenite subsamples.

Technical Report NTB 90-21

Pogos de Caldas Report No.3
Mineralogy, petrology and geochemistry of the Poços de Caldas analogue study sites, Minas Gerais, Brazil.

Summary

The thorium-rare-earth element (Th-REE) deposit at Morro do Ferro is of supergene origin and was formed under lateritic weathering conditions. The ore body forms shallow NW-SE elongated argillaceous lenses that extend from the top of the hill downwards along its south-eastern slope. The deposit is capped by a stock work of magnetite veins which have protected the underlying, highly weathered argillaceous host rock from excessive erosion. The surrounding country rocks comprise a sequence of subvolcanic phonolite intrusions that have been strongly altered by hydrothermal and supergene processes.

From petrological, mineralogical and geochemical studies and mass balance calculations, it is inferred that the highly weathered host rock was originally carbonatitic in composition and was initially enhanced in thorium and rare-earth elements compared to the surrounding silicate rocks. Intrusion of the carbonatite produced a fenitic alteration of the surrounding phonolites, consisting of an early potassic alteration followed by a vein-type Th-REE mineralization with associated fluorite, carbonate, pyrite and zircon. Subsequent lateritic weathering has completely destroyed the carbonatite, forming a residual supergene enrichment of Th and REEs.

Initial weathering of the carbonatite leading to solutions enriched in carbonate and phosphate may have appreciably restricted the dissolution of the primary Th-REE phases. Strongly oxidic weathering has resulted in a fractionation between cerium and the other light rare-earth elements (LREEs). Ce³⁺ is oxidized to Ce⁴⁺ and retained together with thorium by secondary mineral formation and adsorption on poorly crystalline iron­ and aluminium-hydroxides. In contrast, the trivalent LREEs are retained to a lesser degree and are thus more available for secondary mineral formation and adsorption at greater depths down the weathering column. Seasonally controlled fluctuations of recharge waters into the weathering column may help to explain the observed repetition of Th-Ce enriched zones underlain by trivalent LREE enriched zones.

Technical Report NTB 90-20

Poços de Caldas Report No.2
Mineralogy, petrology and geochemistry of the Poços de Caldas analogue study sites, Minas Gerais, Brazil. 1: Osamu Utsumi uranium mine

Summary

The lithology of the Osamu Utsumi mine is composed mainly of a sequence of volcanic and subvolcanic phonolites and nepheline syenite intrusions similar to those of the Poços de Caldas caldera complex; volcanic breccia pipes about 80 m in diameter also occur, characterised by U-Th-Zr-REE mineralisation concentrated in the matrix. A strong hydrothermal alteration, related to the formation of the breccias, has resulted in the potassic alteration and pyritisation of the phonolites and syenites, with a low-grade mineralisation of disseminated pitchblende. The potassic alteration has transformed all feldspars into pure potash feldspars, nepheline into illite and kaolinite, and clinopyroxenes, which are the primary REE-bearers, into mixtures of TiO2-rich minerals, clay minerals and pyrite. The enrichment of K, S, U, Th, Pb, Rb, Ba and Mo was accompanied by a strong depletion in Ca, Na, Mg and Sr. Fluid inclusion data indicate temperatures around 250°C and a KCI-H2O mixture with approximately 7 wt.% KCI for the hydrothermal fluids. For fluids in the breccia pipes that transported additional Zr, Hf and F, the inclusions indicate boiling and give temperatures of 210°C with 40 – 45 wt.% KCI for a KCI-NaCI-H2O brine containing FeSO₄ and KF. Ultramafic dykes (dated to 76 Ma) with carbonatitic affiliation put a younger age limit on the hydrothermal event.

As a result of supergene weathering below a lateritic soil cover 20 to 40 m thick and a saprolite zone 15 to 60 m thick, the pyrites in the rocks have been oxidised to varying depths of 80 to 140 m below surface, resulting in a redox front marked by a contrasting colour change in the rock from oxidised (yellow/buff) to reduced (grey/green) rock. In the vicinity of water-bearing fissures these redox fronts have penetrated to greater depths. Due to mobilisation in the oxidised zone and precipitation immediately below the redox front, a secondary pitchblende mineralisation, partly occurring as nodules associated with secondary pyrite, has been developed. This secondary pyrite is sometimes related with CdS and has a δS of -13 ‰ compared to δS values of -3.63 to +1.24 ‰ for hydrothermal pyrites; the low δS values of the former are attributed to bacterial action. In the immediate vicinity of the redox front, dissolution of the potash feldspar becomes apparent and the kaolinite content begins to increase. The marked colour change at the redox front is caused by the presence of hydrous ferric oxides that, with time, evolve from amorphous Fe-hydroxides to goethite and hematite. On the oxidised side alunite-jarosite minerals are frequent. The porosity increases by 5 to 8 % (almost doubled) at the redox front.

Gibbsite is present when potash feldspar has been removed at the saprolite-laterite contact. Many of the REEs are associated with phosphate-rich clays (including the crandallite group minerals) and seem to be hardly mobilised by the oxidising weathering fluids. Only a slight general loss of REEs was observed between oxidised and reduced rock, with the degree of loss being greater for the light REEs. Certain indications of a fractionation of Ce and Eu from the other REEs are present.

Technical Report NTB 90-19

The regional geology, mineralogy and geochemistry of the Poços de Caldas alkaline caldera complex, Minas Gerais, Brazil.

Summary

The Poços de Caldas alkaline complex, the largest in South America, is circular-shaped with a mean diameter of about 33 Ian. It is one of the Mesozoic alkaline occurrences of south-eastern Brazil that developed from the Upper Jurassic onwards, during continental break-up and drift. It comprises a suite of alkaline volcanic and plutonic rocks (mainly phonolites and nepheline syenites) with normal background amounts of U, Th and rare-earth elements (REEs). The evolutionary history began with major early volcanism involving ankaratrites, phonolite lavas and volcanoclastics, followed by caldera subsidence and nepheline syenite intrusions forming minor ring dykes, various intrusive bodies and circular structures. Finally, eudialyte nepheline syenites, and phonolites, strongly enriched in incompatible elements, were emplaced.

Magmatic evolution included deuteric processes, indicating a volatile-rich parent magma. These processes extended over a large temperature range and resulted in the formation of pegmatitic veins and pneumatolytic and auto-hydrothermal mineral assemblages including rare metal silicates such as giannettite, various zeolites, fluorite and hematite, together with mineral alterations. These alterations developed from magmatic pseudoleucite through alkali-exchange reactions to a "clouding" of the main alkali feldspars under auto-hydrothermal conditions (with fluid inclusion formation, incipient kaolinization and hematite-hydrous ferric oxide pigmentation). Geochemically, the resulting rocks are enriched in potassium when compared to global nepheline syenites and phonolites. Mobilization and concentration of U, Th and REEs could not be detected at this stage.

At least at one location (Morro do Ferro) the intermediate nephelinic suite was affected by a carbonatite intrusion and the formation of a stockwork of magnetite veins.

Very intensive hydrothermal K- and S-rich alteration, associated with pene­contemporaneous formation of magmatic breccias, occurred locally.

These processes led to the formation of several important radioactive and REE-rich anomalies. Two of these, the Th-REE occurrence of Morro do Ferro and the U-Zr-REE-Th occurrence of the Osamu Utsumi uranium mine, comprise the study sites of the Poços de Caldas Analogue Project.

Later major evolutive stages of the Poços de Caldas complex involved the emplacement of mafic-ultramafic dyke rocks and the onset of lateritic and allitic weathering, resulting (at the uranium mine) in supergenic geochemical redistribution and the formation of redox fronts sometimes related to uranium enrichments.

The regional rock studies were focussed on the "status quo" properties of the intermediate nephelinic suite with respect to the subsequent more local hydrothermal and final weathering-related processes. They included petrographic, mineralogical, geochemical and isotopic studies, in addition to petrophysical parameters. Results showed very little variation for the studied intrusive, subvolcanic and volcanic nepheline syenites and phonolites. The lack of a major differentiated series may be seen as an argument for a short emplacement history of the intermediate nephelinic suite. Previous and present radiometric age measurements suggest a time span of about 15 Ma for emplacement, much too long if compared to modern volcanoes. The end of the magmatic and hydrothermal-mineralizing events is likely fixed by the Ar-Ar dating of an unmineralized lamprophyre dyke intrusion at the site of the uranium mine (76 Ma).

Technical Report NTB 90-18

COMPLEXATION OF Cu2+, Ni2+ AND UO22+ BY RADIOLYTIC DEGRADATION PRODUCTS OF BITUMEN

Summary

The radiolytic degradation of bitumen was studied under conditions which reflect those which will exist in the near field of a cementitious radioactive waste repository.

The potential complexation capacity of the degradation products was studied and complexation experiments with Cu²⁺, Ni²⁺ and UO₂²⁺ were performed. In general 1:1 complexes with Cu²⁺, Ni²⁺ and UO₂²⁺, with log K values of between 5.7 and 6.0 for Cu²⁺, 4.2 for Ni²⁺ and 6.1 for UO₂²⁺, were produced at an ionic strength I of 0.1 M. The composition of the bitumen water was analysed by GC-MS and IC. The major proportion of the bitumen degradation products in solution were monocarboxylic acids (acetic acid, formic acid, myristic acid, stearic acid ...), dicarboxylic acids (oxalic acid, phthalic acid) and carbonates.

The experimentally derived log K data are in good agreement with the literature and suggest that oxalate determines the speciation of Cu²⁺, Ni²⁺ and UO₂²⁺ in the bitumen water below pH = 7. However, under the high pH conditions typical of the near field of a cementitious repository, competition with OH-ligands will be large and oxalate, therefore, will not play a significant role in the speciation of radionuclides. The main conclusion of the study is that the radiolytic degradation products of bitumen will have no influence on radionuclide speciation in a cementitious near field and, as such, need not to be considered in the appropriate safety assessment models.

Technischer Bericht NTB 90-17

Langzeitquellenverhalten von Tongesteinen und tonigen Sulfatgesteinen

Summary

The swelling behaviour of shales such as the Opalinum Clay, and of clay-containing sulphate rocks is, in addition to the influence of the overburden pressure, mainly dependent on the mineralogical composition of the rock.

The swelling potential of shales increases with increasing content and specific surface area of the clay. Swelling pressures greater than 2 MPa and swelling strains greater than 10 percent are usually not encountered above depths of up to 300 m below the ground surface.

The relationship between the swelling behaviour of clay-sulphate rocks and their mineralogical composition is more complicated than that of pure clays. In those rocks which contain anhydrite, the texture determines the time-swelling behaviour and the development of the mineralogical swelling potential.

Rocks containing only anhydrite do not swell, whereas, rocks containing 5 percent clay may develop a swelling stress of more than 1 MPa for axial strains less than 1 percent. The maximum swelling potential was found in rocks containing 10 to 15 percent clay and 70 to 75 percent anhydrite. With this latter composition, a swelling stress greater than 4 MPa was determined for axial strain less than 2.5 percent. Samples containing more than 15 percent clay tended to show a smaller swelling potential.

The swelling strain of clay-sulphate rock depends on the quantity of anhydrite which is converted into gypsum. The clay content also plays an important role. Rock without clay shows no swelling strain, even after a testing period of six years.

Samples with a higher clay content exhibit a faster conversion of anhydrite into gypsum than do samples with a smaller amount of clay. The swelling strain of samples containing as little as 5 percent clay was more than 100 percent after a testing period of two years. During this period 90 percent of the anhydrite was converted into gypsum.

Technical Report NTB 90-16

Experimental proposals for procedures to investigate the water chemistry, sorption and transport properties of marl

Summary

The aim of this report is to describe a framework within which laboratory studies on groundwater chemistry, sorption and transport properties might be conducted on samples from rock formations being considered as potential "host rocks" for the disposal of radioactive waste. Here, Valanginian marl has been taken as a specific example, but the general principles should be applicable to other systems.

Some brief notes are given on sampling and handling procedures and mineralogical characterisation. This is followed by a detailed discussion of the procedures considered necessary to determine a groundwater chemistry of a specific rock matrix. The methods described are particularly appropriate to rocks such as marl, i.e. low water content rocks (essentially "dry") with appreciable clay and carbonate contents. An important conclusion drawn is that simple aqueous phase extractions at different liquid to solid ratios, followed by extrapolation procedures, are not always appropriate and can lead to incorrect water compositions.

Some of the uncertainties and difficulties inherently involved in determining sorption parameters from batch, infiltration and diffusion based methods are presented. These methods are then individually discussed in greater detail with some illustrative examples.

In the relatively few studies where sorption has been measured in crushed rock tests and compared with the results from intact rock experiments, it is often found that there are discrepancies. An outline for an experiment is described in which results from the two types of test could be quantitatively related to one another via cation exchange capacity measurements. Using this method it might be possible to explain the reasons for such discrepancies.

Finally, a brief discussion is given on the possible consequences for experimental studies of gas in Valanginian marl and the swelling of the clay rich components.

The authors would like to point out that the opinions, ideas and concepts put forward in this report represent their views at a certain point in time (October 1989) before the experimental programme on marl was fully underway. Practical experience will ultimately be the judge of the contents of this work. Even if some of the ideas are later shown to be in error, this exercise is nevertheless considered to have been useful since it has provided a background to the experimental work and a framework upon which future developments can be built.

Technical Report NTB 90-15

Grimsel Test Site
Uranium migration in crystalline rock:
Capillary solution transport in the granite of the Grimsel Test Site

Summary

The formation of U-containing fluorescent surface exudations in the galleries of the Grimsel Test Site (GTS) have been studied. Mineralogical, chemical and radiochemical analyses were carried out on the exudations and the near-surface rock (2 drill cores). The capillary transport of pore water to the surface was studied by three small in-situ experiments: a) vacuum extraction of pore water, b) surface evaporation under controlled conditions and c) infiltration of water into the granite.

Pore water extracted from fluorescent zones is similar to other GTS­groundwaters but with a much higher U content of up to 1 mg/l. This U is probably mobilized from finely dispersed U minerals in the waterconducting microfractures of the granite near the tunnel wall by the action of dissolved air. Transport to the tunnel wall through an increasingly unsaturated zone is caused by capillary forces and by surface evaporation from very narrow, water-filled microfractures. Water is also sucked into the unsaturated rock at the tunnel wall with negative pressures of 0.5 – 1 bar.

Chemical and radiochemical analyses of the near-surface granite indicate some co-precipitation of U with Fe₂O₃(H₂O)_x from the aerated pore-water during formation of the fluorescent surface layer. Unlike U, Th and Ra appear immobile in the near-surface rock.

Fluorescent zones indicate geochemical conditions which may lead to high mobility and transport of U and possibly of other actinide elements. Such conditions are associated with unsaturated regions close to ventilated rock surfaces and are therefore probably of little direct significance to the safety assessment of deep radioactive waste repositories. They do, however, give indications of the physical and chemical perturbations involved in repository construction and the subsequent operational phase of the repository.

Technischer Bericht NTB 90-14

Hydrogeologische Modellierung im Opalinus-Ton und in der Unteren Süsswasser-Molasse der Nordostschweiz

Summary

With the Sediment Study, Nagra is conducting research into deep disposal of high-level radioactive waste in sedimentary formations as an alternative to the crystalline option. In the first phase, a general assessment of all potentially suitable strata led to the identification of two most promising formations: the Jurassic Opalinus clay (OPA) and the Tertiary Lower Freshwater Molasse (USM). Subsequently, a multiregional site evaluation was conducted in order to identify tectonically and seismically quiet areas. The investigation concept adopted for this purpose as well as the evaluation results are documented in detail in the Sediment Study Report 88 (Nagra 1988). An important part of the assessment process is characterization of the expected regional groundwater flow. For this purpose, a numerical hydrogeologic model was developed for each selected region of first priority. The objectives of the modeling are to provide information about the probable regional distribution of hydraulic heads, the locations of recharge and discharge areas as well as the resulting flow directions and hydraulic gradients. Since both investigated regions are characterized by lack of hydrogeological data, the modeling shall also disclose the extent of uncertainties related to any assessment of the regional groundwater flow.

The model OPA 88 covers an area of 1'600 km² and is composed of 1'600 finite elements. The model area includes the selected OPA regions of the Tabular Jura north of Lägern, Zürcher Weinland and Reiat. The adopted model parameters represent the current "best guess" and were subjected to variations in the sensitivity study in order to cover the associated uncertainties and assess their impact on simulation results.

The head distribution within the Opalinus clay is governed by the contrast in potentials between the regional aquifers of the Upper Malm (above) and Upper Muschelkalk (below). According to the orientation of the vertical hydraulic gradient, the groundwater percolating the host rock is drained either by the Malm or by the Muschelkalk aquifer and exfiltrates in the corresponding discharge zone. In the model area the following discharge locations of regional significance were identified:

The evaluation of hydrogeologically suitable repository areas is based on a relative comparison of the simulated travel times of groundwater flow from the host rock to the biosphere. Only flow paths with origin within the required depth range for a potential repository are considered. The simulations indicate that the best-suited region shall be situated as far in the Southeast part of the investigated area as possible (maximum depth allowance!), i.e. remote to the regional discharge zones along the Rhine river. However, this part of the model area manifests the largest uncertainties with respect to the head distribution in the Malm and Muschelkalk aquifers due to complete lack of reliable test data. Hence, the hydrological model does indeed provide information about the probable groundwater flow regime in the region considered, however, without a reduction of the existing uncertainties, particularly in the aforementioned head distributions, the model results cannot be considered as reliable. For the same reason (i.e. lack of data), the established model can be calibrated only by acquisition of additional field data.

The second numerical model USM 88 developed in the scope of the Sediment Study considers the regional groundwater flow in the Lower Freshwater Molasse (USM) east of the Limmat valley. It covers the Molasse basin between Lake Zurich and Lake Constance over an area of 2'600 km² and consists of 1'700 elements. Conditions similar to the OPA model are found with respect to the availability of hydrogeological field data and to associated uncertainties. A noteworthy exception is the aquifer of the Upper Marine Molasse (OMM) where several potentiometric test data are available from deep groundwater and geothermal exploration wells. Since the OMM heads were not used for the definition of the boundary conditions except for the narrow outcrop area along the eastern margin of the Molasse basin, the observed heads within the model area are used for comparison of the results of the different model runs performed.

Also here, the groundwater flow through the host rock formation is governed by pressure conditions in the adjacent aquifers of regional importance. These are the marine sandstones of the OMM above and the limestones of the Upper Malm below the host rock. The dominating discharge area of the OMM aquifer is the low-lying broad alluvial valley at the confluence of the Thur and Rhine rivers. Other important areas are located in the valleys of Töss (at Rorbas) and Limmat as well as at the Lake Constance outside the model region. The discharge zones of the Malm aquifer have been described already in the OPA model.

The simulated OMM potentials manifest in the Molasse basin (between the Limmat valley and Lake Constance) a flat and smooth surface which is levelled to the peripheral discharge areas in the North and the West of the considered region. Due to the resulting drainage effect of the OMM the prevailing flow direction in the underlying host rock is pointed upwards. The largest vertical hydraulic gradients in the USM arise beneath the low-lying OMM and USM outcrops in the valleys. Since the confined OMM aquifer shows mostly artesian pressures in the lowlands (manifested by exploration wells Aqui, Tiefenbrunnen, Mainau, Konstanz etc.), the occurrence of hydraulic short-cuts towards the surface along permeable faults is possible here. Downwards-orientation of groundwater flow through the host rock can be observed only in the Southeast part of the model area in the hilly region of the Hörnli fan. Accordingly, the particle tracking results provide the longest travel times for groundwater flow paths originating in this area. The model simulations suggest hence that the most-promissive area for a potential repository site is located as far towards the Southeast of the investigated area as possible, i.e. remote to the erosion margins of the OMM and USM strata. Given the SE-oriented dip of the Molasse strata, this requires the repository to be sited at the maximum depth feasible.

Technical Report NTB 90-13

Radionuclide chain transport with matrix diffusion and non-linear sorption

Summary

The present paper describes a two-dimensional model for radionuclide chain transport in inhomogeneous rock.

Advective and dispersive flux takes place in water conducting zones which may consist of a network either of tubelike veins or planar fractures. Out of these flowpaths nuclides diffuse into stagnant pore water of a spatially limited, adjacent zone (matrix diffusion). Sorption on rock surfaces is described by a non-linear isotherm. Under specific conditions matrix diffusion can be represented by an effective (non-linear) surface sorption. Radioactive decay and, in the case of a nuclide chain, ingrowth is also included in the model.

The numerical solutions of transport equations based on the method of lines are developed in detail. The advantages of this approach are the efficiency, the reliability and the general flexibility especially to include arbitrary boundary and initial conditions and arbitrary solute/rock interactions.

For ¹³⁵Cs we present in a comprehensive sensitivity analysis the impact of non-linear (Freundlich) sorption isotherm on break-through curves. It is shown that, provided transport times are comparable or larger than nuclide half-life, non-linear sorption may reduce concentrations at the geosphere outlet by orders of magnitude. Some results are also given for the transport of the ²³⁸U chain.

Technical Report NTB 90-12

A thermodynamic description of the evolution of porewater chemistry and uranium speciation during the degradation of cement

Summary

Portland cement, used as solidification-, backfill-and construction-material, is con­sidered to determine the chemical behaviour of a low-and intermediate level radioactive waste repository for very long periods of time. In order to describe the long term chemical behaviour of such cement based structures, a series of submodels including the hydration of cement, the thermodynamic properties of calcium-silicate-hydrates and the degradation of hydrated cements in natural groundwaters have been developed. As an application, the leaching of uranium from a cement based structure is modelled.

Cement hydration is modelled using Bogue's method. The variety of possible hydrated cement minerals is classified into several groups of model solids. Cement additives (e.g. silica fume, trass etc.) are assumed to react completely with the cement.

The incongruent solubility behaviour of calcium-silicate-hydrates (CSH-gels) is described using an approach directly applicable to common geochemical speciation codes. Based on the assumption that CSH-gels may be described by independent model components, their solubility properties are extracted from a large amount of literature data. Solubility constants are given as a function of the CSH-gel composition.

Within a mixing tank model, the results of the hydration and of the solubility model are used to describe the degradation of cement in natural groundwaters. The mixing tank model assumes thermodynamic equilibrium between hydrated cement and groundwater and recal­culates the speciation of solutes in the pore solution and the remaining chemical inventory in consecutive steps (cycles). It takes some 5000 to 10'000 cycles to completely degrade the hydrated cement. The particular "lifetime" depends mainly on the groundwater compo­sition. The composition of the cement has a secondary effect on lifetime, but it determines the absolute pH level and the relative time period of high pH conditions (pH ≥ 12) in the pore solution. Carbonate concentration in the groundwater is found to be a very important parameter, due to transformation of hydrated cement minerals into calcite.

As a representative for the actinides, the solubility behaviour of uranium in degrading cement is modelled assuming an Eh of -300 mV. The uranium concentration in the pore solution is determined by the "choice" of the solubility limiting phase. If the speciation is calculated by using the thermodynamic solubilities of the pure solids, concentration levels of about 10^-4 M are found for U₃O₈, USiO₄, UO₂, U₄O₉ and of 10^-14 M for CaUO₄. Cumulative uranium leaching from hydrated cement is found to be nearly independent of cement and groundwater composition, but strongly dependent on the properties of the solubility limiting phase. An amount of 0.1 mmoles U/kg hydrated cement is completely leached out after 5 to 10 cycles using U₃O₈ as the solubility limiting solid, after ~ 100 cycles using UO₂ and after ~ 5000 (to 10'000) cycles using CaUO₄.

Recommendations for further development of the submodels and appropriate priorities are given in the conclusions.

Technical Report NTB 90-11

Modelling of a diffusion-sorption experiment on sandstone

Summary

The results of a diffusion-sorption experiment on a sample of Darley Dale sandstone, using simulated groundwater spiked with a mixture of ¹²⁵I, ⁸⁵Sr and ¹³⁷Cs, are modelled by a one-dimensional porous medium approach in which sorption is described by Freundlich isotherms. The governing equations are solved analytically for the special case of a linear isotherm, and numerically using the computer code RANCHDIFF for non-linear isotherms. A set of time-dependent, ordinary differential equations is obtained using the Lagrange interpolation technique and integrated by Gear's variable order predictor-corrector method.

It is shown that the sorption behaviour of ⁸⁵Sr can be modelled successfully by a linear isotherm, using a sorption parameter consistent with batch-sorption tests. The behaviour of ¹³⁷Cs may be modelled by a non-linear isotherm, but the amount of ¹³⁷Cs sorbed is less than that anticipated from batch-sorption tests. ¹²⁵I is assumed to be non-sorbing and is used to determine the porosity of the sandstone.

Technical Report NTB 90-10

Groundwater Flow Through Fissured Rock: Field Investigatons and Interpretation in the Albigna Dam Area, Graubonden, Switzerland

Summary

The knowledge of hydraulic conductivity of rock plays a central role in the planning of underground repositories for disposal of radioactive and other industrial wastes. Estimates of the amount of dissolved substances that could reach the biosphere through groundwater flow in the course of time are based on rock-hydraulic considerations and model calculations. The aim of this research project was to help clarify the characteristics of flow in fissured rock. Activities centered mainly on field measurements and on an interpretation of the measured data using numerical calculations.

The rock foundation of a dam was chosen as the site for repeated observations of flow patterns in a fissured rock. The advantage of this type of location is that the rock is exposed to pressure gradients subject to considerable seasonal fluctuations. As a rule, good accessibility is also assured, enabling a large expanse of rock to be explored by means of boreholes fitted with instruments.

Our project was carried out at the Albigna Dam in the Canton of Graubünden, Switzerland, where the greatest fluctuation in the level of the lake was approximately 100 m. In the area of the dam the rock consists of a fresh, solid granite, whose fissure systems were well known as regards position and distribution. The measurement program for assessing the flow conditions consisted of two elements: piezometrie pressure measurements in boreholes and strain measurements in boreholes. Pressure measurements were accomplished to define the potential field. Strain measurements were accomplished to detect fracture opening and closing as a function of the fluctuations in the lake level. From a combination of the two sets of observations it was hoped to be able to determine the interaction between hydraulics and mechanics, i.e. the coupling of two key processes with a possible influence on a changing permeability of the rock. The instrumentation was positioned in a plane perpendicular to the dam wall in the central area of the dam. Five boreholes between 50 and 60 m long which included approximately 40 measurement points were used for the piezometer measurements. Two additional boreholes positioned laterally to the measuring plane were used for control purposes. Using the "sliding micrometer", the continuous strain distribution was determined along two boreholes in the measuring plane. Pressures were measured using the "Piezodex" system. These two high-precision instruments are portable probes whose proper working and accuracy can be checked at any time.

The results of the piezometer measurements are presented first in tabular form and then in graphic form. The key factors for piezometrie pressure are the position of the given measuring point and the height of water in the lake at the time of taking the reading. The most significant factor influencing the strains is the presence of certain mechanically active large fissures. Comparing the piezodex measurements with the sliding micrometer measurements produces a correlation between the rock deformations and the hydraulic potential. It was discovered that the opening and closing of two large fissures at the foot of the dam on the lake side had a crucial bearing on the distribution of potential. The potential field was constructed by linear interpolation of the values on a network of triangles in the area characterised by a large number of measuring points.

The mathematical investigations were conducted with the AQUA-ROCK finite element program on a 2D-model. Three main conceptually different models were investigated in particular. In the first main model the rock foundation was assumed to be a homogeneous and isotropic continuum. In the second main model the main fissures were idealized as a uniformly distributed and infinitely extending fissure system in accordance with SNOW (1965). In the third main model the foundation was assumed to be a homogeneous isotropic medium penetrated by two large fissures and the drainage boreholes.

From just a visual comparison of the measured and calculated values it is clear that a different model achieves the best correlation depending on the level of the lake. For the maximum lake level, however, it is essential to take account of the action of the fissures either by way of the idealization according to Snow (main model 2) or by considering the individual fissures (main model 3).

Technical Report NTB 90-09

Transmissivities and heads derived from detailed analysis of Siblingen 1989 fluid logging data

Summary

The Siblingen borehole was drilled by the Swiss National Cooperative for Radioactive Waste Disposal (Nagra) to hydraulically characterize granitic basement rocks of northern Switzerland for their potential for the safe storage of high-level radioactive waste. The Siblingen crystalline section of the borehole was tested with an extensive set of hydraulic packer tests and geophysical methods. In addition, the borehole was tested from the interval 525 to 1522 m below ground (b.g.) with non-standard fluid logging techniques.

The fluid logging methods used in Siblingen were the spinner flowmeter (SF), the spinner-packer flowmeter (SPF), the heat-pulse packer flowmeter (HPPF), and temperature and electrical conductivity logging (T/LF). This report presents the detailed analysis of these various fluid logging methods. The result from each fluid logging method is the estimation of the borehole volumetric flow rate as a function of depth. In addition, through electrical conductivity logging discrete isolation of inflowing features is possible. Through fundamental hydraulic analysis these measurements can be used to estimate interval transmissivity and in certain cases interval head. These calculations were performed for all suitable logs. A complete description of analysis methods is included prior to each step of analysis.

A comparison between the various fluid logging methods is made through a review of the analysis results (transmissivities and heads). A comparison between the transmissivities and heads calculated by fluid logging and measured by hydraulic tests is made. In general, the fluid logging techniques predicted transmissivity to within a factor of two when compared to hydraulic tests. The two most accurate fluid logging methods, as defined by hydraulic tests, were conductivity logging and the heat-pulse packer flowmeter for the conditions prevailing in Siblingen during testing. The fluid logging methods gave head estimates which were not in strict agreement with hydraulic test results but are within the variation in observed measurements.

The application of fluid logging methods for the characterization of the Siblingen crystalline has been successful in determination of major transmissive areas (defined as rock having a transmissivity greater than 1E-9 m²/s). In addition, these methods have identified (to within ± 1 m) 13 individual inflow regions within the lower crystalline (1000 to 1522 m b.g.). Fluid logging methods represent a compatible borehole testing method to hydraulic packer tests in allowing for a focused use of hydraulic packer tests. In addition, conductivity logging can be used to attribute discrete intervals to transmissivity measurements.

Technical Report NTB 90-08

Stripa Project
Water flow in single rock joints

Summary

To study the hydromechanical properties of single rock joints a technique to make transparent replicas of natural joint surfaces has been developed. Five different joint samples were replicated and studied. The aperture distribution of the joints was obtained through a measurement method provided by the transparent replicas. The principle behind the method is that a water drop with a known volume, which is placed inside a joint, will cover a certain area of the surface depending on the average size of aperture at the actual point.

Flow tests were performed on the same joint replicas. The tortuosity of the flow and the velocity along single stream lines were measured using colour injections into the water flow through the joints. The equivalent hydraulic apertures determined from the flow tests were shown to be smaller than the average mechanical apertures. The velocity of the flow varies strongly between different paths over the joint depending on the spatial distribution of the apertures. The degree of matedness between the joint surfaces is an important factor influencing the channeling character of the joints.

Technical Report NTB 90-07

Grimsel Test Site
Geophysical methods for the detection of discontinuities ahead of a tunnel face
(out of print)

Summary

In 1988 NAGRA started a research and development program at Grimsel Test Site (GTS) to investigate the potential of geophysical remote sensing methods (radar and seismics) to predict discontinuities ahead of the tunnel face.

This task was initiated as an attempt to provide a more complete knowledge of the rockmass for a safe construction of a tunnel. If we can tell in advance where the main fracture zones are to be encountered during excavations, the safety risks as well costs of tunneling will be greatly reduced.

A number of practical goals were set to guide the studies and experiments during the project:

The first investigations were done using a subset from an existing data base, obtained from both radar and seismic experiments, originally recorded for a tomography project at GTS. Although the data recording, station spacing etc. were optimized for the tomographic application and far from ideal for a reflection survey, it was possible to reach some meaningful results. Reflection events were identified which could be attributed to known fracture zones and lamprophyre dykes.

Radar measurements give good results in rocks with high electrical resistance (e.g. granite, rock salt) but have a very limited investigation depth in materials with lower resistance (e.g. marls, clays). NAGRA's near future interests being focused on measurements in sediments and considering the results of the first tests, we decided to concentrate our efforts on seismics rather than radar.

After discussions with several experts, it was agreed that in this case the most adequate approach for the seismic measurements is a modified "Vertical Seismic Profiling (VSP)" technique which is well known in borehole geophysics. A more suggestive name could have been, for example, "Horizontal Tunnel Profiling" but as we liberally refer to routines developed initially for boreholes, we prefer to conserve the generic name of VSP. For a more objective evaluation of the results, conventional seismic measurements along the tunnel were also recommended.

Theoretical calculations (FD-modeling) were done to determine the optimal test layout. This study showed that, ideally, the source and the receivers should be placed at a certain distance (several meters) away from the tunnel wall. In practice this is hardly possible as it would require many boreholes to be drilled radially from the tunnel. The best compromise between theoretical and operational aspects is to place the source into a side wall drillhole at a minimum distance of about 4 m from the tunnel wall and to attach the receivers directly to the tunnel wall. In this case we expect to reduce the tunnel wave and still achieve a realistic operational time. However, as the measurements at GTS were primarily a scientific experiment, we decided not to measure only the most promising layout according to the theory, but to proceed with different test configurations to prove the validity of the theoretical results optimize the operational routine.

The following VSP configurations were tested:

Additionally to the VSP-type measurements we measured a conventional reflection seismic profile along the tunnel by shooting through the array. Shotpoint and receiver spacings were 0.7 m with an offset between shot and receiver of 0.35 m, leading to a CDP (Common Depth Point) spacing of 0.35 m.

The raw sections of all records showed only very weak reflections. This was caused by the very low reflectivity of the fracture zones at GTS, if compared with surveys conducted in layered sediments. Accordingly, the processing consisted of a relatively large number of steps which are not discussed in this paper in detail. In general, the direct P- and S-waves as well as the reflected S-waves were suppressed by using a median filter while the remaining P- reflections were enhanced by Image Point filtering, a method based on the generalized Radon transform developed by C. Cosma and his collaborators at Vibrometric.

A large effort has been put for the completion of this project in the development of routines for translating the geophysical results into a reliable and unambiguous geological picture. Due to the many experiments carried out previously at Grimsel Test Site (GTS), we had the possibility to compare in different phases the geophysical predictions expressed in terms of geological structure with the exhaustive information available. The agreement between predicted and real location was excellent. We were able to detect discontinuities up to distances of about 150 m.

The conventional reflection section was processed using standard seismic processing routines. Due to the weak source (sledge hammer) the investigation depth was in the order of about 40 m. Within this area a number of reflectors (mainly lamprophyre dykes) were detected. Additionally we got good reflections from a nearby tunnel. Although this test was successful we consider the VSP configuration as the better layout for the prediction measurements.

As a trial of the method in a realistic environment, VSP measurements were conducted in August 1990 in the Leissigen tunnel (a road tunnel in the canton of Bern), which was under construction at the time of the test. The processing sequences tested in earlier experiments were used. Although the circumstances were more complicated and the field conditions more difficult than in GTS, the predictions coincided well with the geological structures known or supposed to exist at the tunnel site.

Technical Report NTB 90-05

Description and results of tracer tests conducted for a deep fracture zone within granitic rock at the Leuggern borehole

Summary

A tracer test program was planned at the Leuggern borehole, to provide hydrogeologic information concerning the fracture zone(s) intersected within the depth interval 1'634.9 – 1'688.9 m. The original design of the tracer-dilution test was to: establish a uniform tracer concentration within the test system, and then monitor (at ground surface) the decline of tracer concentration within the circulated test system fluid. Analysis of the tracer concentration decline pattern was expected to provide an estimate of the natural lateral flux and lateral hydraulic gradient for the isolated test interval. A later pump-back test was also designed to recover tracer that had been "flushed" into the test section, during the previous closed circulation period. Analysis of the tracer recovery pattern was expected to provide an estimate of the dispersivity within the intersected fracture system.

Originally the "tracer-dilution" test was planned to have one tracer injection and removal phase. Results obtained during the first tracer (i.e. Uranine) injection and a subsequent, second tracer (Eosin) injection, however, indicated that a natural vertical hydraulic gradient existed within the borehole test section. Because of the presence of the natural vertical hydraulic gradient, the original test program was modified and a third tracer injection (Naphtionate) and tracer recovery test phase (also referred to as the natural outflow/discharge test) was implemented.

The entire Leuggern tracer test program was conducted between July 5 to December 23, 1988; with the individual Uranine, Eosin, and Naphtionate tracer injection/recovery phases occurring between July 5 to September 2, September 2 to October 31, and October 31 to December 23, 1988, respectively. Following completion of the Naphtionate tracer injection phase on November 17, a natural outflow/discharge test was implemented to recover tracers injected during the previous three injection phases (i.e. 1st injection = Uranine, 2nd = Eosin, 3rd = Naphtionate). To facilitate the recovery of injected tracers, a downhole valve within the central tubing string was opened prior to the start of the natural outflow/discharge phase. The natural outflow/discharge phase was completed over the period 17 November to 23 December, 1988. During the thirty-six day discharge test period, approximately 62 % (12.9 grams) of Naphtionate, 47 % (8.7 grams) of Eosin, and 49 % (38.3 grams) of Uranine were recovered. A total volume of 640'000 liters of water was removed during the natural outflow/discharge test phase, with flow rates declining from an initial high of 35 L/min to an observed 11.2 L/min at test termination.

Results obtained from "arrival-time" information during the Eosin and Naphtionate injection/recovery phases indicate a downward vertical flow of approximately 195 – 225 ml/min in the isolated interval, from an upper fracture inflow zone to a lower fracture outflow zone. Through analysis of the dilution levels of Uranine and Eosin during the injection/recovery periods, and review of field data, the top of the upper inflow zone was determined to be approximately 13 m below the top flow line and the bottom of the outflow zone to be approximately 3 to 5 meters above the bottom flow line.

Hydraulic analysis of the flow rate data during the natural outflow/discharge phase and subsequent pressure recovery following termination of the test indicated that the test zone exhibited a heterogeneous formation, "double-porosity" response behavior. Calculated transmissivity (T) and storativity (S) values for the fracture/shear zone and total system (i.e. fracture zone and matrix) from the flow-rate test analysis are: T = 6.1 x 10^-5 m²/s and 6.4 x 10^-5 m²/s, and S = 4.5 x 10^-5 and 1.5 x 10^-4, respectively. Transient analysis of the pressure recovery response provided an estimate for transmissivity of 7.1 x 10^-5 m²/s, which is in close agreement with the flow-rate analysis test results.

Technischer Bericht NTB 90-04

Zur Tektonik der zentralen Nordschweiz
Interpretation aufgrund regionaler Seismik, Oberflächengeologie und Tiefbohrungen

Summary

This report presents the methodology and results of a geological interpretation of Nagra's reflection seismics data (1982 -1984) for North-Central Switzerland. The area under investigation, which is covered by the entire network of reflection seismics lines, is outlined by the towns of Schaffhausen/Zürich in the east and Säckingen/Olten in the west.

Basement

The structure and geological development of the crystalline Basement complex of Northern Switzerland and the Black Forest must be seen in the context of the Central European Variscan orogeny which, according to plate tectonics models, is the result of a continental collision during the Upper Devonian and the Carboniferous. The regional model implies that the conditions observed in the Southern Black Forest (and the Vosges) also pertain to the subsurface of Northern Switzerland, where borehole data confirm similar structural complexities in the Basement.

The Variscan development of the crystalline Basement of Northern Switzerland can thus be divided into the following phases:

While reflection seismics surveys are unable to provide sufficiently reliable information on the tectonics and lithology of the crystalline Basement, the structure, and particularly the tectonics, of Late Variscan sedimentary trough infills can be mapped seismically. These dovetail with the Basement of Northern Switzerland and available data, particularly from the Nagra boreholes, indicate an intense, deep-reaching (brittle) Basement deformation caused by Permian tectonic events. These Late Palaeozoic fault zones, most of which were rejuvenated during the Tertiary, dominate the structural pattern of the region.

Late Palaeozoic sedimentary troughs

At least two major Late Palaeozoic sedimentary troughs have been identified in the subsurface of Northern Switzerland: a northern trough (Permo-Carboniferous Trough of Northern Switzerland, NPT) beneath the Tabular Jura, and a southern trough (Olten­ – Lenzburg Trough). No attempts have been made to map the latter.

The overall form of the NPT is that of an ENE-WSW-trending asymmetric half-graben (trap-door basin), dextrally offset by WNW-ESE-trending convergent wrench faults (e.g. Eggberg and Vorwald fault zones). These appear to divide the Trough into an eastern and western segment which differ with respect to depth and direction of asymmetry. Thrusting and folding, evident mainly in the western section of the Trough, are the result of a phase of transpressive deformation dated as Lower/Middle Permian (Saalian phase). In the Late Permian, the decreasing Variscan tectonic activity over the whole Trough was characterised by extension and differential subsidence.

The tectonic events are reflected by the sedimentary Trough infill, which can be up to 5 km thick:

Mesozoic and Tertiary sedimentary cover

Marine Mesozoic shelf deposits were laid down during a period of tectonic inactivity and rest unconformably on Basement or Late Palaeozoic sediments. Their distinct vertical lithological differentiation provides a seismic signature which allows detailed mapping.

The Early Mesozoic marine Lower Muschelkalk was deposited on an almost perfect subhorizontal peneplain. The present-day structural configuration of this peneplain (Base Mesozoic marker), which has been mapped using reflection seismics over an extended area, records the vertical, mostly Tertiary deformations of the Basement since Triassic time. A general tilt of the entire area to the SSE (doming of the Southern Black Forest and subsidence of the Alpine foredeep) is noticeable; the Base Mesozoic is dissected by a fault pattern emerging in the west of the Palaeogene Upper Rhine Graben tectonics and in the east of the Miocene tilting and doming. The latter includes reactivated preexisting (Late Paleozoic) fault zones. In the central and western area Late Miocene thin-skinned detachment tectonics (Jura Folding) is superimposed on extensive Basement structures.

Tabular Jura

The structural pattern of the Tabular Jura, located between the Southern Black Forest and the Folded Jura, reveals a superposition of extensional Basement faulting and thin-skinned detachment in the sedimentary cover.

The area west of Frick is dominated by NNE-SSW-trending normal faulting related to (Palaeogene-Early Miocene) Rhine Graben subsidence. Towards the east, the influence of Early to Late Miocene, WSW-ENE and NW-SE-trending faults and flexures becomes increasingly important. These relate to subsidence in the Molasse Basin and the doming of the Black Forest and include reactivated Late Palaeozoic fault zones.

The structural response of the Late Miocene thin-skinned detachment to the pre-existing Basement fault pattern was varied:

Folded Jura

Seismic reflection profiles crossing the Folded Jura have contributed greatly to a new understanding of the tectonics of the eastern Folded Jura. The most important points are:

Northern rim of the Molasse Basin

The style and intensity of basement tectonics observed on seismic sections in the area of the northern rim of the Molasse Basin are comparable to those of the Tabular and Folded Jura.

Due to the synsedimentary regional tilt the Molasse sediments show increasing thicknesses towards the SE (basinwards). In the west they have also been affected by Mio­cene thrusting and folding. While disharmonic thrusting is predominant at deeper levels (Triassic), gentle folding is mainly observed at shallower levels (Jurassic and Tertiary).

In the east of the investigation area, the northern rim of the Molasse Basin borders directly on the Tabular Jura; the deformation of the latter generally continues to the SE below the Molasse. The coincidence of the northern rim of the Molasse and the distribution of the Permo-Carboniferous deposits of Northern Switzerland is noticeable. To the east of the Aare, the Molasse sediments, particularly the Lower Freshwater Molasse, appear to wedge out in the region of the northern rim of the NPT. It is therefore possible that there is a genetic link between the outer rim of the Molasse Basin and the distribution of Palaeozoic sediment troughs in the Basement.

Technischer Bericht NTB 90-03

Zur Tektonik des Südschwarzwaldes

Summary

This report deals predominantly with brittle deformation in the basement of the Southern Black Forest. During the course of a study of the structural geology of this region, it became apparent firstly that there was a need to refine the data acquisition process using new measuring points and, secondly, that the role of fault zones in the crystalline basement required more detailed investigation.

The report concentrates mainly on the second objective. The first part outlines the basic principles of block tectonics and discusses relevant models and development sequences. A description of selected discontinuities (strike-slip faults, tension structures) completes the general overview of block tectonics. Wherever possible, specific examples from the investigation area have been selected. However, it also proved necessary to include descriptions from the literature because, in the basement of the Southern Black Forest, it is difficult to find case studies for which the structural geology can be analysed at sufficient depth.

The second part of the report contains a discussion of general aspects of brittle deformation of the basement; this is complemented by a series of geographically and thematically restricted studies of specific topics. These include an analysis of the data acquired during construction of the underground hydroelectric power plant at Wehr, a regional study in the area between Waldshut and Grafenhausen which looks at the relationship between morphostructural elements and the tectonic structure of the basement and, finally, a special study carried out in the area between Hauenstein and Waldshut which investigates the tectonic significance of a series of NNW-SSE-striking valleys. These special studies are finally discussed from the point of view of acquisition, evaluation and extrapolation of structural geology data.

Technical Report NTB 90-02

Numerical modelling of the creep behaviour of clays with emphasis on tunnels and underground openings
A critical review of the state-of-the-art

Summary

This report presents an interpretive overview and critical assessment of the state-of-the-art for numerical modeling of the creep behavior of clays. The overview and assessment is focused upon application to underground openings. Field and laboratory observations of time-dependent behavior, constitutive modeling of creep behavior, and numerical implementation of constitutive equations are addressed. A critical assessment of the ability of existing models to predict aspects of creep behavior relevant to waste repository design and suggestions for improved analyses that can be developed with existing technology are provided.

Observations of creep behavior in the laboratory and in the field are reviewed to provide a background upon which to assess the adequacy of existing constitutive equations and numerical models for reproducing important facets of creep behavior in clays. The lack of data on creep under long term drained conditions is noted. Attention is also called to the lack of significant data on the creep of stiff clays and clayey rocks, on temperature effects, and on creep under drained conditions for other than one-dimensional stress-states.

Both heuristic and mathematical constitutive models are reviewed. Heuristic models provide a basis for evaluation of the required parameters for the continuum mechanics based mathematical models. The continuum mechanics models are required for numerical analysis. It has been demonstrated that, by using iterative and incremental analysis, virtually any viscous or inviscid continuum mechanics material model can be adapted to consider time-dependent behavior.

Available numerical models for numerical analysis of geotechnical problems involving creep deformations are reviewed. Models for thermo-mechanical coupling are also addressed in this review. Cases where creep-inclusive analyses have been applied to analysis of prototype behavior are cited. However, the lack of well documented case histories of time-dependent deformations over significant time spans is identified as a major obstacle to model verification.

Recommendations are made for an alternative design approach capable of guaranteeing the very long term mechanical integrity of the liner.

Technical Report NTB 90-01

Grimsel Test Site
Grimsel colloid exerciese:
An international intercomparison exerciese on the sampling and characterisation of groundwater colloids

Summary

The Grimsel Colloid Exercise was an intercomparison project which consisted of an in-situ sampling phase followed by a colloid characterisation step. The goal of this benchmark exercise, which involved 12 laboratories, was to evaluate both sampling and characterisation techniques with emphasis on determining the colloid size distribution.

The sampling phase took place at the Grimsel Test Site between February 1 and 13, 1988 and the participating groups produced colloid samples using the following methods.

All samples were shipped to CoCo Club members for subsequent characterisation, with emphasis on size distribution determination.

While shipping and storage may affect the colloid samples, as well as the composition of both unfiltered/filtered water and concentrate samples from this somewhat demineralised water, the major element composition of the fluid samples was comparable between groups. However, air contamination decreases the pH from 9.6 to about 8, while increasing the concentration of total inorganic carbon.

The exercise differentiates the colloid samples produced on site from those obtained after transfer of the fluid samples to the laboratories. The colloid concentration and size distribution was determined by scanning electron microscopy, gravimetry, chemical analysis of fluid samples after micro/ultrafiltration and by transmission single particle counting. The colloid concentration was also evaluated by transmission electron microscopy, static and dynamic light scattering and by laser-induced photoacoustic spectroscopy.

The concentration of colloids (Ø > 10 nm) in Grimsel water is measured to be around 10¹⁴ pt.l^-1 and for Ø > 50 nm about 10¹⁰ pt.l^-1, although reported concentrations range over one order of magnitude between groups and according to the characterisation technique used. Particles (Ø > 450 nm) measured with scanning electron microscopy, gravimetry and transmission single particle counting appear to be around 2 × 10⁷ pt.l^-1. On a weight basis, the concentrations of colloids and particles are around 50 and 100 ppb respectively and the total particle and colloid concentration is 200 ± 100 ppb.

The colloids consist of silica, illite/muscovite, biotite, calcium silicates and organics. A population of about 106 bacteria per litre is also present. The particles are negatively charged in-situ.

The results are discussed on the basis of the detection limit, lateral resolution and counting conditions of the technique (precision) as well as sample preparation, artefact production and measurement optimisation (accuracy). A good agreement between size distribution results was obtained with electron microscopy, gravimetry and particle counting for the original samples.

The main recommendations are the need for in-situ sampling, tracking of artefact production (e.g. ion retention, aggregation ...) and identifying contamination sources during subsample production. The combination of different techniques is finally recommended in order to ensure that no interference affects colloid analysis.

Technical Report NTB 89-28

Numerical modelling of gas migration at a potential repository for low- and intermediate-level nuclear wastes at Oberbauenstock, Switzerland

Summary

Hydrologic impacts of corrosive gas release from a hypothetical L/lLW nuclear waste repository at Oberbauenstock are explored by means of numerical simulation. A schematic two-dimensional vertical section through the mountain is modeled with the simulator TOUGH, which describes two-phase flow of water and gas in porous and fractured media, Two reference cases are considered which represent the formations as a porous and as a fractured-porous (dual permeability) medium, respectively. Both cases predict similar and rather modest pressure increases, from ambient 10 bars to near 25 bars at the repository level. These results are to be considered preliminary because important parameters affecting two-phase flow, such as relative permeabilities of a fractured medium, are not well known at present.

Technical Report NTB 89-27

Interpretation of the tracer testing conducted in the Leuggern borehole

Summary

Tracer testing was conducted in the Leuggern borehole from Jul y to December 1988 to evaluate the hydraulic properties of the crystalline host rock. The tested interval was an approximately 50 m section of fractured crystalline rock at a depth of greater than 1,600 m.

The testing consisted of three tracer injection/recovery periods (uranine – 44 days, eosin – 30 days, and naphtionate – 14 days), which utilized tracer injection/circulation rates, ranging between 25 and 50 ml/min. During these testing periods, tracer was injected in either of two 1/4" flow lines ported at the top or bottom of the interval and recovered from the other. Following the three tracer injection periods, a natural outflow tracer recovery test was conducted from the central tubing at an average outflow of 12 l/min. The central tubing was ported near the centre of the test interval. Data collected during the testing periods included: continuous monitoring of fluid temperature, injection pressure, and electrical conductivity as well as discrete measurement of flow rates, electrical conductivity, fluid temperature, and tracer concentration.

Testing results indicate a downward vertical flow of approximately 195 – 225 ml/min in the isolated interval, from an upper fracture inflow zone to a lower fracture outflow zone. Through analysis of the dilution levels of uranine and eosin during the injection/recovery periods, and review of field data, the top of the upper inflow zone was determined to be approximately 13 m below the top flow line and the bottom of the outflow zone to be approximately 3 to 5 meters above the bottom flow line. The calculated transmissivity value of 6E-05 m²/s from observed outflow rate and pressure recovery data is consistent with results derived from previous hydraulic packer testing in the interval. The effective porosity was determined to be 0.1. Dispersion coefficient values ranged from 1.0 m to 5.0 m. The lateral hydraulic gradient value calculated from tracer recovery analysis from the natural outflow phase was determined to be between 0.001 and 0.0.

Technischer Bericht NTB 89-26

Bohrlochversiegelung
Konzept und Machbarkeitsnachweis

Summary

In this report criteria for sealing boreholes are first considered and the requirements for sealing materials are then specified. For the conceptual design of the seal use is made of a multiple-zone borehole plug consisting of key zones and intermediate zones in between. The key zones are located in competent, low-fissured rock and act as efficient barriers to prevent a drain effect by the borehole. Highly compacted bentonite in the form of cylinders with a diameter slightly smaller than that of the borehole and also in the form of spheres or pellets is used as plug material in the key zones, while cement is employed for the intermediate zones.

A review of the relevant literature has revealed that in past experiments mostly cement and bentonite were used as plug materials. The corresponding results showed that both materials are potentially successful candidates.

For plug emplacement, it is proposed to employ either a specially designed emplacement tool or a perforated pipe. The latter method is somewhat easier to use but the pipe has to remain in the borehole, i.e. in the key zone. It is recommended to have a dry density of the bentonite after swelling of not less than 1.3 Mg/m³. This implies limiting values for the space between the bentonite cylinder and the borehole wall. By means of a bentonite slurry filled into the borehole at the key zone, the allowable ring space can be slightly enlarged. For the use of bentonite in the form of spheres or pellets experimental data are scarce and emplacement procedures are not sufficiently developed to guarantee the required minimum final dry density.

In order to check the quality of the plug, an indirect procedure is recommended in which it is shown, by means of suitable tests, that with a certain emplacement method the plug is able to achieve the required sealing effect. For sealing boreholes penetrating into the repository zone, direct quality control is then limited to checking whether the procedure is adhered to as specified and to recording the quantities of materials emplaced.

Finally, it is shown that the proposed plug design represents an efficient flow barrier in spite of the increased permeability in the disturbed zone surrounding the borehole, and that it can satisfy the specified requirements.

Technischer Bericht NTB 89-25

Nachweis von tektonischen Störungen in 2 Bodengasprofilen in der Nordschweiz

Summary

In the Tabular Jura east of Basel, a field campaign using two soil gas profiles each 1.3 km long was carried out to investigate the possibility of geochemical identification of disturbed zones using gases in the soil. The aim was to clarify to what extent multi-component gas analyses along measurement profiles could be used for rapid identification of disturbed zones in the field. The exercise was also intended to provide relevant experience in approaching similar problems and to allow optimisation of test procedures.

By way of introduction, the report gives information on the origin and place of production of the most important soil gases and then goes on to explain the principle of the field experiment and the measurement techniques and equipment used. The results are then presented, together with a discussion and a cross-comparison with the literature data.

The soil gas profiles are at right angles to the Rheinfelden fault (Rheinfelden soil gas profile) and to the Zeiningen disturbed zone (Zeiningen soil gas profile); both these features have a regional character and a dip slip of several hundred metres. There are no geomorphological indications at the earth's surface of recent movements associated with these two disturbed zones, although there is probably neotectonic activity in the Rheinfelden-Zeinigen area.

The gas samples were taken using a push-in probe. Samples were initially taken for both profiles at fixed points 100 m apart (= low-density gas profiles). In a further stage, the gas anomalies determined could be narrowed down by sampling at points in between, resulting in high-density gas profiles.

The gas investigations involved determining concentrations of the following components: hydrocarbons from C₁ (methane) to C₅ (pentane), CO₂, He, H₂, N₂ and O₂. The measurements were made using two gas chromatographs in a simple field laboratory. The H₂S content of the air in the soil was also determined using a rapid method (Drägerröhrchen); the external temperature and the temperature at a depth of around 1.2 m were also measured. The radon (Rn) content was also determined along the two profiles using α-cups consisting of plastic films which register the α-radiation over a period of several weeks. Information on the origin of the hydrocarbons and the CO₂ was obtained by taking samples for isotope analysis from one measurement location in the vicinity of the Rheinfelden fault.

The investigations show that the most important known disturbed zones can be charted well using the selected procedure. In addition to this, the presumed existence of some disturbed zones could be confirmed and others were discovered for the first time. The gases CH₄, C₂ to C₅, H₂ and the gas ratios CO₂/O₂ and N₂/O₂ proved to be particularly important for identifying disturbed zones. The concen­trations of the gases He and H2S, which are presumably also important from a diagnostic point of view, were unfortunately always below the detection limits (10 and 1 ppm respectively).

If a complete detailed inventory of disturbed zones in an area is to be drawn up and information on the lateral extent of the structures is to be obtained, it would be necessary to have a network of several gas profiles, but the procedure described in this report would seem to be somewhat costly for this type of application. However, it could be continually adapted during extensive reconnaissance exercises to take account of new information, thus minimising the cost involved. Although the best results are to be expected with a combination of as many different gases as possible, preliminary tests in an area carried out under certain conditions (lithological homogeneity, similar types of disturbance, etc.) could reduce the number of measured parameters.

Technical Report NTB 89-24

STRIPA PROJECT
ANNUAL REPORT 1988

Summary

The Stripa Project is organised as an autonomous project of the Nuclear Energy Agency of the OECD. Over the time period 1986 – 1991 (Phase 3 of the Project), an international cooperative programme of investigations is being carried out in an underground rock laboratory in Sweden. Building on experience gained in Phases 1 and 2, the following research will be carried out:

Technical Report NTB 89-23

Stripa Project
Site characterization and validation Stage 2
Preliminary predictions

Summary

The Site Characterization and Validation (SCV) Project is designed to assess how well we can characterize a volume of rock prior to using it as a repository. The programme of work focuses on the validation of the techniques used in site characterization. The SCV Project contains 5 stages of work arranged in two "cycles" of data-gathering, prediction, and validation. The first stage of work has included drilling of 6 boreholes (N2, N3, N4, W1, W2, and V3) and measurements of geology, fracture characteristics, stress, single borehole geophysical logging, radar, seismics, and hydrogeology.

The rock at the SCV site is granite with small lithological variations. Based essentially on radar and seismic results 5 "fracture zones" have been identified, named GA, GB, GC, GH, and GI. They all extend across the entire SCV site. They are basically in two groups (GA, GB, GC and GH, GI). The first group is aligned N40°E with a dip of 350 to the south. The second group is aligned approximately N10°W dipping 60°E.

From the stochastic analysis of the joint data it was possible to identify three main fracture orientation clusters. The orientation of two of these clusters agrees roughly with the orientation of the main features. Cluster B has roughly the same orientation as GH and GI, while features GA, GB, and GC have an orientation similar to the more loosely defined cluster C. The orientation of the third cluster (A) is northwest with a dip to northeast.

It is found that 94 % of all measured hydraulic transmissivity is accounted for by 4 % of the tested rock, not all of this "concentrated" transmissivity is within the major features defined by geophysics. When the hydraulic connections across the site are, examined they show that there are several well-defined zones which permit rapid transmission of hydraulic signals. These are essentially from the northeast to the southwest.

Technical Report NTB 89-21

Biospere modelling for a deep radioactve waste repository
Site-specific consideration of the groundwater-soil pathway

Summary

Scenario evaluations indicate that groundwater is the most probable pathway for released radionuclides to reach the biosphere from a deep underground nuclear waste repository. This report considers a small valley in northern Switzerland where the transport of groundwater to surface soil might be possible. The hydrological situation has been examined to allow a system of compartments and fluxes for modelling this pathway with respect to the release of radionuclides from an underground repository to be produced.

Assuming present day conditions the best estimate surface soil concentrations are calculated by dividing the soil into two layers (deep soil, surface soil) and assuming an annual upward flux of 10 mm from the groundwater through the two soil layers. A constant unit activity concentration is assumed for the radionuclides in the groundwater. It is concluded that the resultant best estimate values must still be considered to be biased on the conservative side, in view of the fact that the more typical situation is likely to be that no groundwater reaches the surface soil. Upper and lower estimates for the surface soil radionuclide concentrations are based on the parameter perturbation results which were carried out for three key parameters, i.e. precipitation surplus, upward flux and solid-liquid distribution coefficients (K_d). It is noted that attention must be given to the functional relationships which exist between various model parameters. Upper estimates for the surface soil concentration are determined assuming a higher annual upward flux (100 mm) as well as a more conservative K_d value compared with the base case. This gives rise to surface soil concentrations more than two orders of magnitude higher than the best estimate values. The lower estimates are more easily assigned assuming that no activity reaches the surface soil via this pathway.

Two appendices are included in this report. The first contains the calculations presented in the BIOMOVS study for the Mol region in Belgium, as defined in Scenario B6b. The second appendix presents an assessment of the potential significance of upward water movement in soil, due to capillary rise from groundwater in Switzerland, based on soil use maps.

Technical Report NTB 89-20

Biosphere modelling for a deep radioactive waste repository:
Treatment of the groundwater-soil pathway

Summary

The effect of radionuclide transfer from near-surface groundwater to the rooting zone soil, via a deep soil layer, is modelled in this report. The possible extent of upward solute movement is evaluated for a region in northern Switzerland.

The concentrations of ²³⁷Np and ¹²⁹I in the deep and top soil, and hence growing crops, are evaluated assuming a constant unit activity concentration in the groundwater. A number of parameter variations are considered, namely variable soil sorption coefficients, reduced infiltration of rain water and decreased groundwater flow. A release to an alternative smaller recipient region in northern Switzerland is also evaluated. For the parameter ranges considered uncertainty in the solid-liquid distribution coefficient has the largest effect on overall uncertainty.

These calculations have been presented within the international Biosphere Model Validation Study (BIOMOVS). A description of the test scenario, and the model calculations submitted, has been included in this report for completeness.

To place the groundwater – soil – crop – man pathway in context, its contribution to the total dose to man is evaluated for the ²³⁷Np – ²³³U – ²²⁹Th decay chain. The results obtained using the two-layer soil model, described in this report, are compared with the single-layer soil model used during Project Gewähr 1985. The more realistic two-layer soil model indicated an increase in importance of the drinking water pathway. It should be noted, however, that not all the critical pathways have been treated in this study with the same degree of conservatism.

Technical Report NTB 89-19

Experiments on container material for Swiss high-level waste disposal projects Part IV

Summary

One concept for final disposal of high-level waste in Switzerland consists of a repository at a depth of 1000 to 1500 m in the crystalline bedrock of Northern Switzerland. The waste will be placed in a container which will be required to function as a high integrity barrier for at least 1000 years.

This report is the fourth and last in the current series dealing with the evaluation of potential materials for such containers. Four materials were identified for further evaluation in the first of these reports: they were cast steel, nodular cast iron, copper and Ti-Code 12.

Copper and cast steel were chosen for further evaluation without prejudice as to the technical merits of the other two materials. The bulk of the experimental work concentrated on the latter material as the data basis for the adoption of copper as a candidate material was more complete.

The main effort so far reported concentrated upon establishing that the general corrosion and the localized corrosion rates of cast steel in groundwaters of the type expected are acceptable. The long term corrosion rate is expected to be less than 20 μm/a and the conditions such that large local variations in corrosion rate or pitting are unlikely.

This report touches upon two other important aspects. The first is the problem of demonstrating that cast steel containers will not fail by stress corrosion cracking. The second is the evolution of hydrogen produced by the reduction of water; the corrosion product hydrogen may be a problem if it is produced faster than it can escape as a dissolved species by diffusion, i.e. if gas pockets or bubbles are formed which could lead to pressure build-up or alter the near-field transport processes by displacement, venting or buoyancy effects.

The experimental results on pre-cracked specimens revealed no susceptibility of cast steel to stress corrosion cracking under model repository conditions. No crack growth was detected on compact DCB specimens exposed in aerobic and anaerobic groundwaters at 80 and 140°C for 16 – 24 months. This suggests that the maximum possible crack growth rates are below 0.1 mm/a or incubation times are exceptionally long under all test conditions. Constant extension rate test results showed a possible effect of inclusions on stress corrosion behaviour. Cast steel remains a candidate material for high-level waste containers.

As expected from thermodynamic considerations no hydrogen could be detected from copper immersed in model groundwaters at 50°C.

Hydrogen is evolved from corroding steel under anaerobic conditions. The highest steady state rate observed in a model groundwater was 2.5 ml (NTP) × m^-2 × h^-1 at 50°C in the model Böttstein groundwater, corresponding to a corrosion rate of 7 μm/a; it was lower at both 25°C and 80°C. Higher values were however obtained in dilute bentonite slurries. The hydrogen evolution rate is independent of chloride concentration between 0 and 8000 mg/l but falls by a factor of 10 on increasing the pH from 7 to 10.

Hydrogen evolution due to corrosion of iron or steel in waste repositories has to be considered in any safety analysis; the amounts produced can be significant.

The report concludes the current phase of corrosion testing for Swiss high-level waste repositories. Evidence to date suggests that both cast steel and copper are suitable container materials. Because the corrosion behaviour of both materials is sensitive to service conditions, in particular length of the aerobic phase, groundwater chemistry and temperature, further testing should be undertaken when a specific site has been identified.

Technical Report NTB 89-18

Interpretation of hydraulic testing at the Kaisten borehole

Summary

This report presents the results of hydrogeologic interpretations of the analyzable single packer, double packer, and H-log tests conducted in the Kaisten borehole. A brief discussion of the testing and interpretation methods is also presented. Data analysis was performed using the INTERA Graph Theoretic Field Model (GTFM) which permits borehole pressure history and thermally-induced pressure effects to be incorporated into the simulations.

Formation pressures (and corresponding equivalent freshwater heads) were determined for nine of the test intervals. The calculated equivalent freshwater heads were at 296.8 m a.s.l. (Buntsandstein) and 323.7 m a.s.l. (Permian) for the sedimentary units. For the upper crystalline, the calculated freshwater heads were at 322.5 m a.s.l. at a depth of 310.38 m, and at 324.6 m a.s.l. at a depth of 482.50 m. For the lower crystalline, from 818.86 m to 1'275.60 m depth, the calculated freshwater heads ranged from 345.8 m a.s.l. to 351.0 m a.s.l.

Hydraulic conductivities were estimated for 53 different hydraulic test sequences in the Kaisten borehole. Interpreted hydraulic conductivities ranged from greater than 1.0E-06 ms^-1 to less than 1.0E-12 ms^-1. Approximately 60 % of the test intervals exhibited hydraulic conductivities greater than or equal to 1.0E-09 ms-1. These high hydraulic conductivity intervals are distributed throughout the borehole. The longest section of the borehole exhibiting intermediate to low hydraulic conductivities (less than 1.0E-10 ms^-1) is 137 m between the approximate depths of 666 m and 803 m. A base case specific storage of 2.1E-08 m^-1 was used for the analyses in the crystalline rock. In several cases, sensitivity analyses were conducted to determine the impact of varying this parameter on the selection of the best estimate for hydraulic conductivity. In these cases, specific storage was increased by two orders of magnitude. For the test intervals which exhibited hydraulic conductivities greater than 1.0E-09 ms^-1, varying the specific storage had no significant effect on the best estimate for hydraulic conductivity. For the test intervals which exhibited hydraulic conductivities less than 1.0E-11 ms^-1, increasing the specific storage to 2.1E-06 m^-1 had the effect of reducing the best estimate for hydraulic conductivity by approximately one-half an order of magnitude.

Technical Report NTB 89-17

Reduction spheres in hematitic rocks from Nortern Switzerland:
Implications for the mobility of some rare elements

Summary

Reduction spheres are small-scale, isolated redox systems occurring in hematite-stained rocks of variable age, origin and provenience. Reduction spheres from Permotriassic continental red beds and from basement rocks of Northern Switzerland have been studied mineralogically and geochemically. More than 40 authigenic minerals occur in the dark, mineralized cores of the spheres. The most common minerals are roscoelite, uraninite and nickel arsenides, but minerals containing the precious metals Ag, Au, Pd and Pt, selenides and REE­rich uraninite have been found as well. The bleached spheres are depleted in ferric iron due to hematite dissolution. Mass balance calculations show that only a minor part of the trace elements present in the bulk rock has been mobilized during halo formation. The relative mobilities are: As > U > Ni > V. The elements precipitated as ore minerals in reduction sphere cores were probably released from iron hydroxides during their alteration to hematite. The red beds which contain reduction halos are unusually rich in As (100 ppm) compared with underlying reduced beds containing less than 5 ppm As. Reduction halo formation took place during the Mesozoic at depths of burial of 500 to 1'100 m. The porewaters were highly saline. Organic matter of unknown origin reacted with dissolved high-valence ions and hematite at discrete, randomly distributed sites in the rocks and caused local precipitation of rare elements as ore minerals and dissolution of hematite. Element migration was diffusive. Oxidation of organic matter and ore mineral precipitation was probably catalyzed by bacteria because the sulphides were formed by in situ low-temperature (< 100°C) sulphate reduction. Reduction spheres can be used to date and trace quantitatively element mobilities during red bed diagenesis. The geological significance of their presence is not yet understood.

Reduction spheres give qualitative information on element mobilities in an environment similar to that of a possible radioactive waste repository in Northern Switzerland. Diffusive element transport of U, REE, Pd and many other elements under oxidizing conditions is evidenced as well as immobilization of these elements by very local reducing conditions. Very high concentration gradients of U and other elements at redox boundaries persisted for roughly 10⁸ years.

Technical Report NTB 89-16

Grimsel Test Site
Hydrologic modelling of the migration site at the Grimsel Test Site
The steady state

Summary

Tracer migration experiments are in progress at the Grimsel Test Site (GTS) in order to test radionuclide transport models and to develop appropriate experimental techniques. In order to assist design and to interpret field tracer tests the knowledge of the hydrology of the site is essential. This report describes the hydraulic modelling efforts with the following objectives: 1) to interpret experimental findings. 2) to provide input, such as flowpaths and flow velocities, for transport modelling. 3) to assist in the design of the tracer experiments. The model concept is based upon the observations, that the fracture, where the migration experiments will take place, is relatively planar, has an average aperture of a few millimetres and is filled with a clayish material: The migration fracture is modelled as a two-dimensional, isotropic, heterogeneous equivalent porous medium. To allow for a sufficiently detailed model a hierarchical approach was followed where the boundaries for the "Iocal" model, i.e. the model in the vicinity of the migration site, were taken by first solving a larger "regional" model with coarser discretization. Transmissivity values as extracted from single borehole tests were used for definition of the heterogeneous transmissivity field in the model calculations. Model results for hydrostatic pressures and discharge rates compare favourably with experimental data, even though some problems remain to be resolved. Calculations for dipole arrangements have been performed; they turned out to be useful tools for deciding on injection- and withdrawal rates to be applied in the migration experiments as well as for qualitatively estimating tracer recovery rates.

Technical Report NTB 89-15

Grimsel Test Site
Hydrogeological characterisation of the migration experimental area at the Grimsel Test Site

Summary

Tracer migration experiments in a single fracture in granodioritic rock are in progress at the underground Grimsel Test Site (GTS), to test radionuclide transport models and to develop experimental techniques. This report recapitulates the exploration of the site and provides information on the instrumentation of the location chosen for migration experiments.

Preliminary estimations of groundwater flow velocities into the tunnel in four different fracture zones (lamprophyre dikes, and mylonitic zones) led to a selection of a location (at AU96m, GTS metering) considered optimally suited for migration experiments. The hydrogeology of the chosen single (proto)mylonitic fracture was unknown at the scale of about 10 × 10 m and had to be explored with boreholes. Ten boreholes proved that the fracture is an almost planar element in the water-saturated rock mass. Eight boreholes yield sufficient groundwater for the measurement of hydraulic pressure and for groundwater sampling. Most boreholes are equipped with a system of triple packers which isolate the fracture and the borehole mouth. Passive hydraulic tests proved that a hydraulic connection exists between the boreholes and that the transmissivity of the fracture is about 0.6 m²/d. Long-term monitoring revealed a decline of hydraulic pressure of about 0.08 bar/y, which indicates that the hydraulic system has not yet fully reached a steady state.

Plastic sheets mounted on the wall of the tunnel at its intersection with the fracture allow the measurement of the discharge of groundwater, which is in the order of 0.5 L/min at the migration location. From the observation of breakthrough phenomena during drilling of boreholes and the miscible displacement of groundwaters of a slightly different composition during injection-extraction experiments, a flow velocity at the migration location was estimated to be in the order of a few meters per hour. The groundwater does not contain tritium in detectable amounts and, therefore, the reservoir volume of groundwater, which discharges to the tunnel, must be high (i.e. more than 200 m³). The fracture of interest is probably hydraulically connected to a major shear zone in the area.

Technischer Bericht NTB 89-14

Mikrobieller Abbau von Bitumen

Summary

Bitumen is used as a matrix to immobilize low and intermediate level radioactive wastes. Since the presence of microorganisms in nuclear waste repositories cannot be ruled out, it is important to quantify the microbial degradation of bitumen. Investigations were carried out both under aerobic and anaerobic conditions using a variety of cultures. It was observed that bitumen degrading microorganisms are ubiquitous and form biofilms on bitumen surfaces. The long term bitumen degradation rates were found to be essentially independent of the culture initially inoculated. Under aerobic conditions this rate lies between 20 and 50 g of bitumen per m² and year, leading to the production of 15 to 40 litres m^-2 a^-1 of CO₂ at STP. Under anaerobic conditions the degradation rate is only about 1 percent of that under aerobic conditions. A linear extrapolation for anaerobic conditions and for a surface to volume ratio typical for nuclear waste (200 litre drums) lead to 0.3 to 0.8 percent loss of bitumen matrix within 1000 years.

Technical Report NTB 89-13

Subcritical crack growth in hot-isostatically postcompacted high-grade alumina

Summary

The subcritical crack-growth behaviour of natural cracks in a hot isostatically post-compacted high-grade alumina ceramic was tested in a concentrated salt solution at 70°C. The result of static bending tests yielded a crack-growth law that can be described by a power law with an exponent n = 73.5. This high value reflects an excellent resistance to subcritical crack growth. A comparison with Al203-materials investigated earlier has been included.

Technischer Bericht NTB 89-12

Wellenberg Arbeitsprogramm
Teil 1 für Untersuchungen von der Erdoberfläche aus und Sondierbohrungen SB1, 3 & 4

Summary

Im hier vorliegenden Arbeitsprogramm wird ein erster Teil der gesamten am Wellenberg vorgesehenen Feldarbeiten behandelt. Dieser Teil umfasst die Untersuchungen von der Erdoberfläche aus und die Sondierbohrungen SB1, 3 & 4 und gehört zur Untersuchungsphase Wellenberg Ia·, welche neben weiteren Bohrungen noch einen Sondierstollen bis Ende Traverse sowie ein Felslabor beinhaltet. Der Bericht erläutert Ziele, Methoden, Umfang und zeitliche Abfolge der vorgesehenen Feld- und Laboruntersuchungen. Es dient auf der einen Seite allen direkt am Projekt Beteiligten als Arbeitsgrundlage und verfolgt andererseits den Zweck, Behörden, Aufsichtsorgane und die interessierte Öffentlichkeit umfassend über die geplanten Arbeiten zu informieren.

Im ersten Kapitel des Arbeitsprogramms werden die grundsätzlichen Aspekte des Untersuchungsvorhabens dargelegt.

Kapitel 2 behandelt die geologischen, hydrogeologischen und geophysikalischen Messungen, welche an der Erdoberfläche (exklusive Sondierbohrungen) durchgeführt werden. Es wird gezeigt, dass diese Untersuchungen neben der Beschaffung von wichtigen erdwissenschaftlichen Daten auch der vorsorglichen Beweissicherung dienen (z.B. Rutschkataster, Quellenüberwachungen etc.). Deshalb werden diese Aktivitäten vor, während und nach dem Abteufen der Sondierbohrungen durchgeführt.

Der Schwerpunkt des Programms – die Untersuchungen in den Sondierbohrungen SB1, SB3 und SB4 – wird in Kapitel 3 vorgestellt.

Aus den Ausführungen in den Kapiteln 3.1 und 3.2 geht hervor, dass die Untersuchungsziele und -methoden grundsätzlich dieselben sind wie bei den bisherigen Standortuntersuchungen für die Endlagerung schwach- und mittelradioaktiver Abfälle.

In Kapitel 3.3 wird der geplante Untersuchungsumfang pro Sondierbohrung quantifiziert. Hierbei gewähren die Ablaufpläne (Arbeitsschritte) sowie die Datenblätter einen informativen Überblick über die operationellen Arbeiten in den Sondierbohrungen SB1, SB3 und SB4.

Schliesslich folgen in den Kapiteln 4, 5 und 6 Erläuterungen zur Berichterstattung, Logistik, Terminplanung und Projektorganisation.

Für Planung, Vorbereitung und Ausführung der operationellen Arbeiten wurde Nagra-intern ein Projektteam zusammengestellt, welches auch das vorliegende Arbeitsprogramm erarbeitet hat.

Technischer Bericht NTB 89-11

Felslabor Grimsel
Neigungsmessungen zur Ermittlung neotektonischer Vorgänge

Summary

Between 1985 and 1989, the defections of the vertical were observed at 6 measurement stations at the Grimsel Test Site using Askania vertical pendulums.

The periodic range observed included the tides of the solid earth (T ~ 12 h and 24 h) and aperiodic tectonic movements (T ~ ∞). The analysis of tides gave differing amplitude factors and phase shifts at the measurement stations. This could be due to a large extent to lateral inhomogeneities between the stations in question. An investigation of the potential effects of reservoirs in the vicinity (T ~ weeks up to 1 year) showed that the effect of the Grimsel reservoir at the stations was extremely small. This could be explained by decoupling of movements along geological disturbed zones. The annual temperature wave, which is determined primarily by meteorological factors, is different at the various stations; this is also interpreted as the effect on the signal of inhomogeneities between the stations. The aperiodic movement, which has not yet been fully investigated, indicates subsidence in the vicinity of the reservoirs. A disturbed zone between the two reservoirs appears to have the effect of decoupling the movements with respect to one another, so that the stations to the north of this zone subside in the direction of the Räterichsboden reservoir (N/E direction), while the stations south of the zone tilt towards the south, i.e. in the direction of the Grimsel reservoir.

Technical Report NTB 89-10

Final interpretation of hydraulic testing at the Siblingen borehole

Summary

This report contains the results of the interpretation of hydraulic testing in the sedimentary and crystalline rocks of the Siblingen borehole. Hydraulic tests were performed between September 1988 and April 1989 in single packer, double packer and open hole configurations. A total number of thirty one separate intervals covering about 90 % of the Siblingen borehole length were tested. Preliminary field analyses documented in "Quick-Look" Reports were performed for all intervals tested. Detailed interpretations were conducted for 16 selected intervals and documented in stand-alone reports. The results of both types of analysis are included in this final report. Data analysis was performed using the ITA Transient Interactive Analysis System (TRIAS) and the INTERPRET analysis code of Scientific Software-Intercomp. Representative test interval parameters including hydraulic conductivity test interval static pressure and equivalent freshwater hydraulic head are interpreted and recommended. Since no reliable in-situ estimation of the specific storage value was possible it was calculated directly from the input parameters. For the crystalline rock a value of 8.6E-08 m^-1 was generally used. Interpreted hydraulic conductivities range from 1.5E-06 ms^-1 to 2.0E-04 ms^-1 for the sedimentary aquifers and from 5.1E-11 ms^-1 to 4.0E-04 ms^-1 for the crystalline rock. Formation pressures and corresponding hydraulic heads were estimated for 26 of the 31 test intervals. The determined equivalent freshwater heads range from 462 m asl for the Muschelkalk aquifer on top to 436 m asl for the lowest part of crystalline rock. Relatively high hydraulic conductivities encountered in the Siblingen borehole are expected to minimize the influence of the pretest borehole pressure history on the test data. Therefore, pressure history was generally not included in the analysis.

This report also contains a brief description of the testing equipment and performance as well as testing and analysis methods used for the determination of hydrogeologic parameters.

Technical Report NTB 89-09

Interpretation of hydraulic testing at the Schafisheim borehole

Summary

This report presents the results of hydrogeologic interpretations of the analyzable single packer, double packer, and H-log tests conducted in the Schafisheim borehole. A brief discussion of the testing and interpretation methods is also presented. Data analysis was performed using the INTERA Graph Theoretic Field Model (GTFM) which permits borehole pressure history and thermally-induced pressure effects to be incorporated into the simulations.

Formation pressures (and corresponding equivalent freshwater heads) were determined for 11 of the test intervals. The calculated equivalent freshwater heads were at 370 m a.s.l. (Malm) and 444 m a.s.l. (Muschelkalk) for the sedimentary units, and were in a tight range from 366 m a.s.l. to 370 m a.s.l. in the crystalline portion of the borehole.

Hydraulic conductivities were estimated for 37 different hydraulic tests in the Schafisheim borehole. A detailed analysis was performed for 16 intervals, and a less in-depth review was conducted of an additional 21 low permeability intervals (interval 1972.6 was tested twice). Interpreted hydraulic conductivities ranged from 5.0E-06 ms­^-1 to less than 1.0E-12 ms^-1. Zones of high hydraulic conductivity (greater than 1.0E-09 ms^-1) were observed at the following approximate true vertical depths: 552.9 m to 563.0 m, 1,227.6 m to 1,286.9 m, 1,564.1 m to 1,607.6 m, 1,882.8 m to 1,903.3 m, and 1,953.7 m to 2,005.8 m.

A base case specific storage of 1.0E-07 m^-1 was generally used for the crystalline rock. A sensitivity analysis was conducted for some tests to determine the uncertainty in the best estimate of K related to the choice of specific storage. In these cases, specific storage was varied by 1 to 2 orders of magnitude from the base case value.

Technical Report NTB 89-08

The coordination chemistry of humic substances

Summary

Dissolved humic materials form ligands which can influence the speciation of certain radionuclides in natural waters and it is necessary to take account of this phenomenon in repository safety assessment model calculations.

In operational terms the dissolved humic materials can be subdivided into humic and fuIvic acids, both groups being poIyfunctional macromolecules with a large structural variety. The description of their solution chemistry is restricted to very simplifying models clue to the inhomogeneity of their properties.

The protolytic behaviour of humic materials is determined mainly by carboxyl- and phenolic OH-groups whose acidities are modified by substituents. A model which assumes a normal distribution of the pK_a values for both acid groups would appear to have a good chemical foundation.

There are, however, no models available which give a basic description of the wide range of complexation properties of humic materials with dissolved metals and which are based on a chemically meaningful background. All of these models make very drastic simplifications and, although capable of reproducing experimental data, they do not allow any extrapolation to ranges of concentration outside those covered by the experiments.

By way of a minimal description, the humate complexes can be treated as a mixture of two to three single complexes with 1:1-stoichiometry. The stability constants and the concentrations of the individual ligands are then adjustable parameters which are dependent on the concentration conditions and are valid only for specific pH-values and ionic strengths. As there is no simple model describing the pH-dependence of complexation, most of the literature data (usually determined for low pH-values) are not in fact applicable in the case of natural waters.

Humic materials affect not only the solution chemistry of the metals but also their sorption onto minerals. The mineral surfaces are modified through adsorption of organic material which in turn aIters their sorption properties. These processes are understood in qualitative terms.

It is unlikely that a comprehensive predictive model of compIexation by humic materials will be available in the foreseeable future. It is therefore suggested that experiments which provide an empirical description of the metal/humic material interactions using realistic pH-values and metal/ligand ratios be carried out. Besides parameter studies and experiments in synthetic or formation waters, analogue studies could also serve as a useful tool.

Technical Report NTB 89-07

Stripa Project
Interim report on the rock sealing project (Stage 1)

Summary

The objective of the Sealing Project is to find new ways of sealing finely fractured rock by grouting. This requires development of new injection techniques as weIl as identification of materials which are sufficiently fluid to be groutable, have an acceptably low permeability and are physically and chemically stable. The present report describes the results of the first two years of investigations (Stage 1) which gave very positive results, as can be concluded from the large-scale field test.

Technical Report NTB 89-06

An Approach to Microbiological Modelling:
Application to the Near Field of a Swiss Low Intermediate-level Waste Repository

Summary

This report presents a general approach to modelling of microbiological activity in a geological system constrained by the availability of nutrients and energy sources. Application of this model to determine the possible consequences of such microbiological activity in a repository for low-and intermediate-level nuclear waste (L/ILW) is demonstrated both for the repository considered as a whole and for a more realistic sub-section containing a particular waste type (ion-exchange resins immobilised in cement). Possibilities for further development of the model to consider the spatial distribution of components within the near field are also discussed.

Technical Report NTB 89-05

Review of HLW disposal concepts in sediments

Summary

A review of research relevant to the disposal of high-level radioactive waste carried out in North America and Europe in argillaceous rocks has been performed on behalf of the Swiss National Cooperative for the Storage of Radioactive Waste (Nagra). This review includes information published up to the end of 1987.

A list of all sedimentary formations (excluding evaporites) considered as host rocks for radioactive wastes in Belgium, West Germany, France, the UK, Italy, the USA, and Canada is presented and the key aspects of research in specific formations in these countries are described. Topics covered include: site selection and investigation methodology, hydrogeology, transport behaviour (radionuclide migration), hydrochemistry, mineralogy-geochemistry, geotechnical problems, thermal effects and engineered barriers. Formations described include: the Boom Clay (Belgium), the sedimentary sequence at Konrad (West Germany), the sedimentary sequences overlying salt deposits at Asse and Gorleben (both West Germany), various formations in France, the Harwell Research Site and various NIREX sites in the UK, various clay formations in Italy and numerous basins in the USA and Canada.

Generic information on argillaceous strata pertinent to waste isolation is also presented. This includes descriptions of the geochemistry of shales and mudstones, physicochemical transport processes in clays, geotechnical aspects of argillaceous rocks, redox potentials in natural systems, the influences of fractures and faults on the permeability of clays and the geomicrobiology of sedimentary rocks.

Recommendations for further research designed to improve knowledge of critical problems are presented.

A comprehensive reference list is included at the end of the text.

Sorption parameters for the critical radionuclides and a reference list on this topic are given in the appendices.

Technischer Bericht NTB 89-04

Modellierung der Grundwasserströmungsverhältnisse am Oberbauenstock

Summary

This report concerns the hydrogeological modelling of the potential waste repository site in the Oberbauenstock (Canton Uri). It includes a description of enhancements made to an existing model and presents the results obtained. The aim of the modelling is the simulation of groundwater flow conditions in the repository host rock and adjacent formations. The modelled region covers an area of approximately 32 km² in the neighbourhood of the overthrust of the Drusberg and Axen nappes.

Each stage of the enhancement of the model is discussed in the present report. The spatial layout of the relevant aquifers and aquitards, together with their hydraulic properties, is defined. The overthrusting structure of the nappes, steeply dipping fractures, the Seelisberg road tunnel and the repository excavation itself are included in the model as discrete structures.

The partial differential equation describing the hydraulic potential field in three dimensions is solved numerically using the finite element code FEM301, which calculates the stationary groundwater flow in a fully saturated porous medium.

This report expands the understanding of hydrogeological conditions in the Oberbauenstock obtained during an earlier phase of modelling performed as part of the "Project Gewähr". Parameter variations have been performed to ascertain the influence of some of the less well founded assumptions concerning hydraulic parameters, the spatial extent of the host rock and various boundary conditions. The modelling results yield values for the water flux through the repository and an indication of the flow trajectories and flow times from the repository caverns to the biosphere.

The results will contribute to the planning of the repository access tunnels as well as serving as the hydrodynamic basis for the preparation of an investigation programme to be performed during the forthcoming exploration phase.

The model described in this report is based on the most recent state of knowledge of the hydrogeology of the Oberbauenstock region. Available data allows independent tests to be made on certain model results. The planned investigations will yield new information that will permit a better definition of some parts of the current conceptual model, and modelling activities should therefore not be regarded as completed. However, this latest series of model calculations covers the possible extent of changes that previously could have inaccurately represented factors influencing the simulated flow conditions.

Technical Report NTB 89-03

Transmission electron microscope study of illite/smectite mixed layers

Summary

A transmission electron microscopy study was undertaken to investigate the microstructural relationships between mixed layer illite/smectite phyllosilicates and their alteration products. The analysis of specimens from Southwestern Sweden suggested that four types of montmorillonite alteration could be distinguished. The first three possibilities involved the dissolution of montmorillonite followed by the precipitation of various phases: silica, fine grained montmorillonite which subsequently recrystallized to illite, or quartz and illite. The fourth possibility involved dehydration and isostructural replacement of montmorillonite with illite. The mobility of amorphous silica in the montmorillonite structure was demonstrated using in-situ experiments with the electron beam, and static observations of two identical but differently prepared specimens. In the latter case distinctions were observed in the residence of the silica phase. Based on the transmission electron microscope observations, it is proposed that the precipitated amorphous silica can affect the swelling capability of montmorillonite both as a replacement of interlayer water, and as an obstruction to the free movement of water into the structure.

Technical Report NTB 89-02

Geochemical database for the sediment study
Intermediate report 1988

Summary

This report documents the geochemical database which was used for the safety assessment of two sedimentary formations: the Lower Freshwater Molasse (Untere Süsswassermolasse, USM) and the Opalinus Clay (Opalinuston, OPA). The database was used for the "Sediment-Zwischenbericht" study of HLW disposal in Swiss sediments (Nagra 1989). The in-situ geochemistry of the groundwaters has been derived from available water sample analyses, assumptions based on mineralogical and literature data, as well as on computer calculations.

The available data (geology, geochemistry/petrography, organics, gas, hydrochemistry) allow the definition of geochemical Reference Conditions (RC) for the potential repository site. This information is then used to calculate the associated Reference Waters (RW).

A sorption database for modelling radionuclide transport in these formations is presented. In addition, possible geochemical perturbations due to the presence of colloids, microorganisms or organic carbon are discussed.

Technischer Bericht NTB 89-01

Felslabor Grimsel
Hydrogeologische Untersuchungen zur Bestimmung hydraulischer Potentiale

Summary

One of the general objectives of developing the Grimsel Test Site is to increase know-how in planning, conducting and interpreting underground tests, and also to increase the level of practical experience in the operation of test equipment.

In relation to the hydrogeologic investigations, the aim was specifically to build confidence in routine tasks planned for hydrologic investigation of potential repository sites. Another purpose was to acquire know-how within the Swiss technical community. The Swiss firm SOLEXPERTS acquired knowledge regarding:

This capability corresponds to the level of technology presently available in the area of hydrologic testing in boreholes.

This report describes the development and application of hydrologic test systems and testing techniques from 1985 to 1987. In addition, the analysis methods and various testing results are presented.