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Technischer Bericht NTB 14-03

SGT Etappe 2: Vorschlag weiter zu untersuchender geologischer Standortgebiete mit zugehörigen Standortarealen für die Oberflächenanlage Charakteristische Dosisintervalle und Unterlagen zur Bewertung der Barrierensysteme

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A.    Overview

The procedure for selecting sites for deep geological repositories for all categories of radioactive waste arising in Switzerland is defined in the conceptual part of the Sectoral Plan for Deep Geological Repositories (BFE 2008). Selection of the sites where the required repositories will be constructed takes place in three stages, ending in the third stage with a general licence procedure for each repository type (HLW and L/ILW repositories). The general licence defines the site and the main features of the facility. In Stage 1, Nagra proposed six geological siting regions based on criteria relating to safety and engineering feasibility. These proposals were entered in the Sectoral Plan with a decision by the Federal Council of 30th November 2011.

The Swiss Federal Nuclear Safety Inspectorate (ENSI) has specified a series of requirements for the provisional safety analyses and the safety-based comparison of the siting regions in Stage 2, in particular as regards the dose calculations to be carried out. In the requirements, ENSI defines a "standardised parameter variation procedure" according to which a number of dose calculations have to be carried out for each repository type and siting region (reference case, alternative cases). From these dose calculations so-called characteristic dose intervals can be derived, which are then used to assess the safety of the siting regions. ENSI has also formulated criteria that are used to evaluate whether a siting region is suitable from a safety perspective and whether the siting regions are equivalent in terms of safety.

This report documents the dose calculations carried out by Nagra and the characteristic dose intervals derived from these. The effectiveness of the barrier systems for the L/ILW and HLW repositories in each siting region is also evaluated quantitatively and the long-term evolution of the barriers is discussed taking into account repository-induced processes. The results documented in this report represent important input for the main report on the safety-based comparison and the proposal of geological siting regions for further investigation in Stage 3.

To make the optimum use of the potential of a siting region, the evaluation of the siting region is done for the so-called priority host rock in the regions for the L/ILW repository where two potential host rocks can be found1. In these cases, the first step is the identification of the priority host rock, using the same method as for the narrowing-down of the geological siting regions. The characteristic dose intervals are determined separately for each host rock and the effectiveness of the barrier systems is evaluated under the assumption that the entire inventory will be located in disposal chambers exclusively in the host rock being considered. These interim results provide key input for the main report.

In the second step2, the characteristic dose intervals are determined and the effectiveness of the barrier systems is evaluated for the HLW and L/ILW repositories in the disposal perimeters within the siting regions as defined in the main report; for the L/ILW siting regions where two host rocks occur the priority host rock is considered.

The dose calculations used to determine the characteristic dose intervals and for the evaluation of the effectiveness of the barrier systems analyse the transport of radionuclides dissolved in porewater from the repository through the host rock and confining units. It is conservatively assumed that from there the radionuclides are transported directly (i.e. without any further retention) to the biosphere. System analyses carried out separately show that, if the repository is designed appropriately, all repository-induced effects on the safety barriers can be kept small. These are therefore not considered explicitly in the dose calculations.

B.    Analyses to determine the priority host rocks for the L/ILW repository

Characteristic dose intervals
The results of the dose calculations (which, as prescribed, chiefly include pessimistic variants) show that, based on the dose intervals, all the L/ILW host rocks considered up to this point are suitable and equivalent in terms of safety. There are, however, differences between the host rocks and these can also be seen in the dose calculations. If the results for the 'Brauner Dogger' and the Opalinus Clay in the siting regions Zürich Nordost and Nördlich Lägern are compared, it is clear that the upper limit of the characteristic dose intervals for the 'Brauner Dogger' is significantly higher than for the Opalinus Clay. This is because the release pathways in the 'Brauner Dogger' host rock are significantly shorter compared to those in the Opalinus Clay. For the Jura-Südfuss siting region with the host rocks Effingen Member and Opalinus Clay, the upper limit of the characteristic dose interval for the Effingen Member is significantly higher than for the Opalinus Clay, again because of the significantly shorter release pathways in this host rock compared to the Opalinus Clay. In summary, in all three siting regions where two potential host rocks occur the Opalinus Clay has clear advantages compared to the other host rock in that siting region.

Evaluation of the effectiveness of the barrier systems
For the L/ILW siting regions with two host rocks, an illustration of the barrier function is presented for each host rock and the barrier function is then quantified using the radionuclide fluxes between the components of the barrier system. A comparison of the results for the host rocks 'Brauner Dogger' and Opalinus Clay in the siting regions Zürich Nordost and Nördlich Lägern shows that, for both siting regions, the two host rocks have an almost identical barrier effect in terms of release from the near-field. However, for release from the confining units, the 'Brauner Dogger' drops down significantly compared to the Opalinus Clay. The picture is similar for the host rocks Effingen Member and Opalinus Clay in the Jura-Südfuss siting region. Both host rocks are practically identical in terms of release from the near-field but the Effingen Member is significantly behind the Opalinus Clay in terms of release from the confining units.

The results documented in the main report for the safety-based comparison of the host rocks confirm the results of the analyses in the present report: The Opalinus Clay is the priority host rock in all three siting regions for L/ILW with two host rocks.

C.    Analyses for the siting regions for the L/ILW and HLW repositories

Characteristic dose intervals
The results of the dose calculations (which, as prescribed, chiefly include pessimistic variants) show that, based on the dose intervals, all the siting regions are suitable and equivalent in terms of safety. There are, however, differences also between the siting regions and these can also be seen in the dose calculations and the upper values of the characteristic dose intervals.

L/ILW siting regions: For the Südranden siting region, it is assumed in the reference situation that, given the relatively shallow depth of the repository and the associated potential for rock decompaction, the upper confining units of the Opalinus Clay are not effective as a barrier. The length of the release pathway considered in the dose calculations is therefore restricted in the upward direction to the section in the Opalinus Clay. In the downward direction, the length of the release pathway is determined by the sections in the Opalinus Clay and the clay-rich Lias, with an assumed release to the biosphere via the so-called Arietenkalk. In contrast, in the siting regions Zürich Nordost and Nördlich Lägern, the critical (shortest) release pathway is longer because both the upper and the lower confining units are effective as a barrier in these cases. In the case of the Jura Ost siting region, in the reference situation, the critical (shortest) release pathway is in the upward direction and release to the biosphere is assumed to occur via the Sandy limestone sequence directly above the Opalinus Clay. In the downward direction the length of the release pathway is determined by the sections in the Opalinus Clay and the clay-rich Lias, with an assumed release to the biosphere via the Arietenkalk. For the Jura-Südfuss region, it is assumed in the reference situation that only the Opalinus Clay is effective as a barrier and that release to the biosphere occurs via the Sandy limestone sequence and the Limestone-rich Lias directly above or below the Opalinus Clay, respectively. This explains the differences in the upper limits of the characteristic dose intervals for the siting regions in Northern Switzerland (i.e. the somewhat higher upper limits for the regions Südranden, Jura Ost and Jura-Südfuss (with a slight disadvantage for Jura-Südfuss) than for Zürich Nordost and Nördlich Lägern. For the Wellenberg siting region, the situation is different: Here, in the reference situation, a minimum transport path length of 100 m is assumed in the host rock; the upper limit of the characteristic dose interval, which is just below 0.01 mSv/a, is determined by the calculation case with increased water flow in the host rock. Because of the associated (non-reducible) uncertainties, the value for increased water flow is significantly higher here than for the corresponding value in the Opalinus Clay. The calculation case that illustrates the effect of erosion (formation of a breakthrough gorge) in the Südranden siting region gives a dose maximum that lies below the upper limit of the characteristic dose interval.

HLW siting regions: In the siting regions Zürich Nordost and Nördlich Lägern, in the reference situation, both the upper and lower confining units function as a barrier. The length of the critical (shortest) release pathway is determined by the sections in the Opalinus Clay and in the clay-rich Lias, with an assumed release to the biosphere via the Arietenkalk. In the case of the Jura Ost siting region, in the reference situation, the critical (shortest) release pathway is in the upward direction and release to the biosphere is assumed to occur via the Sandy limestone sequence directly above the Opalinus Clay. In the downward direction the length of the release pathway is determined by the sections in the Opalinus Clay and the clay-rich Lias, with an assumed release to the biosphere via the Arietenkalk. This explains the characteristic dose intervals for the HLW siting regions: For Zürich Nordost and Nördlich Lägern they are very similar; for the Jura Ost siting region the characteristic dose interval lies slightly higher than for the other two regions because of the slightly shorter critical release pathway. The calculation cases that illustrate the effect of erosion (formation of a breakthrough gorge) in the Jura Ost siting region give dose maxima that lie either clearly below (base case) or slightly above (less favourable variant) the upper limit of the characteristic dose interval. The dose maximum of the less favourable variant, however, lies clearly below 0.01 mSv/a.

Evaluation of the effectiveness of the barrier systems
For the L/ILW and HLW repositories, an illustration of the barrier effect is presented for each siting region and the barrier effect is then quantified using the radionuclide fluxes between the components of the barrier system, with the following results:
L/ILW siting regions: The quantification of the barrier effect using the radionuclide fluxes between the components of the barrier system gives the following picture: Regarding release from the near-field, the Wellenberg region shows a slight advantage compared to the regions in Northern Switzerland. However, for the safety-relevant release from the geosphere, Wellenberg shows a clear disadvantage compared to the other regions. The reason for this is that, for the Wellenberg siting region with its marl host rock, the possibility that radionuclide transport in the geosphere will also occur along fault zones – with a corresponding reduction in the barrier effect of the geosphere – cannot be ruled out.

HLW siting regions: All three HLW siting regions are practically equivalent in terms of the effectiveness of the barrier systems.

D.    Important site-specific geological information for Stage 3 from the viewpoint of safety analysis

An analysis of the results for the two repository types (L/ILW and HLW) in the siting regions to be followed up according to the main report shows that the most important site-specific geological information for Stage 3 relates to the geometry and quality of the host rock and the confining units (thickness of the host rock and location and quality of lithofacial units within the confining units with the potential for strongly increased water flow). It is expected that the exploration work planned for Stage 3 (3D seismics, boreholes) will complement and / or enhance this information so that the associated uncertainties discussed in the present report will be further reduced. For other site-specific geological information to be used directly in the safety analysis and that is important for the dose calculations (hydraulic parameters of host rock, mineralogy of host rock, etc.), it is not expected that the work planned for Stage 3 will achieve a notable reduction in the associated uncertainties. However, reducing these uncertainties is not critical as the parameter values currently being used are known sufficiently accurately. It is nevertheless planned as part of the exploration work for Stage 3 to acquire site-specific information on these parameters. The information base on erosion and glacial gully erosion will also be consolidated in the siting regions to be investigated further in Stage 3.

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1  These are: Zürich Nordost ('Brauner Dogger', Opalinus Clay), Nördlich Lägern ('Brauner Dogger', Opalinus Clay) and Jura-Südfuss (Effingen Member, Opalinus Clay).

2  Only this step is required for the HLW siting regions as the host rock for the HLW repository (Opalinus Clay) was already decided in Stage 1. Similarly for those L/ILW siting regions with only one host rock only this step is required.

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