Preface
The International Stripa Project was a cooperative research and
development project among several member countries of the Nuclear
Energy Agency of the Organization for Economic Cooperation and
Development. The project, which started in 1980 and ended in 1992, was
conducted under the auspices of the Nuclear Energy Agency. The project
was managed by the Swedish Nuclear Fuel and Waste Management Company
(SKB) under the direction of a Joint Technical Committee (JTC) composed
of representatives from participating countries.
The scientific and technical
objectives of the project were to investigate several aspects of
technology concerned with the feasibility and safety of disposal of
long-lived, heat-generating radioactive waste at depth in granitic
rocks. In particular, the Stripa Project addressed:
- the development of instruments and procedures to characterize candidate repository sites;
- the understanding and modelling of groundwater flow and solute transport in fractured crystalline rock; and
-
the design of engineered barriers capable of contributing to waste
isolation by restricting groundwater flow in proximity to the waste
containers and in the surrounding host rock.
Because
the activities and the results of the Stripa Project have been reported
in more than 170 technical reports, the JTC has decided that the final
action of the project should be the publication of an overview report
that would convey, in relatively concise form, the body of information
produced by the project.
The overview report has been subdivided into three volumes:
I. Executive Summary
II. Natural Barriers
III. Engineered Barriers
The Executive Summary
summarizes the contents of the other two volumes with the addition of
some general considerations about the Stripa Project. The authors of
the Executive Summary are the five members of the Overview Reporting
Group, that was established by the JTC for the purpose of producing the
overview report. The Overview Reporting Group consisted of the two
authors of volumes II and III, Paul Gnirk and Malcolm Gray,
respectively, and two outside reviewers, Charles Fairhurst and
Ferruccio Gera. The Project manager, Bengt Stillborg, acted as
coordinator.
Conclusions
- The
hydraulic conductivity of drill holes and excavated openings could be
returned to values similar to those of intact granite by the judicious
use of HCB.
- Models are now available to predict the response
of HCB to changes in stress, thermal and hydraulic gradients. The
models for water transfer are not rigorously precise. In contrast,
thermal properties are reasonably well understood and heat fluxes
through the material can be well described.
- The properties of advanced, high-performance bentonite- and
cement-based materials pertinent to their successful injection as
grouts in fractured rock have been well defined. Equipment and
procedures for injection of the grouts have been developed and are
available for use in repository design and construction. The limits of
the application of the selected materials and methodologies were
defined for the Stripa granite.
- Data at the level of detail
derived for the application and qualification of general groundwater
flow models such as those examined in the natural barrier studies of
the Stripa Project are unlikely to provide sufficient information for
grouting activities intended for repository sealing. As at Stripa, the
information gained from the grouting activities at repository sites
will likely lead to revisions in understanding of the rock mass.
- An
excavation disturbed zone (EDZ) consisting of a blast disturbed zone
enveloped by a stress disturbed zone existed in the rock surrounding
tunnels in the Stripa granite. In the absence of alternative,
preferably repository site specific information, the EDZ of the blast
disturbed zone in granitic rocks similar to those at Stripa can be
taken to have an hydraulic conductivity of about 10-9 to 10-8
m/s. At the locations in the Stripa mine studied by the engineered
barriers research group, the stress disturbed zone appeared to be more
conductive parallel to the axis of the excavations (3·10-10 ≤ k ≤ 9·10-10 m/s) than normal to it (7.5·10-12 ≤ k ≤ 2.3·10-11 m/s).
- Under the low hydraulic gradients expected in groundwater in
a sealed repository site, chemical transformation of the minerals in
clay- and cement-based sealants can be predicted, reasonably, to extend
over tens of thousands to millions of years. The predicted period
depends on the porosity of the as-placed materials and the ionic
concentrations in the groundwater.
- The sealing properties of
both clay- and cement-based sealants are most susceptible to change
under high hydraulic gradients. Thus, both materials will be most
vulnerable to adverse change during seal construction and the period
over which the repository is open for the deposition of waste.
- The
engineered barriers studies for the Stripa Project have increased
confidence in the ability to engineer geological repositories for
heatgenerating radioactive wastes.
- Due to the perturbations
caused by the presence of excavations it may never be possible to
examine physically in underground laboratories, or from excavations at
repository sites, the conditions under which radionuclides will migrate
within either the engineered barriers or the host rocks that form the
waste isolation system.
- A number of unresolved items have
been identified. These could be considered for resolution in future
collaborative studies or national programmes of research and
development related to repository design and construction.
