Stability and Buffering Capacity of the Geosphere for Long-term Isolation of Radioactive Waste : Application to Crystalline Rock.

305 Pages·2009·19.836 MB·English

by  OECD

Radioactive Waste Management
2009
S
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Cover_f.fm Page 1 Wednesday, April 7, 2004 11:00 AM
Radioactive Waste Management
Stability and Buffering Capacity of the Geosphere
for Long-term Isolation of Radioactive Waste:
Applications to Crystalline Rock
Workshop Proceedings
Manchester, United Kingdom
13-15 November 2007
© OECD 2009
NEA No. 6362
NUCLEAR ENERGY AGENCY
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EXECUTIVE SUMMARY
A safety case for a geological repository for high-level and/or long-lived radioactive waste aims
at conveying reasoned and complementary arguments to illustrate and instil confidence in the
performance of the disposal system.
Potential geological host formations and their surroundings are chosen, in particular, for their
long-term stability, their ability to accommodate the waste disposal facility, their ability to prevent or
attenuate potential release of radioactivity (e.g. through their retention capacities), and their buffering
capacity vis-à-vis external and internal perturbations. In building a safety case, it is therefore important
to assess:
• the features, events and processes (FEPs) that could influence the evolution of the geosphere;
• the long-term stability of the favourable conditions displayed by the host formation;
• the buffering capacity of the formation vis-à-vis perturbations.
The key issue is to evaluate the resilience of the main safety functions of the geosphere (including
its flow and transport properties) to natural perturbations. The relevance of various naturally occurring
processes and events will depend upon the timescale to be considered, but timescales on the order of
about one million years are typical.
The NEA Integration Group for the Safety Case (IGSC) initiated the “geosphere stability” project
to address the role and importance of geosphere stability, and to develop a better understanding of the
scientific evidence and arguments that contribute to confidence in geological stability. Under this
initiative, a first workshop in 2003 was devoted specifically to argillaceous settings. The second
workshop, which is the subject of these proceedings, focused on crystalline rocks – a term that, in this
context, encompassed all forms of hard, fractured rocks. The workshop took place on 13-15 November
2007 in Manchester, United Kingdom.
In this workshop, the stability of a crystalline rock was broadly defined as the presence of
thermal-hydrological-mechanical-chemical (THMC) conditions considered favourable for the safety of
a radioactive waste repository. Stability, in this sense, does not imply that steady-state conditions exist.
The geosphere is constantly evolving, although in many cases rather slowly, and such evolution is
perfectly acceptable for safe geological disposal. What is important is that the geosphere's evolution is
fully understood.
The main functions of a crystalline host rock for a radioactive waste repository are to contribute
to the isolation of the waste mainly by providing a favourable environment (e.g. chemically) for the
repository, by contributing to retardation of radionuclide transport and by providing containment of
waste. As a repository programme and understanding of the accompanying processes advance and
mature, it is possible to differentiate these functions into more detailed requirements or sub-functions.
Such detailed requirements of the crystalline host rock are intimately linked to the repository concept.
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The workshop underscored the fact that many crystalline rocks are intrinsically stable
environments, which provide good buffering against external events and processes. It is acknowledged
that the properties and conditions of crystalline rocks evolve with time, but generally this evolution is
limited and the erosion rates of such rocks are often extremely low. Furthermore, the geoscientific
understanding of such evolutionary processes is advanced and it is therefore possible to place
defensible bounds on many of these effects, without simply resorting to a very conservative approach
in assessments.
Regarding external disruptive events and processes for crystalline rock, the workshop concluded
that there is, in general, good confidence in the understanding of their magnitudes, causes,
characteristics and frequencies. There is less confidence in predicting where and when such a
perturbation will occur and in understanding the volume of rock affected by a perturbation.
Nevertheless, the extent to which a repository is affected by a perturbation can often be confidently
addressed by using bounding and/or pessimistic approaches. These can be supported by the results of
natural analogue studies (although relevant site-specific observations are viewed as providing stronger
evidence than those from a generic natural analogue study).
Any assessment of the long-term safety of a repository in crystalline rock will have to take into
account uncertainties relating to geosphere stability. There are good examples of safety cases for a
repository in crystalline rock where comprehensive accounts of geosphere stability issues are provided
and where compliance with regulatory criteria is indicated or expected. The results of assessments of
geosphere stability also provide feedback for repository siting and engineering with regard to factors
such as its location, depth (e.g. providing transport paths with sufficient retardation, maintaining
reducing conditions), layout (avoiding deformation zones and preferred tunnel orientations to
minimise spalling) and design of the engineered barrier system (such as canister thickness and strength
to ensure resilience to mechanical loads). This feedback is an important mechanism for addressing
geosphere stability issues in a waste management programme.
Participants agreed that the workshop had been a considerable success in helping to build
confidence in the stability of crystalline rocks, which could act as hosts for a geological disposal
facility for long-lived wastes. These proceedings synthesise the main outcomes of that workshop and
present a compilation of the related supporting papers.
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Acknowledgements
On behalf of all the participants, the NEA wishes to express its gratitude to the UK Nuclear
Decommissioning Authority (NDA) for serving as host of the workshop and for its attendant support
in logistical arrangements and financing. The NEA also acknowledges the contributions of the
programme committee for the workshop:
• Alan Hooper, NDA, United Kingdom, Chair
• Johan Andersson, JA Streamflow, Sweden (representing Posiva)
• Betsy Forinash, NEA Secretariat
• Raymond Munier, SKB, Sweden
• Koji Umeda, JAEA, Japan
Finally, we extend our thanks to those who provided oral or poster presentations and to all
participants for their enthusiasm and active engagement at the workshop.
These proceedings were prepared under contract by Dr. Tim McEwen.
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TABLE OF CONTENTS
Executive Summary .............................................................................................................................. 3
Introduction .......................................................................................................................................... 9
Scope and Objectives of the Workshop .............................................................................................. 13
Synthesis of the Workshop ................................................................................................................. 15
Poster Session ..................................................................................................................................... 31
Conclusions ........................................................................................................................................ 33
Session I
General Framework: Crystalline Rocks as Host Formations
Safety Functions of Crystalline Rock Formations in Deep Geological Disposal and Their Handling
in a Safety Case – SKB’s SR-CAN Example ..................................................................................... 45
Regulatory Expectations Concerning the Confidence in Geosphere Stability and its Handling in an
Environmental Safety Case ................................................................................................................. 57
Session II
Examples of Key Processes Affecting the Geosphere for Crystalline Rock
Likelihood of Tectonic Activity Affecting the Geological Stability of a Repository in Japan:
Development of NUMO’s ITM Methodology .................................................................................... 67
Climate Change and its Potential Impact on Mechanical, Hydraulic and Chemical Conditions ........ 77
Uplift and Erosion: Potential Impact on the Geological Environment ............................................... 91
Predictability of the Evolution of Hydrogeological and Hydrogeochemical Systems; Geological
Disposal of Nuclear Waste in Crystalline Rocks ................................................................................. 99
Geological and Rock Mechanics Aspects of the Long-term Evolution of a Crystalline Rock Site... 109
Session III
Arguments to Support Confidence in the Stability of Crystalline Rocks
as Potential Host Formations
Lithological History and Ductile Deformation: The Lessons for Long-term Stability of Large-scales
Structures in the Olkiluoto ................................................................................................................ 123
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Stability and Predictability in Younger Crystalline Rock System: Japanese Islands Case ............. 133
Developing Confidence in Stability and Predictability of a Repository in Unsaturated Tuffs ......... 147
Session IV
Response and Resilience of Crystalline Rock to Natural Perturbations
and Geosphere Evolution (buffering)
Fracture Reactivation in Response to Seismic Events ...................................................................... 157
Buffering against Intrusion of Groundwater of Undesirable Composition ....................................... 173
Understanding shield Groundwater Flow Domain Evolution during the Quaternary Period
to Build Confidence in Repository Long-term Safety ...................................................................... 185
Hydraulic and Hydrochemical Response to Seismic Events ............................................................ 197
Poster Session
Discipline-integrated Modelling Approach for Describing the Olkiluoto Crystalline Site
in Finland .......................................................................................................................................... 207
Study on Characterisation of Quaternary Tectonic Movement by Uplift Estimation using Fluvial
Terraces ............................................................................................................................................. 217
Examination of Earthquake Ground Motion in the Deep Underground Environment of Japan ........ 227
Frequency of Fault Occurrence at Shallow Depths during Plio-pleistocene and Estimation
of the Incident of New Faults ........................................................................................................... 235
The Role of the Geoshpere in Posiva’s Safety Case ......................................................................... 241
Understanding the Characteristics of Long-term Spatio-temporal Variation in Volcanism and
the Continuity of the Related Phenomena for Estimating Regions of New Volcano Development . 247
Numerical Assessment of the Influence of Topographic and Climatic Perturbations
on Groundwater Flow Conditions .................................................................................................... 257
Impacts of Natural Events and Processes on Groundwater Flow Conditions: A Case Study in the
Horonobe Area, Hokkaido, Northern Japan ..................................................................................... 269
Groundwater Flow Prediction Method in Consideration of Long-term Topographic Changes
of Uplift and Erosion ........................................................................................................................ 277
An Integrated Approach for Detecting Latent Magmatic Activity beneath Non-volcanic Regions:
An Example form the Crystalline Iide Mountains, Northeast Japan ................................................ 289
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