Building the Safety Case for a Disposal Facility for Spent Fuel, HLW and Long-lived ILW in Switzerland
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Building the Safety Case for a Disposal Facility for Spent Fuel, HLW and Long-lived ILW in Switzerland Paul A. Smith1, Piet Zuidema, Lawrence H. Johnson, Jürg W. Schneider and Peter Gribi2 Nagra, CH5430 Wettingen, Switzerland. 1 Safety Assessment Management Ltd., 20 Manor Place, Edinburgh EH3 7DS, United Kingdom. 2 Vibro-Consult Ltd., Stahlrain 6, CH5200 Brugg, Switzerland. ABSTRACT This paper describes a generic methodology for building the safety case for a geological repository, which is currently being applied to a possible facility for spent fuel, vitrified highlevel waste and long-lived intermediate-level waste in the Opalinus Clay of Northern Switzerland. The methodology involves: 1. the identification of certain basic disposal principles, 2. the choice of a disposal system, via a flexible repository development strategy, 3. the derivation of the system concept, based on current understanding of the phenomena that characterise, and may influence, the disposal system and its evolution, 4. the derivation of a safety concept, based on reliable, well understood and effective pillars of safety, 5. the illustration of the radiological consequences of the disposal system through the definition and analysis of a wide range of assessment cases, and 6. the compilation of the arguments and analyses that constitute the safety case, as well as guidance for future stages of the repository programme. A range of measures, including audits, are used to promote completeness of the phenomena considered in the safety case, and to avoid inadvertent bias.
INTRODUCTION Nagra, the National Cooperative for the Disposal of Radioactive Waste, is responsible for research and development, geological investigations, design studies and safety assessment studies leading to the development of facilities for the disposal of radioactive waste in Switzerland. A methodology has been developed by Nagra for building the safety case for such facilities. The methodology is currently being applied to a possible repository for spent fuel (SF), vitrified high-level waste (HLW) and long-lived intermediate-level waste (ILW) in the Opalinus Clay, a Jurassic shale (claystone), in the Zürcher Weinland of Northern Switzerland. It is, however, a largely generic methodology, and may be applied to other disposal systems. JJ4.5.1
The methodology is designed to test the adequacy of the proposed site and design options in terms of long-term safety. In particular, it tests whether adequate levels of safety are to be expected based on what is known about the proposed system, and whether there are circumstances that cannot currently be ruled out in which safety might be compromised. It is also intended to provide guidance, or at least a platform for discussion, as to how the system might be improved to avoid such circumstances, or knowledge enhanced in order to clarify whether such circumstances could, indeed, arise. The resulting safety case is, as far as possible, - transparent, in that the arguments and analyses that make up the safety case, and the process by
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