Nagra's approach to post-closure criticality safety case development within the high-level waste repository programme roadmap

Performing criticality safety assessments for a high-level waste (HLW) deep geological disposal concept is a regulatory requirement in Switzerland and many other countries. A unique aspect compared to other areas of the fuel cycle are the long timescales of several hundred thousand years after repository closure, over which the safety assessment is to be performed. Nagra, the Swiss National Cooperative for the Disposal of Radioactive Waste, was mandated by the Swiss producers of radioactive waste to develop and implement safe and sustainable long-term solutions for the final disposal of all radioactive waste arising in Switzerland in a timely manner and at reasonable cost. To manage the final disposal programme and to ensure that all internal and external requirements are met on schedule, the Nagra Disposal Programme Roadmap was developed, covering the iterative repository design and safety assessment process and the legally binding repository licensing process. It sets out the overarching framework within which programme activities can be mapped. Regarding criticality safety, the long timespans under consideration are an aspect unique to geological repositories. Challenges regarding the development of a criticality safety case arise from this and additional aspects. To address these, and ensure the timely demonstration of the post-closure criticality safety in line with the milestones set out in the Nagra Roadmap, a specific Post-Closure Criticality Safety Roadmap with a focus on post-closure criticality is under development. The main steps taken so far as well as the identified major workstreams are outlined in this paper. (author)