The safety case for a deep geological repository in Switzerland: system evolution and performance assessment

Abstract

Abstract

The post-closure safety case for a deep geological repository requires a transparent and traceable demonstration that the repository system fulfils its safety functions and remains robust against uncertainties over relevant timescales. This paper presents the performance assessment workflow implemented in the context of the safety case for a general license application of a deep geological repository for both high-level waste (HLW) and low- and intermediate-level waste (L/ILW) at the Haberstal site in Switzerland. The safety concept relies on a staggered system of passive and redundant barriers, the so-called multibarrier system, each performing specified safety functions in the general post-closure safety architecture. The performance assessment addresses the performance of each component of the multibarrier system, as well as the overall performance of the total system, building upon claims, arguments, and evidence in a transparent and traceable manner. Rigorous handling of uncertainty is an indispensable element in this assessment workflow. The uncertainty framework aims to bracket the confidence bounds on the expected system response evaluated through the simplified model representations of the repository system and the internal or external perturbations acting on it. The possible paths of repository performance are formulated into an expected performance and safety-relevant deviations thereof, assessed in a probabilistic framework with an indicator-based approach. Their safety relevance and likelihood of occurrence form the main input for the development of safety scenarios, which is the next step in the safety assessment workflow. The analysis demonstrates a robust performance of the geological repository through a redundant barrier system that fulfils the intended safety functions under both expected repository performance and deviations from it. The presented methodology provides a transparent and systematic framework for evaluating performance in support of the safety case. The assessment integrates quantitative indicator-based analyses, management of uncertainty, and evidencing of argumentation, contributing to confidence building in the long-term safety of the Swiss deep geological repository.