The development of scenarios for quantitative or qualitative analysis is a key element of the assessment of the safety of geological disposal systems. As an outcome of an international workshop attended by European and the Japanese implementers, a number of features common to current methodologies could be identified, as well as trends in their evolution over time. In the late nineties, scenario development was often described as a bottom-up process, whereby scenarios were said to be developed in essence from FEP databases. Nowadays, it is recognised that, in practice, the approaches actually adopted are better described as top-down or “hybrid”, taking as their starting point an integrated (top-down) understanding of the system under consideration including uncertainties in initial state, sometimes assisted by the development of “storyboards”. A bottom-up element remains (hence the term “hybrid”) to the extent that FEP databases or FEP catalogues (including interactions) are still used, but the focus is generally on completeness checking, which occurs parallel to the main assessment process. Recent advances focus on the consistent treatment of uncertainties throughout the safety assessment and on the integration of operational safety and long term safety.

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