Development of Safety Scenarios
Nagra (2024): Development of Safety Scenarios. Nagra Technical Report NTB 24-21.
pdf NTB 24-21 Development of Safety Scenarios(8.88 MB)
In support of post-closure safety assessment and for site comparison, this report documents the development of safety scenarios for assessing the radiological consequences of deep geological repositories for low- and intermediate-level waste and high-level radioactive waste. This process entails the identification and description of a set of safety scenarios that capture uncertainty in the initial state of the repository system and the different ways in which it could evolve over time. In doing so, the potential paths of repository evolution are conceptualised, bounded and formulated into calculation cases that facilitate the quantitative analysis of radiological consequences – the next step in the safety assessment workflow. Thus, the scenarios serve as inputs to both safety analysis in the context of site comparison and the analysis of radiological consequences in the context of post-closure safety assessment of the repository at the proposed site.
Safety scenarios are developed based on information from the assessment basis, which compiles a sound scientific understanding of the waste inventory, repository barriers and the possible long-term geological and climatic evolution of the siting region, to provide a well-developed understanding of the phenomenological evolution of the repository. This is combined with the results of performance assessment to deliver an integrated understanding of the expected initial state and evolution of the repository, its other possible lines of evolution, and the associated uncertainties. Thus, the conditions under which deviations in barrier performance could occur are identified.
Consistent with the national regulatory requirements and international guidelines, the following safety scenarios have been defined:
- a reference safety scenario, which describes the expected initial state and evolution of the repository,
- a set of alternative safety scenarios, derived from a consideration of potentially detrimental phenomena and associated uncertainties that could result in degraded performance of the repository system,
- a set of hypothetical “what-if?” cases that represent extreme or impossible situations in order to test system robustness,
- a set of future human action safety scenarios, that concern the impact of potential actions taken by human society on the performance and safety of the repository.
Each of these safety scenarios has one or more variants that share the same general description of features, events and processes occurring over time, but differ in one or more key factors that significantly affect the radiological consequences of that particular scenario. Subsequently, for each scenario or variant, one or more calculation cases have been defined that incorporate assumptions for factors specific to each case.
This report provides a detailed description of the safety scenario development methodology and documents the application of that methodology through the definition of the safety scenarios, variants and calculation cases. The justification for allocation of each safety scenario to one of the four above-mentioned categories is provided in this report. Moreover, discussion and explanation of the key factors and assumptions relevant to each safety scenario, variant and calculation case are presented, as well as a discussion of completeness and comprehensiveness.