SKI Research Project on Defence in Depth PSA – Assessing Defence in Depth Levels with PSA Methods

摘要

One of the basic requirements for nuclear safety is to maintain and to develop the defence in depth (DiD) principles. The overall aim is to prevent deviations from normal operation from occurring and, if prevention fails, to detect and limit their consequences, and to prevent any escalation to more serious conditions. In principle, the PSA can be used to investigate the application of defence in depth in that it addresses the frequency of initiating events that challenge nuclear safety (DiD Levels 1 and 2), the probability of failure of safety systems and the frequency of core damage (DiD Level 3), the probability of bypass or failure of the containment and the frequency of a large (early) release (DiD Level 4) and effectiveness of the off-site emergency response measures and the frequency of social and economic consequences (DiD Level 5). However, there is currently no direct assessment of DiD levels with PSA. An SKI research project is making an inventory of the potential and possible methods for using PSA in evaluating and ranking the system structures and components (SSCs) being part of the different DiD levels. The project identifies and describes different current evaluations of the DiD levels including by whom, when and how. The current use of PSA results and the possibility for extended use of PSA results in evaluating DiD levels is investigated. The objective is to evaluate the current DiD levels, evaluation of the impact on DiD levels due to plant changes and evaluation of the importance of plant events (LERs, etc) for the SSCs belonging to each DiD level. The project is divided into five steps 1) identification and structuring of the SSCs that belong to each DiD level and that should thus be considered for potential PSA evaluation, 2) mapping of different measures for each level, 3) identification of existing PSA results that can be used to assess the DiD level and SSCs, development of possible new PSA measures that can be used, requirements on PSA model, and need for PSA model extensions and adaptation, 4) test calculations and finally 5) a test of the credibility of the proposed methods and results as well as the interpretation of the results. The project and its preliminary findings and results are presented.