Enhanced approach for CDF quantification in Seismic Probabilistic Safety Assessment of a Research Reactor

Abstract

In Probabilistic Safety Analysis (PSA) applications, it is common practice to employ approximations with the expectation or assumption of a small overestimation in the quantification of the Core Damage Frequency (CDF). The overestimation of the CDF depends on the Minimal Cut Sets (MCS) structures and event probabilities but the amount of conservatism is generally unknown. Hence, when dealing with large and non-coherent fault trees, conventional approaches to model dependencies in event trees analysis using coherent approximations are shown to be inaccurate. The limitation of the techniques in terms of accuracy of the solutions becomes apparent. For instance, the quantification methods using Rare Event (REA) and Min Cut Upper Bound (MCUB) approximations valid in internal event PSA may result in excessively conservative results in Seismic PSA. Therefore, in order to calculate the accurate top event probability from a fault tree rather than improving the direct probability calculation from cut sets, the Binary Decision Diagram (BDD) is introduced for more exact quantification. BDD development highlights the effort to reduce the conservatism caused by RAE and MCUB computations. The analysis carried out in this work, within the Seismic Probabilistic Safety Assessment (SPSA) of a nuclear Research Reactor (RR) case study using BDD framework shows that the approach is feasible and effective in evaluating the seismic risk of core damage and provide reasonable assurance that related decisions on real-time risk status can be taken robustly and with confidence.