EDF Experience in Integrated Deterministic Probabilistic Safety Analysis for Risk Assessment

The benefits of dynamic (integrated) approaches for risk assessment have been discussed in the literature for a long time. Until recently, there were only a few applications of such approaches to realistically sized problems. We consider an application of an integrated approach grounded on dynamic event trees (DETs) for the analysis of two plant damage states. We use a static probabilistic safety assessment (PSA) Level 2 model of a commercial nuclear power plant as a basis. The main difference between the integrated approach and the static approach is that the branching of the sequences in the integrated approach is decided on the fly as the accident evolves by the code (MAAP4) that is used to analyze the progression of the severe accident rather than off-line by an expert. The results of the DET calculations show that the static PSA model gives a fairly conservative estimate of the release frequencies due to severe accident phenomena. At the same time, it is observed that some important dependencies may have been overlooked in the static PSA.