Finnish experiments on level 3 PRA

Level 3 probabilistic risk analysis (PRA) analyses the consequences that radioactive release from a nuclear power plant can have on population and environment. Two level 3 case studies are presented in the paper. The first study experimented with event tree modelling on level 3 PRA using Fukushima accident as the case. This study alternated history so that it was assumed that there was no tsunami, i.e. people near Fukushima had not died or been evacuated. Weather was treated probabilistically instead of using the actual weather from the time of the accident. Atmospheric dispersion and dose calculations were performed using ARANO software, and the calculated population doses were incorporated into the event tree model. The event tree was constructed using FinPSA level 2 software, which enables uncertainty analysis and parametric modelling. The second case study examined the integration of PRA levels 2 and 3 using the results from a level 2 dynamic containment event tree model as a basis. It is not clear what numbers from level 2 should be used in level 3 because uncertainty distributions in level 2 results are wide. Therefore, a moderately large number of source term scenarios were analysed.