2019

Tyrväinen, Tero; Karanta, Ilkka

VTT has developed a probabilistic risk analysis (PRA) model of a generic boiling water reactor nuclear power plant for PRA levels 1 and 2. The model can be used in research, training, education and demonstration. In this report, some refinements to the level 2 (severe accidents) part of the model are described. Particularly, dynamic modelling of timings of depressurization, emergency feedwater system recovery, emergency core cooling system recovery and lower drywell flooding is implemented. Different techniques to model emergency core cooling system recovery time and its effects are presented in a simplified case study. The techniques are evaluated concerning uncertainty analysis. A two-phase method of uncertainty analysis is presented. The purpose is to separate the treatment of aleatoric and epistemic uncertainties, to get rid of certain issues and inconsistencies in more traditional one-phase uncertainty analysis. The method is demonstrated by the emergency core cooling system case study. A drawback of the two-phase uncertainty analysis is that it is computationally very demanding. Therefore, it is proposed that first limited dynamic models would be constructed and analysed by the two-phase procedure; this would give input to a simplified full-scope model, allowing one-phase uncertainty analysis.