Uncertainty evaluation of source terms under severe accident conditions at LWRs

Ishikawa, Jun ; Muramatsu, Ken; Watanabe, Norio

The probabilistic safety assessment(PSA) results can be used for decision making in various areas including risk informed regulation, accident management, off-site emergency planning, and safety goal. Since there exist uncertainties in PSA results, uncertainty evaluation is one of the most important issues in PSA. This paper describes an approach to evaluation of uncertainties in source terms. We proposed a procedure of uncertainty evaluation for source terms by the use of severe accident analysis code THALES2 and applied this procedure to the evaluation of source terms for BWR-5/Mark-II plant. Source terms analysis was performed with THALES2 for containment overpressure failure scenarios involving core damage. From this analysis, uncertainty information of source terms, such as the timing of fission products (FPs) release to the environment and released fraction of FPs. As well, the dominant parameters for individual scenarios were identified: containment failure pressure, in-vessel and/or ex-vessel FP release rate, deposition of aerosols in the containment and reactor building.