Evaluation of level 2 probabilistic risk assessment release calculations for Olkiluoto units 1 and 2 Using MELCOR : Time-dependent Cs-137 release calculations during severe accident progression

Abstract

This thesis evaluates the Level 2 PRA source term modelling methodology for selected severe accident sequences at Olkiluoto units 1 and 2 by comparing it with corresponding MELCOR analyses. Because MELCOR simulations are case-specific and largely deterministic, whereas Level 2 PRA source term models are more abstract and uncertainty-based, direct comparison requires careful alignment of the accident scenarios. A stepwise time-dependent source term model was implemented in Python to reproduce sequence-specific Level 2 PRA release calculations and to compare releases from the reactor pressure vessel and primary circuit, from containment, and to the environment. The analysis focused on caesium-137 and covered five representative severe accident cases. The results support conditional, sequence-specific agreement under carefully matched assumptions, but they do not constitute a general validation of the full Level 2 PRA release calculation chain. The closest agreement was generally obtained for the 24h release from the reactor pressure vessel and primary circuit, while larger discrepancies were observed for releases from containment and especially to the environment. The main differences were associated with non-equivalent boundary conditions, such as assumptions related to leakage paths, venting, and recovery actions. The study shows that sequence-specific comparison between MELCOR and Level 2 PRA is meaningful only when the accident progression assumptions are carefully constrained and interpreted transparently.