Uncertainty analysis of in-vessel retention in a high power reactor during severe accident

The in-vessel retention by external reactor vessel cooling (IVR-ERVC) is one of the severe accident management strategies for mitigation or termination of the accident by holding the radioactive material in the reactor vessel. We made a lumped parameter based computer code to evaluate the thermal load on the reactor vessel, and applied the code for a probabilistic analysis for a high power (>1000 MWe) reactor assuming the APR1400 condition. The present analysis showed the focusing effect in most of cases. The heat transfer models, especially the critical heat flux correlation, showed a significant impact on the result. Consideration of the multidimensional heat conduction effect in the vessel wall can reduce the excessive conservatism in the vessel failure criterion by assuming the same heat fluxes at inside and outside of the vessel wall. The influence of the probability distribution profiles for the uncertainty parameters was also investigated.