Best-practice severe accident analysis for the OPR1000 short-term SBO sequence using MELCOR2.2 and MAAP5

Abstract

A short-term station blackout (STSBO) accident has been considered as one of risk dominant sequences for nuclear power plants (NPPs). Unless appropriate mitigation actions are not taken, a rapid damage of the reactor core and thereby a severe accident may result in. This paper presents best practice analysis results on the evolution of the severe accident sequences and fission product (FP) source terms expected during the STSBO accident of a reference pressurized water reactor. In addition, this study aims to analyze the impact of dedicated severe accident mitigation/management actions on the results of interest through the established modeling approach. As the modeling and simulation tools for the analysis, MELCOR2.2 and MAAP5.05, were used to compare the analysis results of interest such as the integrities of the reactor coolant system (RCS) pressure boundary and the reactor/containment building, the generation of flammable gases (such as hydrogen and carbon monoxide), and the environmental release of the FP cesium. As a key finding, this study demonstrated that both codes similarly predicted the environmental release of the FP cesium as well as the general trend of the evolution of the severe accident sequences and plant responses. Of particular interest was the substantial difference in predicting the generation of the flammable gases by the two codes. Relevant insights are summarized in terms of particular points of interest.