Identification of the containment heating mechanism and temperature distribution by high-temperature gas leakage under severe accident conditions

• Analysis of the leakage of high-temperature gases from the pressure vessel boundary under severe accident conditions. • CFD analysis based on the accident progression analysis. • Analysis of the detailed thermal-hydraulics and structural heat transfer for the temperature distribution of the containment. • Identification of the containment heating mechanism. This study evaluates the thermal-hydraulics in the containment of boiling water reactors caused by superheated gas leakage under severe accident conditions. The research involved characterizing the heat transfer paths, which included identifying superheated gas leakage positions and heat release from the reactor pressure vessel boundaries, classifying the containment volumes, and categorizing potential failure sections of the containment. Based on these insights, the heat transfer paths during severe accidents were clarified, and accident scenarios considering leakage from the safety relief valve and traversing in-core probe tubes were selected as representative scenarios. A three-dimensional computational fluid dynamics (CFD) model of the Mark I containment was developed to evaluate the thermal-hydraulics of an entire drywell of the containment. Special attention was given to modeling the detailed structures of the containment top head flange, radiation from the reactor pressure vessel upper head, condensation in the suppression pool, and heat release from the containment top head to the reactor well. The CFD analyses focused on two scenarios: safety relief valve leakage and traversing in-core probe tube leakage, which can result in significant temperature distribution in the upper and lower drywell, respectively. This study identified the high temperature location on the containment boundary with higher possibility of failure by high-temperature gas leakage under severe accident conditions based on temperature distribution obtained by the present detailed three-dimensional CFD analysis.