Risk effect of the hybrid-SIT system under SGTR accidents

Highlights: • Novel operation of the H-SIT is suggested under SGTR accidents. • Effectiveness of the H-SIT during SGTR accidents is estimated using TH simulations. • Repair probabilities of the PAFS during SGTR accidents are estimated. • Conditional core damage probabilities with H-SIT operation are analyzed. - Abstract: A hybrid safety injection tank (H-SIT) is designed to passively inject coolant into a reactor coolant system under any pressure condition without depressurization, enabling it to be applied in various accident types. In certain accident conditions, if long-term cooling is not achievable because the required active mitigation components are unavailable, the H-SIT can replace the function of the failed components for at least a certain period. Since the H-SIT has limited inventory, related mitigation strategies need to be carefully developed to effectively enhance accident mitigation strategies and contribute to overall plant safety. This study focuses on the risk effect of the H-SIT system under a steam generator tube rupture (SGTR) accident with safety injection system (SIS) failure, a case typically considered to cause core damage. In such cases, the use of the H-SIT provides for a greater diversity of mitigation options and allows more time to repair the failed components. In order to address plant dynamics in a realistic manner, this study considers a variety of secondary-side cooling performances with corresponding repair probabilities. Multiple-tube rupture cases are also considered. The analysis results demonstrate that the H-SIT provides preferred strategies for managing the risks of both SGTR and MSGTR events.