Computational Fluid Dynamics Analysis and Structural Safety Assessment of a Mitigation Device to Minimize Consequence of a Containment Bypass Nuclear Accident

Abstract

The thermally induced steam generator tube rupture (TI-SGTR) accident is a principal contributor to mean early and latent cancer fatality among the containment bypass accidents. To mitigate the consequence of a TI-SGTR accident, use of a bypass mitigation device has been proposed. This study investigated the feasibility of using the proposed bypass mitigation device based on computational fluid dynamics (CFD) analysis and structural safety assessment using a commercial simulation software (Fluent). As TI-SGTR accident may occur if main steam safety valve (MSSV) for preventing over pressurization is stuck-open in station black out (SBO) scenario, the analysis included the modeling of the flow of dry steam from MSSV to the capturing pipe of the mitigation system. According to CFD analysis results, after passing MSSV, the inlet pressure was decreased to the atmospheric pressure. The structural safety analysis was based on evaluating the equivalent stress distribution of the capturing pipe. Under three inlet pressure conditions, the largest concentrated stress on the capturing pipe was found to be less than 10% to tensile strength of the steel. For the concrete support, the safety margins may not be sufficient for 8.7 MPa inlet pressure condition. The thermal-mechanical analysis was performed for the period of 15 min, indicating that the effect of thermal expansion is small and that the resulting strain does not pose a concern. The results of this study can also be utilized to study externally released flow through MSSV or to identify directions for supplementing or reinforcing the migration system.