CFD Analysis and Structural Safety Assessment of a Bypass Mitigation Device Used During a Ti-SGTR Accidental Release From a MSSV

Abstract

In Nuclear power plants, Main steam safety valve (MSSV) is a barrier to prevent overpressure of steam flow by opening the secondary cycle to the atmosphere. Since MSSVs operate at condition of high temperature and pressure, they have possibility for stuck-open failure. If this accident occurs, large amount of steam or gases release through failed MSSV. It may lead Thermally-induced Steam generator tube rupture (TI-SGTR) due to sudden high gradient of temperature and pressure. With loss of electrical power, TI-SGTR occurs, Core will start to melt in 2–3hours after loss of electrical power. When TI-SGTR occurs with core melt, Leakage of radioactive material occurs through MSSV to environment. Though the probability of an accident is very low, the release of radioactive material can lead large cancer risk to the public. Therefore, many studies to mitigate the radioactive materials are in progress such as diversion to containment building or capturing with external mitigation system. In this study, we are focusing on this capturing device. The objective of this study is to analyze integrity of mitigation device using fluid behavior from MSSV to capturing pipe. Hydraulic conditions at safety valve inlet were used from previous researches. Using commercial simulation software, computational fluid dynamics (CFD) analysis was performed for distribution of fluid temperature, pressure, velocity in MSSV and pipes. For structural safety assessment, 1-way Fluid-Structure interaction (FSI) method was used. CFD result was applied for load on structure surfaces to simulate transient structural analysis of mitigation device. As a result, stresses, strains of capturing pipe were calculated and integrity was discussed.