The thermally-induced steam generator tube rupture (TI-SGTR) accident is 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 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 minutes indicated 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.
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.
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