Thermal-fluid-structure analysis of fast pressure relief valve under severe nuclear accident

Abstract

Fast pressure relief valve (FPRV) equipped at the pressurizer of nuclear power system is used to release steam with high pressure and high temperature rapidly from the primary circuit loop and to prevent high temperature core melting in case of severe accident condition. When releasing the steam, the time-varying temperature, mass flow and interior pressure of the inlet will affect the reliability of FPRV. In this study, the thermal analysis and the static structural analysis of FPRV under severe transient thermal shock are conducted by a 3D thermal-fluid-structure coupling numerical simulation considering the transient coupled flow, heat transfer and mechanical load. In the analysis, one-way coupling simulation is conducted. Firstly, the fluid flow and heat transfer in the valve and valve body are simulated with time. Then the instantaneous temperature fields of FPRV structure obtained from the CFD analysis at different critical moments, at which the valve is bearing high temperature, high mass flow or high interior pressure, are imported into static structural analysis as thermal load for stress analysis. Through comparative analysis of stress under mechanical load, thermal load and their combined effect, it is found that the thermal stress is the primary component of the coupled stress of FPRV and the thermal loads rather than the pressure loads are responsible for accidents related to stress overload in nuclear power plants. Therefore, the coupled stress including thermal stress should be less than the allowable stress intensity for the safety evaluation of the FPRV in a nuclear power plant.