CFD simulation of thermal hydraulic phenomena in enclosed cavity of nuclear power plants

The “dead-leg” in nuclear power plants (NPP) are pipes, which are connected with the primary loop and isolated by two check valves setting at both ends. It is a typical enclosed cavity. As the temperature and pressure in primary side is higher, the coolant in tube is heated by the valve and becoming two phase state, resulting in the corrosion of valves. In present work, the CFD method was employed to study the single phase and two phase thermal hydraulic phenomena in enclosed cavity. The buoyancy was calculated by solving compressible governing equations, and the mixture multiphase model and improved compressible phase change model were used to calculate the boiling process. The models are validated against with the experiment data and the simulation results show great agreement. The coolant natural convection occurs and the significant thermal stratification exists in the vertical direction. The void fraction increases at first and tends to a stable value finally. The vapor accumulates in the upper part of the pipe and the corrosion in the region is more serious. The trend of pressure increasing is consistent with void fraction and self-pressurization phenomenon is caused by the compression of vapor. Increasing the initial pressure could reduce boiling process greatly and it is an effective means to alleviate “dead-leg” phenomenon. This work is meaningful for the deep understanding of thermal hydraulic phenomena in enclosed cavity and provides valuable guidance for engineering.