CFD Estimation of the heat transfer due to the natural convection in a CEA extension shaft guide tube

SMART is an integral reactor and its pressurizer is at the top of the reactor vessel. The temperature of the water in the pressurizer is about 100 deg. C, which is lower than the temperature of the reactor coolant system. To reduce the heat transfer from the reactor coolant and to sustain the low temperature of the pressurizer water a wet thermal insulator covers most of the pressurizer surface. However, the bottom of the pressurizer has holes due to the CEA extension shaft guide tubes that penetrate from the top of the reactor vessel to the bottom of the pressurizer Through these holes, the circulating reactor coolant can flow into the CEA extension shaft guide tubes. This phenomenon causes heat transfer through the walls of the guide tubes. The flow inside of guide tubes is driven by natural convection due to the high temperature difference between the pressurizer water and the reactor coolant, and by the presence of the holes. The amount of heat transfer through the surface of the guide tubes is required in the thermal design of the pressurizer cooler This estimation is conducted by CFD using the commercial software, FLUENT to solve the flow and temperature fields a simplified geometry of a CEA extension shaft guide tube is modeled. To provide a boundary condition, a reactor coolant passage is added to the bottom of the guide tube. Two bottom flow conditions at the bottom of the guide tube are chosen: (1) the cross flow and 2) the axial flow to the shaft. An unsteady simulation is conducted to estimate the heat transfer The simulation results show that the amount of heat transfer is so high that the opening at the bottom of the guide tube should be reduced and that the bottom flow conditions show d effects on the heat transfer rate. The opening can be reduced by a thick orifice-like structure. Other simulations are conducted with reduced opening geometries. These simulations show that the heat transfer rate can be dramatically diminished by reducing the opening at the bottom of the guide tube. The amount of the reduction in heat transfer by the reduced opening depends on the bottom flow conditions: The axial flow case shows a higher reduction in the heat transfer rate than the cross flow case. (authors)