Taking Accurate Account of Radiative Heat Transfer for the Simulation of Core Reflooding

The reflooding of the reactor core during a loss-of-coolant accident (LOCA) in a pressurized water reactor is a rather complex conjugate heat transfer situation. In the mist flow regime downward from the quench front, the rod wall can reach temperatures up to 1400 K, and radiative heat transfer can play a significant role. The present study concerns the accurate numerical computation of radiative heat transfer throughout a subchannel with LOCA representative flow conditions resolved at a computational fluid dynamics–scale spatial discretization thus allowing the large gradients of two-phase-flow properties to be determined. The accuracy of several methods to solve the radiative transfer equations has been compared both in canonical test cases and in low-pressure LOCA conditions. The role of radiative transfer is obvious in all variables including those related to the dynamics of the flow. Analysis of the gap between the present estimation and a standard correlation has been performed. It leads to the conclusion that radiative transfer can be taken into account accurately by correlation as soon as well-defined radiative properties are considered. The transfer is very sensitive to droplet size and concentration and can be as large as the convective heat transfer.