Numerical Simulation of Reflooding after a Nuclear Loss-of-Coolant Accident

Abstract

A loss-of-coolant accident (LOCA) in a Pressurized Water Reactor (PWR) can occur because of a breach in the primary circuit, leading to a sudden pressure drop and evaporation of water in the core. Consequently, fuel rods start to heat up and deform, and to prevent bursting, emergency core cooling systems (ECCS) are necessary to reflood the core. However, because of the fuel rods temperature, which can reach 1200°C, the injected liquid water starts to boil immediately. As a result, the core is mainly cooled by a mix of water steam and dispersed liquid droplets, which is commonly referred as dispersed film flow boiling (DFFB). Heat transfer in DFFB is very complex, as it involves many different physical phenomena: convection, radiation, droplet rebound in Leidenfrost regime, turbulence, and mass transfer between phases. In this framework, core reflooding has been simulated with a CFD tool called neptune_cfd, and the results are compared to those of COLIBRI, an experiment that models a sub-channel between fuel rods. A special emphasis is placed on finding a fitting correlation for the heat flux resulting from drop impact on a hot surface, which can represent up to half of the total flux in some configurations. The heat flux simulated with neptune_cfd is closer to experimental results than previous simulations, however the cooling is still underestimated. These tendencies are favorable from a safety standpoint, because it means that the CFD tool is conservative and can be used for the design of safety systems, however the margins could still be reduced by improving the physical modeling, and particularly the radiative transfer.

Keywords: LOCA, Reflooding, DFFB, CFD, Multiphase flow