An exploratory work on the use of a Lagrangian-Eulerian model for simulating heat transfer of subchannels under reflooding conditions

Abstract

In case of a Loss-of-Coolant Accident (LOCA) in a Light Water Reactor (LWR), the reactor core becomes partially uncovered and the quick degradation of heat removal—from the nuclear fuel elements—constitutes a critical factor in the integrity of the reactor core. As time elapses from the beginning of the accident, the temperature of the fuel clad continues to rise until the increased cooling produced by the injection of water, reduces the clad temperature and propitiates the rewet of the rods. If the surface temperature of the fuel elements exceeds the activation temperature of the Zr-O2 reaction, the core is damaged (partial or full meltdown). Hence, the need of having analytical tools to accurately predict the cladding and steam temperature under a LOCA scenario, is paramount to design palliative measures to prevent a possible core damage. The multi-scale nature of the physics behind reflooding makes the modelling of this type of problem a challenging task. In a first attempt to study a reflooding problem, within a Lagrangian-Eulerian framework, we focus our attention on the cooling effect of small droplets entrained in the superheated steam. We assess the effect of drag, lift and thrust forces—acting on individual droplets—on the steam temperature and liquid fraction distribution. Different correlations for the lift force are evaluated and uncertainties associated to the drag and lift coefficients of evaporating droplets are discussed.