FILM BOILING IN A POROUS PARTICLE BED

摘要

In order to predict the safety margin of nuclear reactors under severe accident conditions, the possibility to reflood highly overheated particle beds must be understood. Since particles may reach very high temperatures when reflooding occurs, the film boiling regime is of major interest to understand the propagation of water. The present paper proposes a consistent modelling of the non isothermal two-phase flow of water in a porous medium initially at high temperature. The paper presents the extension of a previously developed six-equations thermalhydraulic model to the film boiling regime. This model was already derived and assessed in previous papers, in the case of a limited over-heating of the particles. It is assumed that, when the average particle temperature is above the Leidenfrost temperature, the flow structure corresponds to separate steam and water channels, which leads to large local variations of the particle temperature. Under those conditions, formulations of absolute and relative permeabilities and heat exchange coefficients are derived. Onde-dimensional calculations are compared with experimental data. The results show a fairly good agreement of the produced steam flow rate and the progression of the quench front. The model is able to describe two different behaviours: first, the cases where the quench front progresses steadily and second, the cases where the quenching process is unsteady.