Multi-dimensional numerical study of core debris bed reflooding under severe accident conditions

N. Trégourès, F. Fichot, F. Duval, G. Repetto (1), M. Quintard (2).
The 10th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-10) Seoul, Korea, October 5-9, 2003

A multi-dimensional three-temperature model for two-phase flow through debris beds is presented. It is implemented in the ICARE/CATHARE code which has been developed by the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) for the study of severe accident scenarios in PWRs. The model is based on generalized Darcy momentum equations for each fluid phase and three energy equations. The momentum equations are closed by empirical relations. The fundamental difference with many other models is that the energy balance equations are obtained by volume averaging the local conservation equations. Such a method allows the consideration of local non-equilibrium among the three phases and a theoretical expression of the closure laws. Heat transfer coefficients are calculated based on a simplified “flow regime map” which describes the transition from a liquid “wetting” phase at relatively low solid temperature to the situation where the steam is the “wetting” phase at high solid temperature. Comparisons between numerical solutions of the model and one-dimensional quenching experiments under bottom or top flood conditions are given with encouraging results. Directions for model improvements have been identified. Finally, a two-dimensional simulation of a debris bed in a reactor vessel under top flood condition is presented, clearly demonstrating the benefits of using a multi-dimensional three-temperature model.

(1) : IRSN
(2) : IMF Toulouse