Molecular approach to sodium vapour condensation, rewetting and vaporization in LMFBR bundle under hypothetical accident conditions (Q2706053)

This interesting paper is devoted to an application of the Monte Carlo method. In the frame of safety analysis of Liquid Metal Fast Breeder Reactors (LMFBRs) under hypothetical Loss of Coolant Accident (LOCA) conditions, two phase flow of sodium is simulated in a reactor core. The basic laws of the kinetic theory are applied to the determination of the evaporation and condensation rates of vapour in a reactor bundle. This microscopic approach is based upon a Monte Carlo simulation of the molecular trajectories, collision rates among molecules and of the molecules with the material diffusely reflecting surfaces. Rewetting of dried out regions of the cladding surfaces is simulated with a macroscopic dynamic film model. Evaporation of sodium molecules from the film yields a source of molecules which re-enter into the kinetic Monte Carlo calculation. The coupling of the micro- and macroscopic models has been applied to the numerical simulation of an out-of pile sodium boiling experiment. The model and algorithms proposed in this paper for computing dryout and rewetting sequences eliminate an important deficiency of computer programs based on a macroscopic description of the phenomena.