Evaporation of materials from the molten salt reactor fuel under elevated temperatures

Abstract

The release of fission product and salt compounds from a molten salt reactor fuel under accident conditions was investigated with coupled computer simulations. The thermodynamic modeling of the salt and fission product mixture was performed in The Gibbs Energy Minimization Software GEMS and the obtained compound vapor pressures were exchanged with the severe accident code MELCOR, where the evaporation from a salt surface located at the bottom of a confinement building was simulated. The fuel salt considered in the simulations was LiF-ThF4-UF4 with fission products Cs and I. The composition of the fuel salt material was obtained from an equilibrium fuel cycle simulation of the salt using the EQL0D routine coupled to the Serpent 2 code. The results were compared to simulations using pure compound vapor pressures in the evaporation simulations. It was observed that by modeling the salt mixing the release of fission products and salt materials was reduced when compared to the pure compound simulations. The mixing effects in the salt, when compared to the pure compound simulation also affected evaporation temperatures and therefore the timing of the release of compounds. In an additional simulation in which the depressurization of the confinement was considered, the total evaporated mass of compounds increased due to increased mass transfer at the salt surface. The simulation process described in this paper can be used for a more comprehensive accident analysis of molten salt reactors once the detailed description of the reactor confinement and accident sequences are available and more fission product elements have been added to the analysis.