Fundamental study on the vaporization of cesium and iodine dissolved in LiF-NaF-KF molten salt

Abstract

As a basic study related to a severe accident of Molten Salt Reactor (MSR), the equilibrium vapor pressures over LiF-NaF-KF eutectic salt (46.5–11.5–42.0 mol%, FLiNaK) containing Cs and I as volatile Fission Product(FP) were measured by a transpiration method under pure Ar gas flow at the temperature ranging from 900 to 1100 K. As the result, the vapor pressures of Cs and I in molten FLiNaK were evaluated to be higher than those of other elements in the salt, and the vapor pressure of the solvent salt elements such as Li and K also increased with addition of CsI and CsF. The activity coefficients of CsI and CsF were derived from the partial vapor pressure of CsI and CsF calculated by assuming the gas phase equilibrium. Activity coefficients of CsI, 6 to 13 in this temperature range, showed strongly positive deviation from ideal solution while those of CsF were around unity. This peculiar activity coefficients suggest instability of CsI in the FLiNaK salt as suggested for LiF-ThF4-CsI salt by literature. The vaporization behaviors measured for the FLiNaK with different Cs/I ratio indicated strong interaction between Cs and I in FLiNaK salt.