The dependence of equilibrium partition coefficient of cesium and iodine between sodium pool and the inert cover gas on the concentration in the pool

► We measured equilibrium partition coefficients of cesium and iodine between liquid sodium pool and the inert cover gas. ► The obtained empirical equations were consistent with Castleman's theoretical equations. ► The effect of cesium concentration upon the partition coefficients was consistent with the theoretical values. ► That of iodine concentration was incompatible with the theoretical consideration due to the formation of Na 2I 2 in the cover gas. Equilibrium partition coefficients were experimentally measured for volatile fission products of cesium and iodine between liquid sodium pool and the inert cover gas. In the experiments, the “transpiration method” was utilized in which the saturation vapor of sodium with cesium and iodine vapor in an isothermal evaporation pot was transported by inert carrier gas and trapped by filters outside the pot. The objectives of the experiments are to: (a) obtain the equilibrium partition coefficients of cesium and iodine at high temperature between 600 and 850 °C and, (b) study the dependence of the partition coefficients upon the concentration in the sodium pool. From the results of previous work and this study, the following empirical equations between the partition coefficients of cesium and iodine and the sodium pool temperature could be obtained: log K d ( Cs ) = 2173 T − 1.0487 ( from 450 to 850 ° C ) , log K d ( I ) = − 215 T − 0.271 ( from 450 to 850 ° C ) . These equations are consistent with Castleman's theoretical equations. The partition coefficients of cesium measured at five different points of mole concentration in the pool were almost consistent with the theoretical values and decreased with the increase in the concentration. On the other hand, the measured partition coefficients of iodine increased with the increase in the concentration in the pool and this tendency was incompatible with the theoretical consideration. This discrepancy could be explained considering the formation of dimmer of NaI (Na 2I 2) in the cover gas.