Studies On Behavior And Separation Of Fission Product Cesium In Pyroprocessing Of MSR Fuel

On-line fission products(FPs) separation is one of the main research contents for Thorium Molten Salt Reactor nuclear energy system(TMSR). Cesium, an important FP, which has similar physicochemical properties with the carrier salt 7Li F-Be F2, is stable in the liquid fuel salt in the form of Cs F. The removal of Cs F from fuel salt regularly has benefit to the neutron economy of the reactor.This study aims to investigate the evaporation behavior of Cs F and the factors influencing Cs F separation efficiency under different experimental conditions by using FLi Na K(Li F-Na F-KF molar ratio 46.5%-11.5%-42%) molten salt as simulated carrier salt. Three different evaporation methods,namely bubbling, vacuum distillation and reductive evaporation by metal lithium, were employed in this work. Using selfdeveloped bubbling device, the separation rate of Cs F in 50 g FLi Na K-Cs F(1wt%) increased significantly from 3% to 13% at 1.01×105Pa and 850℃ with argon bubbling flow of 250cc/min in 150 min, and the loss of carrier salt was also significant. Considering the low boiling point of metal cesium, reductive evaporation by metal lithium was investigated to separate Cs F from FLi Na K. The amount of evaporated Cs F increased with the increase of lithium at 700 ℃. The reaction Li(l)+Cs F(l)® Li F(l)+Cs(g) occurred, and the separation rate was 91% at a molar ratio of lithium to Cs F of 120. Reductive evaporation can effectively improve the separation efficiency of Cs. The results from the experiments of vacuum distillation at 5 Pa revealed a linear correlation between evaporated mass of Cs F and evaporation temperature. At 780 oC, the Cs F concentration in molten salt decreased from 1% to 0.14% with a separation rate of 86% despite of the loss of 9.5% of the carrier salt. The addition of Th F4 inhibited the evaporation of Cs F and carrier salt in the vacuum distillation process. The amount of evaporated Cs F and carrier salt decreased with the increase of Th F4 added into the molten salt.Furthermore, the behavior and trend of Cs and other main FPs in fluoride volatility process and vacuum distillation process were discussed. The results from the experiments of simulated fuel salt in fluoride volatility-vacuum distillation process showed that Cs F was remained in the salt carrier with decontamination factor 1.Fundamental data are provided to understand the evaporation behavior of Cs F in fluoride molten salt, which could benefit to the establishment of efficient separation technique for Cs F removal from irradiated liquid fuel.