Distillation Behavior Of Fluoride Doped LiCl-KCl Molten Salt

With the continuous development of Generation IV reactor systems such as molten salt reactor,pyroprocessing has received more and more attention from various countries.Pyroprocessing is a kind of chemical separation during which spent fuels are processed at high temperature and under water free condition.It has many advantages including radiation resistance,compacted reprocessing units,low proliferation risk and less high level radioactive waste.The pyroprocessing program led by Shanghai Institute of Applied Physics?SINAP?aims at separating thorium by electrochemical method in molten chorine.The electrolyte after electrolysis contains rare earth fluorides and other impurity elements both of which should be removed form LiCl-KCl so that the electrolyte can be recycled and the amount of waste can be minized.Low pressure distillation is one of the candidate methods for this purpose due to the fact that it is a simple continuous process without the formation of by-products,and the energy consumption is low as well.It utilizes the difference in volatility between the carrier salt?chloride?and the fission product?fluoride?,and can effectively recover the carrier salt that is nearly free of impurities and fission products.In this thesis,the effect of rare earth and alkaline earth metal fluorides on the distillation behavior of LiCl-KCl was studied.The effects of temperature and pressure on distillation were first investigated,which led us to carry out all the following experiments at 1050 K and 130 Pa.For the influences of evaporation ratio and fluoride concentration on the decontamination factors?DF?,the results showed that the content of rare earth in the recovered salt by distillation increases with the increase of evaporation ratio,and the DF of rare earth elements in LiCl-KCl mixture decreases.It is speculated that the entrainment of salt and the polarization of rare earth concentration during distillation lead to the evaporation of rare earth fluorides.At 1050 K and 130 Pa,the DFs of Nd,Ce,La,Sr in the recovered salt are up to 10³ at an evaporation ratio of95%.However,the DFs of EuF₃ and ZrF₄ are significantly lower than those of other fluorides in LiCl-KCl,which is consistent with the high vapor pressure of ZrF₄.To further improve the DF of fission elements in the collected salt,we added oxides into the salt mixture which caused the precipitation of rare earth fluoride in the form of oxide,and the distillation behavior of the mixture was studied under low pressure condition.NdF₃ was used as the representative of rare earth fission product,and CaO,Li₂CO₃ and Li₂O were chosen as precipitants.Based on the thermodynamic analysis,the Gibbs free energies for the precipitation reactions are less than zero at 600-1400 K,which indicate that it is thermodynamically feasible for the precipitation reaction to occur.In the LiCl-KCl molten salt system containing NdF₃,it was found that the rare earth fluorides were converted into oxide precipitations with high conversion rate upon addition of CaO.The reaction of rare earth fluoride and CaO?molar ratio:1:2?was carried out under argon atmosphere at 900 K for 2 h.After the fluoride was converted into an insoluble rare earth precipitation,the reaction condition was changed to 1150 K and 16 Pa in order to carry out distillation experiments.The results show that the DFs of neodymium and europium obtained by precipitation-distillation are 1.63×10³ and 1.73×10³respectively,which are significantly higher than the DFs without CaO.Based on these results,the distillation behavior of molten LiCl-KCl containing 5%ThF₄ was carried out using the same precipitation-distillation procedure.Under the conditions of 1150 K and 16 Pa,the DFs before and after the addition of CaO were 2.2×10³ and 2.7×10³,respectively.X-ray diffraction?XRD?patterns show that there are peaks of ThO₂ and CaF₂ in the residual salt after distillation,which indicates that ThF₄ reacted with CaO at 900 K.As a result,addition of CaO can convert both rare earth and thorium into insoluble oxide precipitations,which help to improve the purity of recovered LiCl-KCl salts.The present work provides preliminary results on the effect of fluoride on the distillation behavior of LiCl-KCl molten salt.The research results show that the DFs of rare earth?except Eu?and alkaline earth in the collected salt are 10³,while that of Eu and Zr are about 10².The combination of precipitation and distillation was used to explore the effect of precipitant on the DF of rare earth.It was proved that it is feasible to improve the DF by using such a strategy which could be further eventually employed for the separation and purification of molten salt in industry scale.