ZIT Composite Material Immobilization Of Lanthanides（Pr,Eu） And Doped WO₃ Modification Research

There are a large amount of lanthanide elements in the form of oxychloride in the high-level waste at the end of the retreatment of MOX spent fuel.After the removal of chlorine,the lanthanides exist mainly as oxide precipitates.Therefore,the oxide precipitates are immobilized and buried deep underground,which is very important for the steady development of the closed nuclear fuel cycle.To achieve the goal of stable and efficient solidification of waste oxides and geological treatment,appropriate materials need to be developed.ZIT（Zinc Titanate）composite has attracted attention due to its advantages of large loading capacity,low sintering temperature and strong stability.ZIT is made of Zn₂Ti O₄,Ca HPO₄,B₂O₃ and Si O₂ after grinding and calcining.However,there are few studies on the immobilization of ZIT composites at present,especially the studies on the immobilization of multiple Lns have not been reported.In this paper,the lanthanide elements Pr and Eu were immobilized using ZIT composite as matrix.The effect of WO₃ doping on the properties of immobilized products was studied.In this paper,unary lanthanide oxides(Pr₆O₁₁)or binary lanthanide oxides(Pr₆O₁₁ and Eu₂O₃)were immobilized respectively.The ZIT immobilized product with monazite structure was prepared by high temperature calcination,and the physical and chemical properties of the immobilized product were characterized by a series of tests.The influence of different sintering processes on the solidified body was studied.It was determined that the performance of the immobilized product could be improved by the sintering process of pre-sintering at 700℃for2 h and formal sintering at 1200℃for 4 h followed by 2 h heat treatment.The effects of loading lanthanide oxides with different mass fractions on the properties of immobilized products were studied.The maximum loading capacity of single or binary lanthanide oxides was 35 wt.%,the maximum apparent density was 4.48 g·cm^-3,and the normalized leaching rate could reach 10^-6 g·m^-2·d^-1 after 28 days of immersion.The effects of Pr₆O₁₁ and Eu₂O₃ on immobilized products were investigated,and it was confirmed that the phase structure,microstructure and properties of immobilized products were not affected by the mixing ratio when different kinds of lanthanide elements were solidified at the same time.Analysis of immobilized products at different temperatures showed that Ca HPO₄ decomposed at 600～800℃.Zn₂Ti O₄ reacted with Si O₂ to produce Zn₂Si O₄ and rutile Ti O₂ at 800～1050℃.Pr PO₄ and Eu PO₄ appear at 950～1000℃.1050～1200℃is the process of crystal growing gradually.The flatness and density of immobilized products also increase gradually during the process of temperature rise.However,only the density property of the solidified body changed with different sintering time,and it remained around 4.46～4.56 g·m^-3 after 4 h.In order to further improve immobilization performance,WO₃ was introduced into ZIT composites as dopant.It was found that WO₃ doping promoted the conversion of Zn₂Ti O₄ to Zn₂Si O₄ and the occurrence temperature of Zn₂Si O₄ decreased from 800℃to 700℃.The maximum content of WO₃ was 3.5 wt.%.The introduction of WO₃ improved the overall performance of the cured body.When 1 wt.%was doped,the maximum density was 4.56 g·m^-3,which was significantly higher than that of undoped immobilized products.After 28 days of leaching test,the lowest leaching rate of Pr and Eu normalized was 2×10^-6 g·m^-2·d^-1.The experimental results show that the modified ZIT composites with higher density were successfully obtained by doping WO₃.