The Application Of Accelerators In The Pyrohydrolysis Of Fluorides From Thorium Molten Salt Reactor

The tailings of pyroprocessing based on Thorium Molten Salt Reactor?TMSR?is made up of a large number of insoluble fluorides,which contains some high heat and high-level radioactive waste.Long-term storage of these products may lead to the formation of harmful substances,such as F₂ and HF,which add significant difficulties to final waste treatment.The classical aqueous processing is based on the oxide system,which can not be directly used on spent fuel from TMSR.In order to further recycle useful materials TMSR fuel after pyroprocessing,it is necessary to explore a new technology to convert fluorides into their corresponding oxides.A study on pyrohydrolysis of different fluorides at high temperature was carried out.With the use of this method,some indissolvable fluorides are converted to oxides,dissolved in nitric acid solution,and treated in the well-established aqueous separation technology.On the other hand,the converted oxides can be stored after vitrification.Alkali,alkaline-earth and rare-earth fluorides which are difficult to be converted can be transformed in to their oxides by use of accelerators.Firstly,the pyrohydrolysis experiment of LiF was conducted under different thermal conversation temperature and within different reaction time.In this paper,effects of addition of accelerators,types of accelerators,dosage of accelerators and reaction time on pyrohydrolysis conversion efficiency of LiF were studied.The results showed that reaction temperature and amount of accelerators have some influence on the pyrohydrolysis of LiF.The acceleration effects of the four accelerators are in accordance with the following rule:WO₃>U₃O₈>Cr₂O₃ and?-Al₂O₃.Our previous result showed that UF₄ can be converted into U₃O₈ at a relative low low temperature.As impurities should be avoided in the further treatment,no additional elements were introduced.So U₃O₈ was chose as a suitable accelerator in this study.As a kind of important fission products,the pyrohydrolysis behaviors and dynamical behaviors of neutron poison represented by SmF₃?light rare earth?and DyF₃?heavy rare earth?in moist air,have been studied by means of TG.The pyrohydrolysis yield under different reaction time and temperature was studied.Reaction order and activation energy of the pyrohydrolysis process were simulated.Meanwhile reaction products in solid state were characterized by means of XRD,TG and SEM.Then,the corresponding reaction mechanisms were expounded.Through the simulation calculation,we found that the two stages of the reaction can both meet the second-order reaction kinetics,and the rate equation obeyed the phase boundary rate law,which was expressed by[1-?1-x?^1/3]=kt.Further studies indicated that the rate-determining step was the surface reaction control,and the shrinking core model could be used to describe the mechanism.In the low-temperature stage?from 650^oC to 850^oC?,DyF₃ was converted to DyOF.The results of the simulation calculation showed that the rate equation was derived as a second-order reaction.In the stage from DyOF convert into Dy₂O₃,the hydrolysis rate is proportional to reaction time.The results showed that the rare earth fluorides could not be converted completely in2h,even though the temperature reached up to 1050^oC.So the addition of accelerator U₃O₈ promoted the completely pyrohydrolysis of Sm F₃ and DyF₃.The dosage of accelerator U₃O₈ on pyrohydrolysis conversion efficiency was investigated.The reaction mechanism was summarized according to the analysis of components of solid products.These experimental results indicated that SmF₃ can be transformed completely,and the solid hydrolysates were Sm_1.33U_2.67O_9.12,Sm₂U₂O_8.34,Sm_2.67U_1.33O_7.56.56 and U₃O₈,when above a 3 to 1 weight ratio of accelerator U₃O₈ to SmF₃ was used.While DyF₃ can be transformed completely,and the solid hydrolysates were Dy₂O₃,Dy₆UO₁₂2 and U₃O₈,when above a 1 to 1 weight ratio of accelerator U₃O₈ to DyF₃ was used.The carrier salt LiF-BeF₂ was simulated by Li₃AlF₆ in our experiments.A series of Li₃AlF₆ of different components were prepared,followed by their structure characterization,the structural characterization of conversion products.The solid products were characterized by means of x-ray diffraction?XRD?,raman spectroscopy,thermogravimetry?TG?,and differential scanning calorimetry?DSC?respectively.The pyrohydrolysis behavior of Li₃AlF₆ was carried out on our self-made apparatus.The influences of accelerators U₃O₈ and?-Al₂O₃ on the pyrohydrolysis behaviors of Li₃AlF₆ were investigated.Then,the products were characterized by means of XRD,and the corresponding reaction mechanisms were proposed.The results showed that Li₃AlF₆ with excessive Li F and pure Li₃AlF₆ could not be completely hydrolyzed.While Li₃AlF₆ with excessive AlF₃ could be completely hydrolyzed.Li₃AlF₆ can be converted to its oxide by use of accelerators such as adding appropriate amounts of AlF₃,U₃O₈ and?-Al₂O_3.A series of mixing salts of different components were prepared,such as Li₃AlF₆-ThF₄,Li₃AlF₆-UF₄ and Li₃AlF₆-ThF₄-UF₄.The pyrohydrolysis behaviors of these molten salts in the state of mechanical mixing and molten salt state were investigated.XRD results of Li₃Al F₆-ThF₄ showed that the solid products were composed of LiTh₄F₁₇,LiTh₂F₉,Li₃AlF₆ and Al F₃.The final hydrolysates of different states are the same,all of which are ThO₂,Al₂O₃ and LiF.XRD results of Li₃AlF₆-UF₄ showed that the solid products were LiU₄F₁₇7 and Li₃AlF₆.The final hydrolysates were Li₂UO₄,Al₂O₃ and LiF in the molten salt state and Li₂UO₄,LiU_0.83O₃,Al₂O₃ and LiF in mechanical mixing state.XRD results of Li₃AlF₆-ThF₄-UF₄ showed that the solid products were Li₃ThF₇,LiTh₂F₉,Li₃UF₇,AlF₃ and Li₃AlF₆.There is a great difference in solid hydrolysates between the mechanical mixing state and the molten salt state.The former were ThO₂,Li₂UO₄,Li₄UO₅,Al₂O₃ and Li F,and the latter were Th_0.75U_0.25O_2.02,LiAlO₂,Al₂O₃ and LiF.In this paper,a variety of studies on the pyrohydrolysis behaviors of different fluorides were carried out in detail.The results of pyrohydrolysis can provide theoretical guidance and reference for the disposal of tailings from TMSR pyroprocessing and fluorinated waste.