Electrochemical Behavior Of Th（Ⅳ） And Its Electrochemical Separation In ThF₄-liCl-KCl Melt

Molten salt reactor,the unique liquid fuel reactor in Generation IV reactors,has been internationally recognized as one of the most suitable reactor type for implementation of Th-U cycle.Due to the property of Th-U cycle,the utilization rate of thorium in the"once through"fuel cycle is less than 1%,and it can be increased by recovery and reuse of thorium spent fuel.The effective utilization of thorium is one of the important goals of Thorium Molten Salt Reactor（TMSR）nuclear system,and a concept flowsheet of TMSR fuel reprocessing has been proposed by Shanghai Institute of Applied Physics（SINAP,Chinese Academy of Sciences）,including the methods of fluoride volatilization,low-pressure distillation and molten salt electrochemistry for the separation and purification of uranium,carrier salt and thorium,respectively.Separation of thorium from lanthanide fission products in fluoride is a key process to the thorium recovery in this roadmap.Compared with various molten salt media,the LiCl-KCl molten salt has wider electrochemical window,lower melting point and more positive reduction potential for Th（Ⅳ）/Th,and it is also extensively applied in the field of spent fuel treatment,therefore,it is a good choice as the medium for electrochemical separation of ThF₄ and fission products.Although many systematic researches on electrochemical behavior and separation of lanthanides and actinides in LiCl-KCl molten salt have been conducted all around the world,almost all studies were carried out in the pure chloride molten salt system,and there were many issues in the electrochemical separating of ThF₄ from other fluorides in LiCl-KCl molten salt as follows:the solubility of ThF₄ in LiCl-KCl molten salt;the electrochemical behavior of Th（Ⅳ）in Th F₄-LiCl-KCl;the impact of existence and accumulation of F^-in LiCl-KCl molten salt to the electrochemical behavior and electrolysis of thorium;and the influence of concentration change of Th（Ⅳ）in medium to the electrolysis efficiency;and so on.This paper is in response to these problems.First of all,ThF₄-LiCl-KCl molten salts with different thorium concentration were prepared through a repeat melting-coagulating method,and it was determined as the preparation flowsheet of ThF₄-LiCl-KCl molten salt.The dissolution of ThF₄ in LiCl-KCl molten salt was analysis via inductively coupled plasma-atomic emission spectroscopy（ICP-AES）,oxygen detector and X-ray diffraction（XRD）,which showed that ThF₄ was well-distributed in ThF₄-LiCl-KCl molten salt with ThF₄ concentration less than 10 wt.%.Preliminary electrochemical measurements indicated that the concentration of ThF₄ would influence the electrochemical signal of system,for example,at a high Th concentration（over 5 wt.%ThF₄）,the electrochemical window of the molten salt in the CV curve was beyond the scanning range.So ThF₄concentration was determined as 3 wt.%for the following electrochemical experiment.Electrochemical behavior of Th（Ⅳ）in ThF₄-LiCl-KCl molten salt was systematically investigated by cyclic voltammetry（CV）,square wave voltammetry（SWV）,chronopotentiometry（CP）and electromotive force method（EMF）under 733-793 K.The results showed that the reduction of Th（Ⅳ）was a diffusion controlled four-electron transfer process,namely Th（Ⅳ）+4e^-→Th,and the introduction of F^-did not change the electrode reaction of Th（Ⅳ）.Compared with the pure chloride system,the generating of stable large size complex after the addition of F^-into chloride system would cause the decrease of diffusion coefficient and the increase of activation energy of Th（Ⅳ）.The electrolytic separation of thorium from ThF₄-LiCl-KCl molten salt was carried out by the method of pulse potential electrolysis for 6 hours at 773K,and the primary separation ratio of thorium reached 86.8%.The study on the electrochemical behavior of Ln（Ⅲ）（Ce（Ⅲ）,Gd（Ⅲ）,Nd（Ⅲ）,Sm（Ⅲ）and Eu（Ⅲ））in the LnF₃-LiCl-KCl system showed that the reduction of Ce（Ⅲ）and Gd（Ⅲ）is a three-electron process to metal;the reduction of Nd（Ⅲ）is divided into two steps:first from Nd（Ⅲ）to Nd（Ⅱ）,then from Nd（Ⅱ）to metal Nd;the reduction of Sm（Ⅲ）and Eu（Ⅲ）is a single-electron process to low valent ions Sm（Ⅱ）and Eu（Ⅱ）.The introduction of F^-does not change the electrode reaction of ions,but it will significantly affect the diffusion coefficients of them in LnF₃-LiCl-KCl molten salt,where the diffusion coefficients of Ln（Ⅲ）are almost one order of magnitude smaller than that in LnCl₃-LiCl-KCl.The deposition potential difference between Th（Ⅳ）and Ln（Ⅲ）was more than 0.19 V,which theoretically evidenced that it was feasible to separate of Th and Ln in LiCl-KCl molten salt.Finally,the impact of ThF₄ concentration on the electrode process and electrolysis of Th（Ⅳ）was studied in CeF₃-ThF₄-LiCl-KCl molten salt under 773 K.The results showed that the reduction potential variation of Th（Ⅳ）was related to the Th（Ⅳ）and F^-concentration--the initial reduction potential was-1.75V in CeF₃（0.3 wt.%）-ThF₄（3 wt.%）-LiCl-KCl molten salt system,and it became-1.72V when ThF₄ concentration was 10 wt.%.Optimized pulse potential electrolysis method was adopted to separate thorium in two CeF₃-ThF₄-LiCl-KCl molten salt with different ThF₄ concentration（3wt.%and 10 wt.%）,and the separation ratio of thorium reached 98.9%and 99.7%,respectively.The initial electrolysis rate and amount of cathodic deposition in 10 wt.%ThF₄ system was much larger than those in 3 wt.%system,indicating Th（Ⅳ）concentration in molten salt is one of the major factor on the thorium electrolysis efficiency—the higher Th（Ⅳ）concentration is,the higher the electrolysis efficiency will be.The average electrolysis rate in 10 wt.%ThF₄ system was significantly smaller than that in 3 wt.%system,which means F^-concentration is another influencing factor of the electrolysis efficiency:with the similar Th（Ⅳ）concentration,the higher F^-concentration is,the lower the electrolysis efficiency will be.In summary,both concentrations of Th（Ⅳ）and F^-are the influence factors affecting the electrochemical behavior of Th（Ⅳ）and the efficiency of electrolysis,and the effects are opposite.These results will be of great importance in the design of the separation process of spent fluorides fuel in chloride system.To ensure the effective separation of thorium and energy conservation,it is essential to keep a relative high Th（Ⅳ）concentration during process by adding spent fuel continuously.Meanwhile a regular de-fluorine process of the molten salt is also necessary.