| Realizing the efficient utilization of the thorium-uranium cycle and thorium resources is an important goal of the thorium-based molten salt reactor nuclear energy system（TMSR）.In order to achieve this goal,The Shanghai Institute of Applied Physics,Chinese Academy of Sciences（SINAP）put forward the technical route combing molten salt reactor and pryprocessing of fuel salt.The TMSR fuel salt pryprocessing flowsheet proposed by SINAP includes three separation technologies,fluoride volatility,low-pressure distillation and molten salt electrochemistry.The main compositions remained in residue after fluoride volatility and distillation processes are ThF4,FPFx and a small amount of TRUFx.The separation of ThF4 from rare earth fission products is the key issue in recovery of ThF4.Based on the wide range electrochemical window,low melting point,high precipitation potential difference between Th and rare earth,and extensive application experience in the field of spent fuel treatment,LiCl-KCl has been chosen as a medium for electrochemical separation of ThF4 from fission products.To separate ThF4 from LiCl-KCl melt by electrochemical technique,it is necessary to understand some fundamental issues,such as:the dissolution of ThF4 in the LiCl-KCl molten salt;whether the introduction of F-will affect the electrochemical behavior of Th（IV）in the LiCl-KCl molten salt;the influence of the relative concentration of Th（IV）and F-on the electrolysis efficiency during the electrolysis process and so on.The research group has already carried out the study on the preparation of ThF4-LiCl-KCl molten salt,which showed that ThF4 dissolved uniformly in the LiCl-KCl molten salt with the concentration of ThF4 within 10 wt.%,and it was found that the existence of a small amount of F-in ThF4/LnF3（3wt.%）-LiCl-KCl molten salt,will not change the reaction process of Th（IV）and Ln（III）on the inert electrode.The study also showed that,compared to the pure chlorine molten salt,a relatively stable complex was formed between F-and Th（IV）/Ln（III）due to the strong complexation of F-,which reduced the activity of Th（IV）and Ln（III）in the salt and then their diffusion properties.On the basis of the above research,the influence of different F-concentration on the electrochemical behavior and electrolysis separation of Th（IV）in the ThF4-LiCl-KCl molten salt was studied in this paper.The following results were obtained:1.The various F-contents in the ThF4-LiCl-KCl molten salt were secured by adding LiF and KF,The preparation process of ThF4-LiCl-KCl molten salts with various[F-]/[Th（IV）]were prepared through a repeat melting-coagulating method,The homogeneous,low impurities ThF4-LiCl-KCl molten salt was obtained with the[F-]/[Th（IV）]ratios of 4,6,7,8,9,10 and 12.2.The cyclic voltammetry（CV）,square wave voltammetry（SWV）,Chrono potential（CP）and electromotive force method（EMF）were used to study the electrochemical behavior of Th（IV）in ThF4-LiCl-KCl molten salt with various ratios of[F-]/[Th（IV）]under 733-813K,and the reduction potential,diffusion coefficient,electrode reaction and activation energy of Th（IV）in different molten salts were obtained.The results showed that with the increase of[F-]/[Th（IV）]in the molten salt（from 4 to 12）,the activity coefficient of Th（IV）decreased,which caused the negative shift of the reduction potential of Th（IV）,and the negative shift of the apparent standard electrode potential of Th（IV）/Th.Except the molten salts with the[F-]/[Th（IV）]ratios of 8 and 9,the reduction of Th（IV）was one step of four electron transfer process.In the salts with the ratio of 8 and 9,the reduction of Th（IV）exhibited two steps both with 2 changed electrons:Th（IV）+2e-→Th（II）and Th（II）+2e-→Th,showed the existence of Th（II）,but the mechanism was not yet clear.With the increase of[F-]/[Th（IV）]in molten salt,the diffusion coefficient of Th（IV）in molten salt first decreaseed and then increased,and there was an inflection point at ratio of 8.All these results indicated that the electrochemical behavior of Th（IV）in ThF4-LiCl-KCl molten salt is greatly influenced by the ratio of[F-]/[Th（IV）in molten salt.3.The effect of[F-]/[Th（IV）]ratio on the recovery of Th（IV）in ThF4-LiCl-KCl molten salt was studied by pulse potential electrolysis.The results showed that the recovery rate of Th decreased monotonously with the increase of[F-]/[Th（IV）]ratio under same electrolysis time at the same electrolysis potential.The electrolysis recovery rate of Th in the molten salt with[F-]/[Th（IV）]ratio of 4 was 88.74%after 8h’s electrolysis,the recovery rate was 67.61%when the ratio was 8,and the recovery rate was only 56.28%when the ratio was 12,which indicated that F-would reduce the electrolytic recovery efficiency of Th in the ThF4-LiCl-KCl molten salt.When analyzing the relationship between F-concentration,diffusion coefficient and recovery rate,it was found that the effect of the introduction of fluorine ions in the chlorine salt is complicated for the separation of Th（IV）electrolysis.When[F-]/[Th（IV）]increased from 8 to 12,the Th（IV）diffusion coefficient increased,while the current density decreased with the electrolysis time.The final performance was that the recovery rate of Th decreased.The decrease of the electrolytic current density might be related to the increase of the stability of the Th-F cluster caused by excessive fluoride or the decrease of the anode conductivity caused by the reaction between graphite anode and the fluorine ion.In conclusion,the concentration of F-in the molten salt will affect the electrochemical behavior and the electrolysis efficiency of Th（IV）.The increase of F-concentration causes the negative shift of the reduction potential and the reduction of the electrolysis rate.This is of great guiding significance for the design of the separation process of fluoride spent fuel in the chlorine salt.In order to realize the effective separation of thorium and save energy,the defluorination treatment of molten salt should be carried out regularly during the treatment process. |
PDF Full Text Request