钍熔盐堆闭式循环在线后处理的探索设计及其化学问题研究

A new on-line reprocessing scheme in TMSR (thorium-based molten salt reactor) was proposed, in view of the innovative TMSR-NM reprocessing researched in French has the defects of safety and extraction efficiency. In order to achieve the effective separation between thorium and lanthanides and thorium fuel cycle, a reductive extraction in high temperature molten salts/liquid metal, a selective electrodeposition and anodic oxidation were employed in continuous/alternate way in LiF-BeF2(CaF2)-LnF3-CeF4-ThF4 molten salts. In order to improve the separation efficiency of Th/Ln, the key chemical issues were explored as follow: (1) The thermodynamics and kinetics of a reductive extraction of Th in high temperature molten salts/liquid Bi-Li, the relationship between ion valence state and extraction efficiency and valence state controlled by electrochemical method. (2) The electrochemical and diffusion behavior and nucleation mechanism of Th and Ln ions at a liquid Bi cathode; the electrochemical mechanism of selectivity electrodeposition of Th ions. (3) The electrochemical mechanism of selectivity anodic oxidation of Th at Bi-Th anode. A comprehensive analysis on the material balances of Th and Ln, and separation effects of Th/Ln were made during the whole cyclic processing. Based on the research results above, the innovative concept of the closed cycle on-line reprocessing in TMSR were explored. The innovative concept reprocessing scheme proposed, having originality and self-owned intellectual property, provides feasible and essential datum for TMSR.

鉴于法国提出的新型TMSR-NM后处理方案存在的安全性不够高、萃取率不高的缺憾，本项目提出了安全性更好的TMSR在线后处理新方案：以LiF-BeF2(CaF2)-LnF3-CeF4-ThF4为研究体系，采用高温熔盐/液态金属萃取、选择性电沉积及阳极溶解连续/交替的方式，实现Th/Ln的有效分离和钍循环。并对（1）高温熔盐/液态金属Bi-Li还原萃取Th过程的动力学和热力学、离子价态与萃取效果的关系及价态的电化学控制；（2）Th（Ⅳ）、Ln离子在液态阴极Bi还原的电化学行为、扩散行为及成核机理；进而探讨Th选择性电沉积的电化学机理；（3） Th选择性阳极溶解的电化学机理等化学问题进行研究，提高Th/Ln的分离效率，综合分析循环过程中Th、Ln的物料衡算及影响Th和Ln的分离因素，开发TMSR闭式燃料循环在线后处理的新概念流程，为我国具有自主知识产权的熔盐堆后处理技术提供可行性和基础数据。

采用高温熔融盐/液态金属Bi-Li还原萃取、选择性电沉积及阳极溶解连续/交替的方式，实现Th/RE分离和钍循环，探索设计了钍熔盐堆闭式燃料循环的概念流程和概念工艺流程。围绕“在线检测”在国内率先进行了氟化物熔融盐/液态金属萃取、电沉积、阳极溶解过程的在线检测研究，研究结果如下：.(1) 钍(Th)、稀土(RE)离子分离的可能性研究：采用电化学方法研究了Th、RE离子在熔盐中的电化学行为，在定性分析的基础上测定了Th和RE在熔盐中的平衡电极电位差，依据电位差计算Th与Pr、Er、Gd分离效率接近100%。(2) LiF-CaF2-REF3(RE= Ce，Eu，Sm)的电化学行为及RE(III)的定性分析和定量分析：依据定性分析的结果，通过方波伏安法绘制稀土离子浓度与峰高的关系标准工作曲线，依据标准工作曲线对稀土离子浓度变化进行定量分析。(3) 在线检测了 LiF-CaF2-REF3（RE=Ce、Eu、Sm）/液态Bi-Li还原萃取稀土离子的进程:采用电化学方法在线检测了1123K时LiF-CaF2-REF3 /液态Bi-Li还原萃取过程中，RE(III)离子浓度随时间的变化。结果表明：Ce、Eu、Sm的萃取速率在0.004-0.009mol/h范围内，5-7h后RE(III)离子的萃取率都达到98%左右；Ce的萃取速率比Eu、Sm小，说明还原萃取主要与稀土金属活动顺序有关，即还原萃取过程是热力学控制过程。(4) 在线检测了LiF-CaF2-EuF3熔盐中电沉积Eu进程： 1123K时Eu(III)提取速率约为0.0015mol/h，2.5 h提取率为72%。(5)在线检测了Bi-Ce、Al-Sm合金在1123 K的LiF-CaF2熔盐中阳极溶解的进程，恒电流为+0.1A时，Ce、Sm的阳极溶解速率分别为0.396mol/h和0.06mol/h。说明阳极溶解也主要与金属活动顺序有关。