熔盐电化学分离核能系统中镧系金属离子的研究

Based on the TMSR(Th-molten salts reactor )system, electrochemcial method is applied to deal with the overall scientific issues concerning the separation of the lanthanides from the molten salts and actinides in the reprocessing of the molten salts reactors. The results will provide a significant important theoretical support for the technical design of the reprecessing for TMSR system..By electrochemcial methods including cyclic volatmmograms, square-wave voltammograms,and chronoamperograms and chronopotentiograms, combined with chemical analysis method，the redox potential，electrochemical reduction mechanism and controlled steps and valences of lanthanides in the molten salts are characterized. And the equilibrium constant between two oxidation states also will be obtained.The correlation between the stability of divalence and trivalence for the lanthanides ions with the anion ions of the supporting electrolyte will be studied.The lanthanides ions structure will be measured by high-temperature Raman spectroscopy,and the correlation between ions structure and anion ions of supporting electrolyte, as well as the inherent relationship between ions structure and the reduction potentials of lanthanides also will be discussed.Therefore,flourides, chlorides and mixture of floruide-chloride with the most stable trivalence and unstable divalence can be expected. Potentiostatic/galvanostatic modles are employed to separate lanthanides from the molten salts,in which stirring the molten salts is used to enhance the electrochemical reaction rate,and Nitrogen ions are introduced to the flourides to avoid anodic effects in carbon anode, and the effects of supporting electrolyte type, potential,current density, temperature and divalence ions of lanthanides in the melts on the electroseparation rate and efficiency will be duscussed.

课题以钍基熔盐堆系统为背景，拟采用电化学方法全面系统解决熔盐反应堆后处理过程中镧系金属离子与锕系金属离子以及熔盐电解质分离的科学问题，为反应堆后处理工艺的设计提供重要的理论依据。课题拟通过电化学方法，包括循环伏安、方波伏安以及计时电流和计时电位，结合化学分析手段，表征镧系金属离子在不同熔盐体系中的氧化/还原电位、电化学还原机理和控制步骤、存在的价态，以及不同价态之间的平衡常数，获得镧系元素价态稳定性与熔盐支持电解质阴离子的关系。通过高温拉曼光谱表征镧系元素的离子结构与支持电解质阴离子的关系，以及与还原电位之间的内在联系。从而分别获得三价镧系元素最稳定、二价最不稳定的氟化物、氯化物以及氟-氯化物电解质体系。采用恒电位/恒电流电解方式分离镧系金属离子，采用搅拌的方式提高反应动力学，在氟化物电解质体系中添加氮离子，考察电位，电流密度、低价离子、温度、电解质体系对镧系元素分离效率和分离速度的影响。

采用电化学方法去除和表征LiF-NaF-KF体系中的氧离子，并首次报道了将氟化物熔盐中氧离子的含量降到了200ppm。LiF-NdF3-Nd2O3熔融盐中氧离子的氧化过程分为吸附和气体产生两部分，研究了影响临界电位的因素，确定了氟离子起始氧化电位。方波伏安法表征了Nd2O3在NdF3-LiF体系中的溶解行为，得出了方波伏安曲线的峰值电流Ip与Nd2O3浓度的关系曲线，以及得出了溶解度与电解质组成之间关系式。.采用电化学研究表明：证明了Nd(III)离子在NdF3-LiF体系中还原过程为两步还原，即Nd(III)→Nd(II) 和Nd(II)→Nd(0)，两个还原过程均受扩散控制；首次明确了在氟化物体系中通过Nd(III)和Nd(0)的歧化反应可以生成稳定的NdF2，NdF3-LiF组成对NdF2浓度影响较大，电解制取钕的效率为50%。在LiF-CaF2-SmF3熔盐体系中，Sm(III)/Sm电化学还原过程分为二步，即Sm(III)/Sm(II)和Sm(II)/Sm。Sm(III)/Sm(II)的还原过程受扩散控制，扩散系数为2.9×10-5 cm2/s。在LiF-CaF2-Sm2O3(0.15mol/L)-AlF3(8.89mol/L)熔盐体系，首次通过恒电流电解，通过形成Al2Sm和Al3Sm的沉积物部分钐可以分离。.在LiF-CaF2-SmF3-Sm（过量）熔盐体系，研究了Sm(III)、Sm(II)和Sm之间的表观平衡常数。稀土镝离子Dy（III）在氟化物体系中为一步还原转移三个电子的过程，该过程受扩散控制，Dy（III）的扩散速率为3.41×10-5 cm2.s-1。在LiF-CaF2-DyF3-Fe2O3-Dy2O3体系中恒电流电解，得到了球状镝铁合金，其中Dy占40.71 at%，Fe占34.43 at%。借助Raman光谱法研究了该熔体结构。. 以上研究结果为熔盐中分离和提取稀土离子提供了重要的理论基础。