非直流熔盐电解精炼U-Zr(Al)-Ln合金的电化学机理研究

In DC (Direction Current) electrorefining of metallic spent nuclear fuel, the anodic passivation and cathodic dendrite growth result in that electrorefining efficiency decreases and even the electrorefining fails to perform normally. Non-DC electrorefining was adopted to solve this problem. Firstly, the electrochemical behavior, reaction controlling step and electrochemical mechanism of anodic dissolution of U-RE-Zr(Al) alloys were explored in DC electrorefining process. The passivation reaction and mechanism of anodic dissolution of U-RE-Zr(Al) alloys were confirmed by analyzing the composition of passivation layer. Then, electrorefining by AC(Alternating Current)/DC superimposed or periodic reversal current was explored in order to restrain anodic passivation and cathodic dendritic growth. The electrochemical behavior, dissolution sequence, change of U valence state, the control step and electrochemical mechanism of anodic dissolution were investigated in anodic dissolution of U-RE-Zr(Al) alloys. The major study was focused on the relationship of ratio of forward and reverse current with the passivation reaction and the composition of passivation layer. The effects of the ratio of forward and reverse current were investigated on sequence of cathodic deposition and the recovery rate of U, in particular, inhibition of the cathodic dendritic growth to reveal the electrochemical mechanism of Non-DC electric crystallization. The purpose of studying the Non-DC electrorefining is to improve the anodic dissolution rate and the separation coefficient of U and Ln, break through the key technology of electrorefining process and laid foundation for developing the metallic spent nuclear fuel reprocessing with independent intellectual property rights.

针对金属乏燃料直流电解精炼过程中，阳极易发生电化学钝化和阴极枝晶生长影响电解精炼的效率甚至造成电解精炼无法正常进行的关键问题，本项目拟采用非直流电解精炼解决此问题。首先研究直流电解精炼过程中，U-Zr(Al)-Ln（镧系元素）合金阳极溶解的电化学行为、控制步骤和电化学机理，再通过分析钝化层的组成，确定钝化反应和钝化机理。然后，为了抑制阳极钝化和枝晶生长，采用交直流叠加或周期反向进行电解精炼。研究阳极溶解的电化学行为、溶解顺序、U价态变化、控制步骤等电化学机制，重点研究正反向电流比与阳极溶解钝化反应、钝化层组成的关系；研究正反向电流比对阴极沉积的顺序、U的回收率的影响，重点研究正反向电流比对抑制枝晶生长的影响，揭示非直流条件下电结晶的电化学机理。通过研究非直流电解精炼，提高阳极溶解速率和U/Ln分离系数，突破电解精炼的关键问题，为开发具有自主知识产权的金属乏燃料电解精炼技术奠定基础。

随着核电技术的进步、燃耗的加深，干法后处理因其优势受到广泛关注。干法后处理的核心技术是电解精炼，包括阳极溶解和阴极沉积两步，而其控制步骤是阳极溶解，为了提高电解精炼的效率，开展了U-Zr(Al)-Ln（镧系元素）合金为模拟金属乏燃料的研究。研究了直流电解精炼过程中，U-Al合金在 LiCl-KCl 熔盐中阳极溶解的电化学行为，以及发生阳极溶解过程各组分的溶出顺序、U价态变化；研究了Al-Ln(La、Sm、Gd、Y)和Zr-Ln（Ln=La、Dy、Gd）合金在 LiCl-KCl 熔融盐中阳极溶解的电化学行为，阳极溶解速率与电参数、合金组成的关系等动力学问题；溶解顺序、价态变化、控制步骤和电化学机理。研究了阳极溶解过程的活化、钝化等电化学行为，测定了阳极溶解电位、临界钝化电位和钝化区、钝化过渡区、钝化稳定区等电位范围。通过分析钝化现象、钝化层的组成，确定了钝化反应；探索了熔盐中阳极钝化机理；研究固态Al-RE(Y/Sm)合金和Zr阳极溶解过程中添加氟离子F-对钝化的影响，发现Al-Y合金阳极钝化膜单位面积的电阻降低两个数量级从108.68 Ω cm-2降低至1.30 Ω cm-2，提出了F-抑制钝化的机制：表明氟离子可以有效抑制阳极钝化，提高了电解精炼的效率。研究了已溶解的 U(IV)、Zr(IV)、Ln(III) 离子在惰性 W 和活性电极上的电化学行为，探索了阴极沉积的电化学机理； 研究了U(Ⅳ)/U(Ⅲ)和U(Ⅲ)/U(0)电对在W电极上的动力学过程及形核机制；测定了Al-U合金的热力学性质；恒电位/恒电流电解过程中铀的回收率达93%以上，实现了 U/Ln 的分离。研究了非直流电解精炼，对脉冲和直流熔盐电沉积金属铝Al进行了比较；研究了电沉积金属铝过程中金属Al晶体生长和晶粒大小的变化、电结晶的电化学机理；研究了铀U在W电极上的形核机制；探索了脉冲电源电解法抑制枝晶的生长。这些结果可以为熔盐电解精炼乏燃料的溶剂盐的筛选以及提高电解精炼的效率提供基础数据，在乏燃料干法后处理领域具有一定的应用前景。