熔盐电解金属铀早期成核及形貌演变过程研究

In the pyrochemical electrorefining process, the majority of uranium in spent nuclear fuel can be recovered on a solid cathode. However, the nucleation of uranium at very early stage, especially their morphology and evolution on the solid cathode, are poorly understood and few studies have been reported on this issue. Our previous research has found that the yield and morphology of uranium dendrites are greatly controlled by factors such as cathode material, temperature and overpotential after electrolysis under long-term extreme diffusion conditions. Recently, preliminary experiments have illustrated that nucleation morphology of uranium at early stage is also significantly different under varied experimental conditions. Above results have indicated that the formation and state of uranium at early nucleation stage play an important role in subsequent uranium metal deposition, dendrite growth and electrolysis efficiency. Therefore, based on the preliminary results, this project aims to clarify the early electro-crystallization process of metallic uranium in molten salt and to establish an efficient method on the uranium nuclei formation control. Combined with scanning electron microscopy and atomic force microscopy techniques, this study will adopt a special disk electrode suitable for high temperature molten salt system, and will advance the understanding of nucleation morphology, evolution process as well as its influencing factors of uranium, and finally would find out the relationship between nuclei growth and subsequent uranium dendrite growth. Basic experiment data and scientific background for the system data selection, process design, monitoring and simulation in the electrorefining process will also be provided.

干法后处理电解精炼流程中，乏燃料中铀的回收主要是通过固态阴极进行电解实现；然而，铀在固态阴极基底上的早期电结晶成核过程、尤其是早期晶核形貌及其生长演变过程却仍不清楚，目前这方面的研究鲜有报道。前期研究工作发现，长时间极限扩散条件下电解后，铀枝晶的宏观产量、微观尺度和形貌受阴极材料、温度、过电位等因素的影响巨大。近期预实验结果发现，铀的早期晶核形貌也因上述因素不同而具有显著的差异。以上结果表明，铀早期晶核的形成和状态对后续铀金属沉积、枝晶生长和电解效率具有重要的作用。本项目将在前期工作基础上，采用适合于高温熔盐体系，便于扫描电子显微镜和原子力显微镜技术表征分析的抛光圆盘电极进行电解沉积，探究铀的早期电结晶过程，查明成核机理、晶核形貌、生长演变过程及其影响因素，明确晶核生长模式和后续铀枝晶生长的关系，为干法后处理熔盐电精炼流程中体系材料选择、流程设计、监测和仿真提供科学依据与基础数据。

干法后处理电解精炼流程中，乏燃料中铀的回收主要是通过固态阴极进行电解实现；然而，铀在固态阴极基底上的早期电结晶成核现象、尤其是早期晶核成核机理、形貌及其生长演变过程却仍不清楚，很难有效解决铀枝晶生长带来的工艺问题。 本项目中，我们结合实际成核形貌演变过程，对铀成核理论模型进行优化，对成核机理和成核动力学进行了系统的研究，得到相关的动力学参数；并对阴极材料（W，不锈钢和玻璃碳）、温度、铀离子浓度和杂质离子（Cs）的影响进行了比较分析和研究。首先设计并改进了适用于高温熔盐体系的可拆卸式圆盘电极，并对其性能进行评价和优化。该圆盘电极实现对电结晶微纳米晶核高质量采集，促进了晶核样品形貌的清晰表征，有力的支撑了铀成核机理的研究。对于铀的早期电结晶成核，铀在惰性W阴极表面首先会迅速形成一层铀膜，以此作为基底，铀的晶核成核后会重排和聚集融合，形成尺寸较大的晶核。铀的晶核密度活性位点受过电位和浓度的影响，低浓度条件下，晶核密度随过电位增加而增加，尺寸则减小。高浓度条件下，则相反。对于机理，S-H模型不适合熔盐电解电结晶成核机理的分析，利用考虑电荷转移阻抗结合H-T成核模型可以很好地计时电流曲线进行拟合，得到合理的成核动力学参数。另外，升高温度可以提高晶核密度，基底对晶核形貌，活性位点的影响比较大，杂质Cs对晶核形貌影响不大。根据铀成核机理和影响因素的研究，发现升高温度，低浓度下增加过电位，高浓度下降低过电位可以增加铀晶核的活性位点，有望减少特异性枝晶生长。总的来说，本项目设计的圆盘电极和研究方法，对于其他高温熔盐中的电沉积，电合成，电镀等领域的早期成核表征和研究，提供了一种简单通用的分析方法，具有广泛的适用性。另外，关于铀早期电结晶成核的研究为干法后处理熔盐电精炼流程中体系材料和工艺选择、流程设计和仿真提供了科学依据与基础数据。