含锕系熔盐体系的极化力场开发及理论模拟

High-temperature molten salts have been widely used in the field of nuclear energy such as molten salt reactor and spent nuclear fuel reprocessing. The fuel and coolant in molten salt reactor as well as the pyroprocessing of nuclear fuel both involve molten salts containing actinides. As important aspects in high-temperature molten salts, the studies of physicochemical properties and structures can serve as important references for engineering design. However, the experimental study is a challenging job due to the intense radiation, high temperature and corrosive character. The knowledge of physicochemical behavior of actinide molten salts is limited and the understanding at the molecular level is lacking. The physicochemical properties of molten salts can be calculated exactly by using theoretical methods that can also provide a reasonable explanation of the behavior. The difficulties encountered in theoretical studies include relativistic effects, electron correlation in actinides, and accurate description of the dynamic process of ions in molten salts and their influences on the macroscopic properties. In this project we will develop the polarizable force field and perform molecular dynamics simulations and relativistic quantum chemical calculations on some molten salts containing actinides. By studying the microscopic structure，bonding character and physicochemical property, we will investigate the possibility to build the physicochemical database based on theoretical results, and analyze the differences between different kinds of actinide molten salts which can provide theoretical foundation for separations between actinide ions.

高温熔盐广泛应用于熔盐反应堆和核废料后处理等领域，熔盐堆中的燃料、冷却剂以及干法后处理均涉及锕系元素。作为高温熔盐化学的重要研究内容，微观结构和物理化学性质等基础科学问题的研究可以为工程设计提供必要参考。但含锕系熔盐体系往往具有强辐照、高温和强腐蚀等特点，给实验研究带来了极大的挑战，这导致对其物理化学行为的认识相对有限，并且缺乏分子层次的微观理解。采用理论计算与模拟可获得可靠的物化性质参数，并合理地阐释物理化学行为。但理论研究的难点主要有锕系元素的相对论效应、电子相关问题以及如何准确描述内部离子的动态过程及其对宏观性质的影响。本项目拟自主开发极化力场拟合程序，对若干含锕系的熔盐体系进行分子动力学模拟和高精度的相对论量子化学计算。通过微观结构、成键规律和物理化学性质的研究，探讨通过理论计算构建物化性质数据库的可行性，并分析不同锕系熔盐体系的异同，为实现锕系离子的相互分离提供理论指导。

本项目根据熔盐在反应堆以及核废料后处理过程中的实际应用，选取具有代表性的锕系体系进行了系统的理论研究。我们自主开发了熔盐体系的极化力场拟合程序，优化了若干类重要锕系熔盐体系的力场参数，解决了经典分子动力学模拟的关键问题。通过在不同温度和浓度条件下对熔盐体系进行的一系列模拟，研究了温度和浓度对熔盐结构和输运性质的影响。得到了锕系离子第一配位层中的卤素离子的寿命以及相应的平均力势，根据过渡态理论，拟合了卤素离子寿命的计算公式，探讨了寿命与活化能之间的关系。采用相对论密度泛函理论方法和考虑电子相关的post-HF 等方法进行了电子结构的计算，分析了键级、电荷、轨道能级图，研究了锕系元素的5f 和6d 轨道对锕系-配体化学键的作用以及贡献。研究成果将有助于从分子层次理解熔盐体系的微观结构及其对宏观性质的影响，有助于深入理解锕系化合物的成键规律，为实现镧锕分离提供理论指导。