Investigation of the effect of the microstructure upon the oxidative long-term dissolution of uranium dioxide: a fundamental approach including synthesis and corrosion of simplified model systems as well as electron microscopy and atomistic simulation

拟议项目的目的是获得对uo2基陶瓷氧化腐蚀过程的深刻基本理解，这种材料被广泛认为是乏核燃料（SNF）的合适模型系统。由于容器（通常是不锈钢）的腐蚀，假定深层地质核废料储存库的地球化学环境正在减少。然而，在与水接触的SNF表面，由于放射性衰变而产生的放射性溶解产生氧化物质。H2O2被认为是其中最相关的。锕系元素的氧化作用使它们从不溶性转变为高溶性。在基本层面上，这种氧化过程类似于金属的腐蚀。因此，本项目的工作假设是晶界在该腐蚀机制中起主要作用。本文将采用宏观和微观相结合的实验方法，包括简单UO2的合成、表征和腐蚀作为参考，以及掺杂稀土元素的更复杂的UO2陶瓷，并进一步掺杂金属颗粒的UO2陶瓷。这些材料将通过经典电子显微镜（SEM， EDX）和EBSD进行表征，以了解陶瓷结构的晶界和其他薄弱点如何促进溶解。先进的透射电子显微镜技术将用于对新鲜和腐蚀表面进行详细的高分辨率调查。对腐蚀前后的晶界和晶粒取向进行系统表征。此外，原子模拟的晶界将被执行，以确定点缺陷热力学。基于这些数据，将建立连续统模拟，以预测空间电荷区的缺陷浓度（以及沿空间电荷区的扩散速率），因为点缺陷在固态反应中起着核心作用。最终，从这种结合实验和理论的方法中观察到的结果将用于推导uo2基陶瓷腐蚀行为的模型。这样的模型可以用数学方法描述基于uo2的乏核燃料的长期腐蚀行为，并得出这种陶瓷的长期稳定性和放射性核素释放的后果。

The aim of the proposed project is to obtain a deep fundamental understanding of the process of oxidative corrosion of UO2-based ceramics, materials that are widely considered as suitable model systems for spent nuclear fuel (SNF). Due to the corrosion of the container (typically stainless steel), the geochemical environment of a deep geological repository for nuclear waste is assumed to be reducing. However, at the surface of SNF in contact with water, radiolysis due to radioactive decay produces oxidizing species. H2O2 is considered to be the most relevant of these. Oxidation of the actinides transforms them from insoluble to highly soluble species. On a fundamental level, this oxidation process is similar to the corrosion of metals. Therefore, the working hypothesis of this project is that grain boundaries play a major role during this corrosion mechanism. A combined macroscopic and microscopic experimental approach will be followed including synthesis, characterization and corrosion of simple UO2 as a reference as well as more complex UO2 ceramics doped with rare earth elements, and in a further step, doped UO2 ceramics with additional metallic particles. These materials will be characterized by means of classical electron microscopy (SEM, EDX) and EBSD to understand how the grain boundaries and other weak spots of the ceramic’s structure contribute to the dissolution. Advanced transmission electron microscopy techniques will be employed for detailed high-resolution investigations of the fresh and corroded surfaces. Grain boundaries and grain orientations will be systematically characterized before and after being corroded. In addition, atomistic simulations of grain boundaries will be performed to determine the point defect thermodynamics. Based on these data, continuum simulations will be set up which can predict the defect concentrations in (and diffusion rates along) the space charge zones, since it is point defects that play a central role in solid-state reactions.Ultimately, the observations and findings from this combined experimental and theoretical approach will be used to derive a model of the corrosion behavior of UO2-based ceramics. Such a model could be used to mathematically describe the long-term corrosion behavior of UO2-based spent nuclear fuel and derive consequences for the long-term stability and release of radionuclides from such a ceramic.