核燃料内部气泡演化行为的相场研究

The fission gases xenon and krypton are unceasingly generated in irradiated nuclear fuels, which is a sintered compact of granular uranium dioxide in current commercial light water reactors (LWRs). As a consequence of their extremely low solubility in fuel, the fission gases tend either to precipitate into bubbles or to be released to the free volume in the fuel rod. Fission gas release and gas bubble swelling are critical issues for LWR fuel performance. Released fission gas can reduce the thermal conductivity of the fuel-clad gap and cause temperature and.pressure increases in the fuel pellet; Gas bubble swelling may increase the contact pressure between fuel and clad, and lead to clad failure. Current nuclear fuel performance codes such as FRAPCON and FASTGRASS root on empirical formula of gas bubble behavior and its impact on thermo-mechanical properties derived from simplified assumptions which significantly limit their predictive capability, especially for the development of new concept reactors and advanced nuclear fuels. Therefore, understanding and predicting fission gas behavior and.evolution kinetics are essential for enhancement of fuel performance codes and the development of high burn-up reactor technologies..The rapid development in computational methods and the knowledge accumulation of radiation damage mechanisms show great promise in theoretical predictions of micro- and meso-scale structural evolution in irradiated materials. Atomistic simulations such as first principles, molecular dynamics and Monte Carlo methods enable one to accurately calculate thermodynamic and kinetic properties such as the generation, resolution, and clustering of defects in irradiated materials. Mesoscale phase-field method shows its powerful capability in predicting phase stability and microstructure evolution kinetics in chemically and microstructurally complicated systems, not only for systems close to the equilibrium state, but also for systems far from equilibrium. One of the strength of this method lies in the natural and straightforward way with which multiple material processes such as multiple spices diffusion under non-uniform temperature and stress field, migration of gas bubbles, and elastic-plastic interaction are accounted for. Our team has been doing world leading research in micro and mesoscale modeling using phase-field method since 2000. In this proposal, we propose to develop an atomistically-informed (based on theoretical research using DFT, MD and so on), or experimentally proved and microstructurally-resolved, quantitative phase-field model for predicting the fission gas behavior in nuclear fuels and to provide accurate thermodynamic and kinetic data to enhance the prediction capability of nuclear fuel performance codes. This project will also provide a good opportunity for training young scientists and engineers in this technologically important area for China. The analytic tools and methodologies developed in this project may find wide applications in irradiation related microstructural evolution in structural materials in nuclear, functional materials in micro-electronics, photonics, and biomedical industries.

裂变核电站的核燃料在裂变时会不断产生裂变气体氙和氪。这些裂变气体在燃料中溶解度极低，会在燃料内聚集成气泡或释放到燃料-包壳之间的缝隙，从而导致燃料温度和压力升高，进而导致包壳受膨胀压力而损坏。目前计算核燃料老化行为的程序都是以气泡热力学性质的某些重要简化假设为前提，这严重限制了程序的预测能力，尤其不能用来指导新概念反应堆和先进核燃料的研发。我们的团队从本世纪初开始用相场方法研究材料微观和介观结构演变，取得了多项国际一流成果。本项目拟发展一个基于原子理论研究或有实验根据的、微观尺度充分精细的相场模型。该模型同时考虑多组元缺陷在非均匀浓度、温度和应力场里的扩散、相变、气泡迁移、弹塑性相互作用、晶粒形貌和晶界效应等多种物理过程的耦合，预测核燃料中裂变气体气泡的演化行为，为核燃料老化研究提供精确的热力学和动力学数据，强化核燃料性能的预测和设计能力，同时也为国家培养年轻的科技人才。

核燃料中裂变气泡集合（由很多气泡组成）的微观结构演变是一个极其复杂的过程。该过程不仅涉及到多个时间和空间尺度，如辐照级联过程中原子尺度的点缺陷的产生属于飞秒量级和纳米尺度，而长程扩散和宏观材料性能的变化则属于月年的量级和毫米甚至米的尺度，而且同时还涉及到多个物理过程的耦合，例如，辐照诱导气泡持续形核、气泡溶解合并、气泡与迁移空位以及间隙原子之间的交互作用、高压气泡之间的弹性相互作用、气泡与固态燃料之间的温度梯度（由气泡与晶体之间不同的热导率造成）、气泡在温度梯度和应力场内的运动、位错和晶界作为间隙原子和空位的陷阱等等。所有这些都对于气泡的演化动力学、材料肿胀、裂变气体释放起着重要的作用，进而影响核燃料的性能和安全性。虽然人们已经在某些方面做了一些研究工作，但离全面考虑上述因素还相差很远。事实上，能够完整考虑上述因素、多场耦合、定量的理论模型，在本项目之前尚不存在。..本项目的主要研究目标为：发展一个有实验根据的、微观尺度充分精细的、定量的、能用于预测多物理场耦合（如温度场、应力场、缺陷浓度场等）的、UO2中裂变气体行为的相场理论框架和计算体系，并用它研究辐照条件对气泡形核、迁移、溶解、合并、长大，和多晶燃料中晶界对缺陷的影响。同时为国家的核电工业和材料科学领域的发展培养博士和博士后等高级人才。上述研究目标已经完成。本项目开发出来的部分计算方法，已经或正在被用于研究其他相关现象，如核材料（如锆合金）氧化、合金的电化学腐蚀。