高燃耗下核燃料颗粒内部的应力状态及关键影响因素研究

Stress State and Key Influencing Factor in High Burnup Nuclear Fuel Particle

高燃耗是先进反应堆堆芯的发展方向,高燃耗下核燃料内部微结构的精细化建模是燃料强度分析和性能评估的基础。本研究开发了高燃耗燃料颗粒微结构的自动化建模和力学计算程序,综合考虑燃料颗粒内气孔尺寸、位置的非均匀分布特征,系统分析了基体材料的力学性能、燃料颗粒间距、运行环境静水压力及裂变碎片损伤层对燃料颗粒开裂行为的影响规律。结果表明:燃料颗粒间距越大,燃料颗粒越不易开裂;裂变碎片损伤层的存在使得燃料颗粒开裂风险小幅增大;燃料颗粒内气孔尺寸、位置分布的非均匀性,会导致燃料颗粒从多处开裂,且颗粒在外层开裂的概率更大;开裂危险区普遍具有气孔尺寸较大且大气孔串联的特征;基体材料对燃料颗粒表面作用的约束压应力具有较大的波动性,但应力均值随燃料元件所受静水压力的增大而近乎线性增大;增大弥散燃料基体材料的弹性模量可在一定程度上抑制燃料颗粒的开裂行为;燃料元件所受静水压应力越大,燃料颗粒越不易开裂,而燃料颗粒间距缩小能削弱环境压应力的影响程度。本工作为高燃耗条件下弥散燃料安全评估及优化设计提供了分析方法及数值参考。

Advanced nuclear reactors are currently moving toward longer fuel cycles and higher burnup levels. Besides, high burnup levels can improve fuel economy and reduce the total amount of spent fuel in reactors. The formation of high burnup fuel particle is closely related to the failure behavior of ceramic dispersion fuel, and it is of great significance to focus on the strength analysis and cracking behavior of high burnup fuel particle. The representative volume element of dispersion fuel was innovatively established in this paper, and the heterogeneity of pore size and position in high burnup fuel particle was considered, corresponding parametric modeling and mechanical simulation procedure of high burnup fuel particle was realized automatically. The impacts of mechanical property of matrix, distance between fuel particles, ambient hydro-static pressure and damaged layer on the strength of the fuel particles were analyzed systematically. The results show that the distance between fuel particles increases, the less possibility of crack initiates from the fuel particle. The existence of damaged layer will increase the risk of cracking of the fuel particles slightly. Due to the stress interference of the heterogeneous pores, crack initiates from multiple sites within the fuel particle, and the danger zones are more possibility located at the outer space, which is consistent with the experimental observations. Besides, danger zones within the high burnup fuel particle were characterized by larger pores and pore strings. It can be inferred that the formation of larger pores is probably due to the continuous coalescence of small pores and their surrounding pores. The constraint pressure of matrix on the fuel particle is not a constant, specially, it fluctuates significantly, and its average value increases linearly with the ambient hydro-static pressure. Increasing the elasticity modulus of the matrix will inhibit the cracking behavior of the fuel particle, which reveals that the matrix should be designed more stronger and tougher. The higher the ambient hydro-static pressure, the less likely the fuel particle will crack. While shortening the distance between fuel particles, and the impact degree of ambient hydro-static pressure reduces. This work realizes the accurate calculation of the internal stress field of high burnup fuel particle and establishes a high burnup fuel cracking strength evaluation model based on fine microstructure modeling. Besides, this paper provides an analytical method and numerical reference for the failure study and optimal design of dispersion fuel under high burn-up conditions.