高燃耗燃料包壳多轴热-机械疲劳特性及环境退化机理研究

To ensure the longlife and reliable service of some key components used in nuclear power, the project attemps to address the thermo-mechanical failure mechanism and fatigue life prediction model of high burn-up fuel cladding in advanced nuclear power plant due to the pellet cladding mechanical interaction (PCMI) during a reactivity-initiated accident (RIA) occurs. The following issues will be focused in the present project: (1) Illuminate the effect of multiaxial stress on deformation mechanism and fatigue resistance, clarify the influence of temperature, loading rate, stress ratio and multiaxial stress on ratcheting strain and its rate, realize the ratcheting and fatigue interaction mechanism. (2) Specific the thermo-mechanical fatigue failure of Zr alloy, investigate the roles of particular complicated loading conditions and temperature on hydride content, orientation,and distribution in detail, clarify the rules of competition behavior among thermo-mechanical loading, hydride concentration and oxidation on ductile to brittle transition of Zr alloy. (3) Develop a life prediction model through the combination of damage-coupled cyclic visco-plastic constitutive model and strain energy density theory. The effect of multiaxial stress, ratcheting strain, thermo-mechanical loading, dydride-induced degradation and oxidation on the fatigue life can be reflected in this model. In this interdisciplinary program, the mechancial, material and chemical analyses were included. Through the accomplishment of the project,the basic rule of PCMI during a RIA through the view point of thermo-mechanical- chemical analysis can be found and the life prediction model which can be used for engineering application can be developed. Also, this project could provide some important insights on the improvement of the long-life design and manufacturing of high burn-up fuel cladding in advanced nuclear power.

面向先进核电关键部件长寿命可靠运行的需求，针对反应堆反应性引入事故时高燃耗燃料包壳的热-机械疲劳失效机理及寿命预测模型开展研究。主要包括：（1）阐明多轴载荷对包壳材料变形机制及疲劳抗力的影响，澄清温度、加载率、应力比、多轴应力对棘轮应变及其累积速率的影响规律，揭示棘轮变形与疲劳载荷的交互作用机制；（2）开展锆合金热-机械疲劳失效研究，澄清复杂应力、温度对析出氢化物形态、分布及指向的作用机制，揭示热-机械载荷、氢含量、氧化对锆合金延脆转化行为的影响规律并建立映射关联；（3）基于损伤耦合的粘塑性循环本构模型和应变能密度理论，建立虑及多轴应力、棘轮应变、热-机械载荷、氢致退化、氧化效应的寿命预测模型。本项目体现了力学、材料和化学等多学科领域的交叉，可望从热-力学-化学角度阐明反应性引入事故时芯块与包壳相互机械作用的基本规律并建立预测方法，为高燃耗燃料包壳的寿命设计方法和制造水平提升提供科学支持。

面向先进核电关键部件长寿命可靠运行的需求，针对反应堆反应性引入事故时高燃耗燃料包壳的热-机械疲劳失效机理及预测模型开展研究。项目取得了如下重要成果：（1）在室温与高温条件下对锆包壳的单轴与多轴棘轮变形及失效行为进行了研究，澄清了平均应力、应力幅、加载率、加载历史、内压、温度和蠕变对棘轮应变及其累积速率的影响，结果表明棘轮应变随平均应力与应力幅的增大而增大，随加载率的增加而减小。高应力加载历史以及内压均对包壳管的后续棘轮应变累积具有显著的抑制作用。温度越高，锆包壳的棘轮应变及其速率显著增大。峰值应力保持时间越长，锆包壳的疲劳寿命越短，说明蠕变会显著加速锆包壳的疲劳失效。以上研究表明，棘轮变形累积若未出现安定，将会显著促进锆包壳的疲劳失效进程，导致包壳的变形失效甚至提前开裂破坏。（2）在单轴与多轴应力作用下对锆合金的变形机制系统地进行了研究。结果表明，包壳管在轴向拉伸作用下，柱滑移是主导变形机制；而在轴向压缩作用下，以柱滑移和{10-12}拉伸孪生为主，其中{10-12}拉伸孪生可协调包壳壁厚方向的应变。双轴加载会激活大量{10-12}拉伸孪生，表明拉伸孪生是双轴加载的主导变形机制。然而，孪生只可在一定程度上协调锆合金厚度方向的变形，双轴加载下试件会因为变形不协调而提前开裂。（3）微观组织对渗氢包壳管的疲劳及失效机制研究，探讨晶粒大小、织构对氢化物析出方向及开裂机制的影响，结果表明：相比于纤维态微观组织包壳管，等轴态微观组织包壳管的切向织构增加，导致更多的径向氢化物析出。径向氢化物容易产生微裂纹，并加速裂纹扩展，降低锆包壳的疲劳寿命。（4）提出损伤耦合的粘塑性循环本构模型，对锆合金在热-机加载下的应力应变响应进行了描述。结果表明，修正模型可准确描述锆合金在热机载荷下的应力应变滞环，尤其适用于应变幅较大的场合。本项目体现了力学、材料和化学等多学科领域的交叉，为高燃耗燃料包壳的寿命设计方法和制造水平提升提供科学支持。