核燃料多级包容结构冲击条件下破坏的研究

The structural integrity of the nuclear fuel containment under impact accident is an important safety requirement. To avoid the radioactive material escaping into the environment, hierarchical structures, include ceramic coated particle, graphite matrix element and metallic canister, are utilized in high temperature gas-cooled reactors. Under impact loading, the stress waves will propagate in the layers of structures and reflect at the interfaces. For the complicated distribution of energy, it is difficult to determine the behavior of failure and the percentage of damage. To solve this problem, this project proposes a step-by-step analysis method based on the material properties which achieved by experiments. The procedure is as following. Firstly, by coupling discrete element method and finite element method, the displacement of canister and the interaction between fuel elements and canister will be calculated. Secondly, the damage of fuel element will be computed with the peridynamic model. Finally, the crack propagation direction at the interface of graphite matrix and particle surface will be analyzed to obtain the failure probability of the ceramic layer. After verified by typical experiments, the above procedure will constitute an integrated method for analyzing hierarchical structures and evaluating the damage of the inclusive boundary. The achievement of this project will be used to design the containment structures of nuclear fuels and research on the similar hierarchical structures.

核燃料在冲击事故工况下的包容完整性是保障核安全的关键。为了实现核燃料包容的高安全性，在高温气冷堆中采用了陶瓷包覆颗粒、石墨基体元件和金属贮罐的多级包容结构。然而，此种结构下，在冲击发生时应力波会在各级包容结构间形成复杂的传播和散射，为确定其破坏形式和破损概率带来了困难。本项目针对这一问题，提出了在利用实验测定各级材料参数后，逐级进行数值和理论分析的研究方法，首先利用有限元和离散元耦合的方法，确定燃料元件与金属贮罐间的作用力和贮罐的破坏行为；再采用非局部模型，对燃料元件的破坏进行分析；最后对裂纹扩展到陶瓷包覆颗粒与石墨基体界面时的扩展行为进行理论分析，从而得到冲击条件下，陶瓷包覆颗粒的破坏概率。通过以上分析并结合有代表性的验证实验，建立起核燃料多级包容结构的冲击破坏分析方法以及包容边界破坏概率的评估方法，为核燃料包容结构的改进设计以及多级材料结构的冲击破坏机理研究奠定理论基础。

核燃料在冲击事故工况下的包容完整性是保障核安全的关键。为了实现核燃料包容的高安全性，在高温气冷堆中采用了陶瓷包覆颗粒、石墨基体元件和金属贮罐的多级包容结构。然而，此种结构下，在冲击发生时应力波会在各级包容结构间形成复杂的传播和散射，为确定其破坏形式和破损概率带来了困难。本项目针对这一问题，利用实验测定及文献中已有的各级结构材料参数，对层级结构进行了数值和理论分析，采用有限元和离散元耦合的方法，不仅得到了容器的屈曲和破坏行为，而且得到了燃料元件与金属贮罐间的作用力，进而对燃料元件的破坏概率进行了评估。这一方法为类似核燃料贮存和运输容器在事故工况下的评估提供了依据，也为容器的优化设计提供了基础。