ABSTRACT After the termination of a loss-of-coolant accident (LOCA), the reactor continues to be cooled for a long term until fuel assemblies are withdrawn from the reactor core. The fuel cladding tube degrades in strength due to high-temperature oxidation during a LOCA event. It is important to confirm that fuel rods exposed to LOCA conditions can withstand earthquakes during the long-term cooling in terms of preserving the coolable geometry of the reactor core. Finite element method analyses were performed to estimate the deformation of fuel rods in a fuel assembly under vibrations simulating an earthquake as well as the stress applied to the fuel cladding tube with a rupture opening. The localized stress at the rupture opening in the analyses was compared with the strength assessed through bending tests of the cladding tube samples that were ruptured and oxidized to less than 15% equivalent cladding reacted (ECR) in advance. As the result, the fuel rods are expected to be prevented from fracture due to bending at earthquakes during the post-LOCA cooling unless the oxidation of cladding tubes exceeds the limit defined in the current Japanese LOCA criteria, 15% ECR and a deflection of the fuel rodexceeds approximately 40 mm.

中文翻译：

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水平振动作用下熔覆管破裂应力分析

摘要 失冷事故（LOCA）结束后，反应堆会继续长期冷却，直到燃料组件从反应堆堆芯中取出。由于 LOCA 事件期间的高温氧化，燃料包壳管的强度会降低。重要的是要确认暴露于 LOCA 条件下的燃料棒可以在长期冷却期间承受地震，以保持反应堆堆芯的可冷却几何形状。进行有限元方法分析以估计在模拟地震的振动下燃料组件中燃料棒的变形以及施加到具有破裂开口的燃料包壳管的应力。将分析中破裂开口处的局部应力与通过预先破裂并氧化至小于 15% 等效包层反应 (ECR) 的包壳管样品的弯曲试验评估的强度进行比较。因此，预计在 LOCA 后冷却期间，燃料棒不会因地震弯曲而断裂，除非包壳管的氧化超过当前日本 LOCA 标准中定义的限制、15% ECR 和燃料棒超过约 40 毫米。