Stress and Fatigue Analysis of Thermal Shock for Cladding of Center Measurement Column Lower Head in the Fast Reactor Under Shutdown Condition

The center measurement column lower head, which is under long-term influence of the liquid sodium, locates just above the core outlet in the fast reactor. Under the shutdown condition, the sudden drop of coolant temperature will cause the center measurement column lower head to be affected by the strong thermal shock that will threaten the integrity of the lower head. Cladding is always added to lower head to protect it from thermal shock damage. Therefore, the calculation and analysis of stress and fatigue damage for cladding is significant. Based on the built complete model and shutdown temperature curve of reactor core outlet, using ASME fatigue evaluation method, the stress and fatigue damage of thermal shock in cladding is calculated. And the influence of different layers of cladding on the von Mises equivalent stress distributions is analyzed in the paper. The results show that under a given shutdown curve, the maximum von Mises stress emerges at the joints of cladding and the sleeve. As the number of layers increases, the von Mises equivalent stress of each layer decreases. Due to the existence of thermal resistance, the von Mises equivalent stress has similar distributions between two designs of layer and different distributions between layers. The study can provide design reference on cladding for relevant engineering design.