Abstract In the normal operating conditions of sodium-cooled fast reactors (SFRs), the coolant near the core outlet keeps high temperature. However, in the scram conditions, the temperature of coolant from core outlet decreases sharply due to the reduction of core power. It makes the surface temperature of the central measuring shroud above the core outlet, which is used to supply the guiding of control rods and in-vessel measuring equipment, suddenly decrease. The maximum temperature change of the lower head of central measuring shroud (LHCMS) can reach 175 K in 20 s. Such condition will bring a severe thermal shock to the LHCMS. Several thermal shocks may bring fatigue damage to the LHCMS and threaten the safety of the reactors. Cladding can be used to protect the LHCMS from the fatigue damage of thermal shocks. Currently, there is no standard procedure for designing cladding of the LHCMS. In this paper, a numerical design procedure for designing cladding of LHCMS was proposed. The thermo-structural coupling method and ASME fatigue assessment method were used. Firstly, the models with different cladding thicknesses were established in finite element method (FEM). Secondly, the fatigue damage factors were calculated based on the strain results and fatigue assessment method, then, the design cladding thickness was determined according to Miner’s linear fatigue rule. In the meanwhile, if there is contact between the LHCMS and cladding, there will exist friction and squeeze that increase the stress and strain on the structures. Therefore, a reasonable gap distance should also be designed between the LHCMS and cladding after determining the cladding thickness. It should be noted that when designing the cladding thickness, in order to avoid the influence of the possible contact on the deformation of the LHCMS, special model settings were used so that the LHCMS and cladding can deform without interaction. Thirdly, the deformation values of the model with the design cladding thickness at different times were obtained, then the gap distance was determined according to the relative deformation values of the LHCMS and cladding. Finally, a model with the design cladding thickness and gap distance was established to verify design results. From the simulation results, under the boundary conditions of this paper, the cladding thickness should be 4 mm, and the gap distance between the LHCMS and cladding should be 4 mm. In addition, the effects of different cladding thicknesses on the temperature, stress and strain of the LHCMS were also studied. The results showed that under the scram conditions (temperature decrease), the temperature of the outside surface of the LHCMS increases by 10 K or 1.5%, the stress and strain of the outside surface of the LHCMS (away from the stress and strain concentration area) decrease by about 9% with every 1 mm increase of the cladding thickness at 20 s. Besides, the stress and strain concentration will increase the stress and strain by 17% to 21%. The numerical design procedure and results of this paper can provide reference and data for the design of the LHCMS cladding in SFRs, and thereby having good potential application.

中文翻译：

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SFR 中抗热冲击的中央测量罩下封头的包层设计

摘要 在钠冷快堆（SFR）正常运行条件下，堆芯出口附近的冷却剂保持高温。然而，在紧急停堆条件下，由于堆芯功率的降低，堆芯出口冷却剂的温度急剧下降。它使核心出口上方的中央测量罩的表面温度突然降低，该测量罩用于为控制棒和船内测量设备提供导向。中央测量罩 (LHCMS) 下封头的最大温度变化可在 20 s 内达到 175 K。这种情况会给 LHCMS 带来严重的热冲击。多次热冲击可能会给 LHCMS 带来疲劳损坏，威胁反应堆的安全。包层可用于保护 LHCMS 免受热冲击的疲劳损坏。现在，LHCMS 的包层设计没有标准程序。在本文中，提出了一种用于设计 LHCMS 包层的数值设计程序。使用热结构耦合方法和ASME疲劳评估方法。首先，采用有限元法（FEM）建立了不同熔覆层厚度的模型。其次，根据应变结果和疲劳评估方法计算疲劳损伤系数，然后根据Miner线性疲劳法则确定设计熔覆层厚度。同时，如果LHCMS与包层接触，就会产生摩擦和挤压，从而增加结构上的应力和应变。因此，在确定熔覆层厚度后，还应设计合理的 LHCMS 与熔覆层之间的间隙距离。需要注意的是，在设计熔覆层厚度时，为了避免可能的接触对LHCMS变形的影响，采用了特殊的模型设置，使LHCMS和熔覆层可以在没有相互作用的情况下变形。再次，得到模型在不同时间与设计包层厚度的变形值，然后根据LHCMS和包层的相对变形值确定间隙距离。最后，建立了具有设计包层厚度和间隙距离的模型以验证设计结果。从模拟结果来看，在本文的边界条件下，熔覆层厚度应为4 mm，LHCMS与熔覆层之间的间隙距离应为4 mm。此外，不同包层厚度对温度的影响，还研究了 LHCMS 的应力和应变。结果表明，在紧急停堆条件下（温度降低），LHCMS 外表面温度升高 10 K 或 1.5%，LHCMS 外表面（远离应力应变集中区）的应力应变) 在 20 s 时，覆层厚度每增加 1 mm，减少约 9%。此外，应力应变集中会使应力应变增加 17% 到 21%。本文的数值设计过程和结果可为SFRs中LHCMS包层的设计提供参考和数据，具有良好的潜在应用价值。LHCMS 外表面（远离应力和应变集中区）的应力和应变随着 20 s 熔覆层厚度每增加 1 mm 减少约 9%。此外，应力应变集中会使应力应变增加 17% 到 21%。本文的数值设计过程和结果可为SFRs中LHCMS包层的设计提供参考和数据，具有良好的潜在应用价值。LHCMS 外表面（远离应力和应变集中区）的应力和应变随着 20 s 熔覆层厚度每增加 1 mm 减少约 9%。此外，应力应变集中会使应力应变增加 17% 到 21%。本文的数值设计过程和结果可为SFRs中LHCMS包层的设计提供参考和数据，具有良好的潜在应用价值。