A numerical procedure for determining the layer-to-layer gap distance of the cladding protecting lower head of central measuring shroud from thermal shock

Abstract

In a sodium-cooled fast reactor (SFR), there is a central measuring shroud locating above the core outlet, which can supply the guiding of the control rods and in-vessel measuring equipment. So its integrity is very important for the safety of the fast reactor. In normal operating conditions, the central measuring shroud keeps in high temperature. While, in some abnormal conditions, such as the scram conditions, the temperature of coolant from the core outlet will sharply decrease due to the decrease of the power. The cold coolant will flow past the hot central measuring shroud, especially the lower head of the central measuring shroud (LHCMS), which leads to strong thermal shocks to the LHCMS and threaten the integrity of the LHCMS. Cladding is often added on the outside of the LHCMS to protect it from thermal shock. The cladding can reduce the temperature change on the LHCMS, and thereby reducing the thermal stress caused by the sudden temperature drop. Usually, the cladding consists of several layers. The study (Zheng et al., 2019) showed that tight contact between the LHCMS and cladding layers is not acceptable. Therefore, the layout of cladding, such as the layer-to-layer gap distance, is crucial to the design. In the meanwhile, since the top of the cladding and the LHCMS are separately welded on upper supporting structure, gaps between the cladding and the central measuring shroud can be achieved.In order to design the gap distance between the LHCMS and cladding layers, a numerical procedure was established in this paper. In this procedure, first, a model without gaps was established using finite element method (FEM) to simulate the deformation of the LHCMS and each cladding layer. In this model, a special boundary condition was applied to ensure the nearly same deformation as that of the model with design gap distance. Second, the relative deformation between each layer could be obtained. After analyzing the relative deformation, the gap distance could be determined. Finally, the model with design gap distance was established to verify the design results. The model with larger gap distance was also established to study the effects of the different gap distances.The results of numerical procedure showed that the gap distance should be 1.35 mm according to the model without gaps, but it still needs to be given at least 14% margin according to the model with gaps. In the model with gaps, the stress distribution of each layer appears to be symmetrical because the two sides of the LHCMS and cladding layers have no constraints. Based on the fatigue assessment method from ASME, the number of allowable cycles of the model with gaps is much larger than that of the contact model, indicating the gaps are useful to reduce the stress level. In addition, the stress of the model will not change once the gap distance reaches the reasonable value. This paper provides a simplified design method and thereby having potential application for cladding designs.