Contact model analysis on cladding designs against thermal shock for the lower head of central measuring shroud in a fast reactor

Abstract

The central measuring shroud in the fast reactor has the function to support the in-vessel measuring equipment and control rods. Ensuring its integrity is an important part of guaranteeing the safety of the reactor’s operation. When the fast reactor is under the scram condition, however, the coolant temperature drops rapidly at almost 14 K per second. Such a large temperature change will bring strong thermal shock to the lower head of central measuring shroud (LHCMS), which is located above the core outlet. Since thermal shock is one of the factors to cause initial crack or even damage of the LHCMS, cladding is always added to the LHCMS to protect it from thermal shock damage. However, there are no general design guidelines for the cladding. In the process of design, selection of the number of layers and layout schemes between layers require detailed numerical calculations, and the stress levels of different design schemes need to be analyzed. At present, most of the research focuses on the influence of material properties on thermal shock, and lacks of research on the effect of different cladding design on the stress of the cladding and LHCMS. In the paper, therefore, the finite element software was used to establish two models to analyze the stress difference between one-layer cladding model and two-layer cladding model with contact. The temperature and stress distribution of the two models were calculated according to the scram temperature curve of the fast reactor. And fatigue analysis was applied to give fatigue damage factors of two models. The results indicate that the maximum stress in the thickness direction appears on the surfaces in contact with the external fluid at the last moment, rather than the time temperature drops the fastest. Furthermore, because of irregular thermal expansion between layers, stress step shows on the contact surfaces, and the stress of the outermost cladding of the two-layer model is greater than the stress at the same position of the one-layer model, which means when the cladding layers are contacted with each other, even if the cladding is divided into multiple layers, which should reduce the stress, the stress caused by thermal shock cannot be reduced, but increased. Besides, the results of fatigue analysis also prove that the stress of two-layer model is greater than that of one-layer model. The study is useful for the cladding design of components in a fast reactor.