Yuanzineng kexue jishu (Feb 2024)
Study on Thermal Ratchet Effect of Nuclear Double Walled Vessel and Its Supporting Structure Based on Nonlinear Method
Abstract
The nuclear double walled vessel is a common structural form of high temperature reactor body, which assumes the dual safety functions of primary pressure boundary and primary coolant "defense in depth" containment. However, the temperature of the inner container changes dramatically under transient conditions, and the thermal conductivity of the inner and outer containers is poor. The temperature difference in the radial direction (between the inner and outer containers) and the height direction of the double walled vessel is large, and the temperature of the double walled vessel is easy to appear in the phenomenon that the temperature of the inside is higher than that of the outside in the heating stage and the temperature of the inside is lower than that of the outside in the cooling stage. The temperature distribution of the double walled vessel is not uniform and the thermal deformation is not coordinated, the thermal ratchet effect is a possible important failure mode for the main discontinuous position of the nuclear double walled vessel. In this paper, a reasonable axisymmetric finite element model of a nuclear double walled vessel and its supporting structure was established, and the accuracy of the finite element model's temperature and stress analysis was verified. The temperature field under transient conditions was calculated by considering the three heat transfer modes of conduction, radiation and convection. The nonlinear stress and strain response of the structure under the heat-mechanical cyclic load was analyzed by considering the elastic-plastic of the material and geometric large deformation of the structure. The law and characteristics of ratchet effect distribution were studied based on the above analysis results. The results show that the thermal ratchet effect is significant, which is the main failure mode of this type of vessel. The thermal ratchet sensitive zone which locates at the main connection position of the double walled vessel of the supporting structure has the characteristics of multi-point distribution, and different parts of the inside and the outside are involved among which the connection position of the inner container's head and supporting ring is more sensitive. The plastic strain accumulation behavior in the danger zone mainly appears in the cooling stage of the cyclic temperature load, and the strain increment gradually becomes stable with the increase of the number of cycles. It is pointed out that the thermal ratchet effect is an important problem to be paid attention to in the design of nuclear double walled vessels and the nonlinear analysis method is an effective method to solve this problem. This study can provide important method guidance for the thermal ratchet' design of nuclear double walled vessels, the optimal design of operating parameters of primary circuit and the arrangement of strain monitoring points.
Keywords
