Creep Analysis of Hemisphere Shell Structure Under High Temperature Gradient

Abstract

The In-Vessel Retention (IVR) of molten core debris has been part of the severe accident management strategy for advanced pressurized water reactor (PWR) plant. Generally, the pressure is supposed to successfully be released, and the externally cooled lower head wall mainly experiences the temperature difference which may be more than 1000°C. However, the Fukushima accident shows that a certain pressure (up to 8.0MPa) still exists inside the reactor pressure vessel (RPV). Therefore, in order to make the IVR succeed, it is necessary to investigate the structural integrity of the RPV under the combined internal pressure and the thermal load on the lower head. Therefore, it is supposed that the lower head of RPV is a Hemisphere Shell Structure (HSS) with 2150mm external radius and 25mm wall thickness. Similarly, the outer wall temperature is supposed to be 127°C, the wall temperature difference 1200°C, and the material properties (i.e., thermal conductivity, specific heat capacity, yield strength, and so on) are dependent on the temperature. In this paper, the main subjects discussed are as below. First, the heat transfer analysis is carried out to obtain the temperature distribution of the HSS wall. Due to the high temperature gradient, there may present different failure modes along the HSS wall thickness, i.e., plastic failure and creep failure. Then, the stress and strain distributions along the wall thickness are analyzed by ANSYS finite element method where the Norton-Bailey type creep law is adopted. The result shows that the stress in the lower temperature part (LTP) whose temperature is lower than 405°C is lower than the yield stress of the material with the corresponding temperature. That is to say, the LTP still can bear a certain internal pressure. At last, to consider the combined impact of internal pressure and temperature difference on the HSS, the finite element analysis is carried out by adopting the isochronous stress-strain curves of the Chinese RPV material. The relationship between the stress in the LTP at 100 hours and the internal pressure is discussed, and the limit internal pressure is determined. It is concluded that HSS still can maintain its integrity under IVR, even if there exists a certain internal pressure.