Analytical approach to estimate the thermal stress distribution of reactor pressure vessel nozzle corners with a constant cooldown rate

Abstract

Concerns about the integrity of the nozzle corners of reactor pressure vessels have been raised due to the long-term operation of nuclear power plants. Because ASME BPVC Section XI Appendix G (2013 edition) provided a stress intensity factor solution for nozzle corners, little progress has been made in relevant research. Using the Finite Element (FE) method for stress analyses appears to be successful; nevertheless, FE analyses of three-dimensional models are still computationally expensive. Therefore, this study develops a simplified approach to estimate the thermal stress distribution of nozzle corners. The thermal load is assumed to be caused by a change in the coolant temperature during normal cooldown operation. The cross-section temperature gradients on the nozzle corners are predicted based on the geometric shapes and through-wall temperature distributions of a cylinder and a semi-infinite plate. Thereafter, the cross-section thermal stress distributions are estimated based on the predicted temperature gradients; using the theory of circular plates and cylindrical shells. Three-dimensional FE analyses are carried out to validate the proposed approach. Six different nozzle geometries are considered based on pressurized water and boiling water reactor designs. The comparisons show that the estimates of the temperature and thermal stress distributions are in good agreement with the FE results. In addition, differences among the stress intensity factors calculated in accordance with ASME BPVC Section XI Appendix G are discussed.