A Study on Pressure-Temperature Limit Curve for Beltline and Outlet Nozzle According to New Regulation Requirements

Recently, the public interest in the safe operation of nuclear power plants (NPPs) is increasing due to severe accidents like Fukushima accident in Japan. As the operating lifetime of NPPs increases worldwide, it is not surprising to encounter material deterioration in NPPs. Therefore, it becomes more important to guarantee the safety of operation conditions in NPPs under high temperature and high pressure. The reactor pressure vessel (RPV), one of the principal components of NPPs, contains the neutron fuel and the internal structure. Material deterioration would be present in the RPV caused by neutron irradiation. Embrittlement of the material yields an increase of nil ductile transition reference temperature and reduces the fracture toughness of the material. Therefore, it is very critical issue to determine operating pressure and temperature limits in order to prevent brittle fracture at low temperature.

To ensure the structural integrity of RPV, pressure-temperature (P-T) limit curves guide the proper combination of operation pressure and temperature. ASME Code Section XI, Appendix G suggests the procedure of generating P-T limit curve with the maximum cooling rate of 100 °F/hr. P-T limit curves are generated based on the relationship between the critical fracture toughness and the stress intensity factors determined by mechanical and thermal loads. They grasp the operating margin of NPPs graphically. Previously, US Nuclear Regulatory Commission admitted the beltline as a representative evaluation region considering neutron irradiation. However, a recent announcement stated that structural discontinuity of the nozzle could weaken the structural integrity of RPV according to Regulatory Issue Summary 2014-11. Therefore, the nozzle should be included as an important evaluation region in order to satisfy the new regulation requirements.

In this study, we create finite element(FE) models for the beltline and outlet nozzle of a RPV and then perform the internal pressure analysis. To verify the proposed FE models and procedure, mechanical stress intensity factors at the crack tip are compared with the suggested values from the code. After this verification, P-T limit curves are generated from 20 °F/hr to 100 °F/hr. Furthermore, more P-T limit curves are added at higher cooling rates above 100 °F/hr in order to investigate safety capability beyond the design standard in the event of accident. Consequently, the safe operating condition for a RPV is widely investigated by each P-T limit curve from 20 °F/hr to 200 °F/hr, resulting in a 2D P-T contour map which would be more intuitive and advanced guidance to the NPP operators.