Demands for the safety of the Nuclear Power Plant (NPP) are increasing because of some accidents during decades. For the safety of the NPP, maintaining integrity of the reactor is one of the most important part. For the integrity evaluation of the Reactor Pressure Vessel (RPV), evaluation methods such as Upper Shelf Energy (USE), Pressurized Thermal Shock (PTS), and Pressure-Temperature (P-T) limit curve, etc. have been suggested by the ASME Code with consideration of neutron irradiation embrittlement. Among them, The P-T limit curve suggests limitations for the temperature and pressure during the operation of the RPV. The ASME Code Section XI Appendix G (Sec. XI App. G) suggests a method to generate P-T limit curves of the RPV [1]. There is restriction on the operation procedure; the cooling rate of the reactor is limited to 100 °F/hr or less and the available temperature range for the equations at the ASME Code is also limited to 100 °F/hr. However it is needed to cool down the reactor very fast at the severe accident condition to control the reactor to the stable condition and this sudden temperature drop can cause a thermal shock in the reactor. Therefore it is important to compensate the risk by accurately prepared P-T limit curve with high cooling rate for severe accidents in the NPP. In this study, researchers try to expand the limitation of the cooling rate for the P-T limit curve from 100 °F/hr to 200 °F/hr. Finite Element Analysis (FEA) for integrity evaluation and comparison of results using ASME Code equations were carried out.

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