Evaluation of Pressure-Temperature Limit Curves for Reactor Pressure Vessel Nozzle using Ex-Vessel Neutron Dosimetry(EVND) and Surveillance capsule Data

Nuclear power plants are regulated by the law to identify the tendency of irradiation embrittlement according to the surveillance test results for the reactor pressure vessel material during the operating period. The regulation in Korea for surveillance tests is Nuclear Safety Commission Notice No. 2021-28 "Standards for Surveillance Tests fur Nuclear Reactor Pressure Vessels." This regulation requires that the fast neutron fluence be monitored regularly during the life of the reactor pressure vessel and that the material fracture toughness value be evaluated according to the neutron fluence to verify the integrity of the reactor pressure vessel until the end of its life. Surveillance capsules are installed in the reactor pressure vessel during the construction stage of the nuclear power plant and are regularly withdrawn for testing. After withdrawing the surveillance capsule and the Ex-vessel neutron dosimetry(EVND), the test results are used to evaluate the Pressure-Temperature(P-T) limit curve for the reactor pressure vessel. The P-T limit curves have been traditionally evaluated based on the beltline region, which is most affected by neutron irradiation. Due to the geometric discontinuity, the inside corner regions of the vessel nozzles are the most highly stressed regions of the reactor vessel. These higher stresses can potentially result in more restrictive P-T limits. Therefore, the NRC issued Regulatory Issue Summary(RIS) 2014-11, which requires the consideration of reactor vessel nozzles in P-T limits curve evaluation. In this paper, the neutron fluence for the Westinghouse 3-loop and OPR-1000 reactor pressure vessel nozzle were evaluated using the neutron dosimeter close to reactor pressure vessel nozzle. The P-T limit curves for Westinghouse 3-loop and OPR-1000 reactor pressure vessel nozzle at 48EFPY were evaluated. And then, Westinghouse. 3-loop and OPR-1000 reactor pressure vessel nozzle P-T limit curves were compared to the beltline region P-T limit curves for those plants.