Modification of embrittlement correlation method of reactor pressure vessel steels. Calibration to the surveillance test data at high fluences

Neutron irradiation embrittlement of reactor pressure vessel steels (RPV) is a phenomenon that needs to be properly monitored and predicted for the safe operation of nuclear power plants. The current embrittlement correlation method to predict the amount of embrittlement of RPV steels in Japan was developed by CRIEPI in 2007, and is specified as the domestic embrittlement correlation method in JEAC4201-2007. Recently, some new surveillance data obtained after the development of the method showed a trend of a larger amount of embrittlement at high fluences, which led us to revise the current correlation method. Microstructural characterization of the recent surveillance materials indicates that the mechanism observed in the materials is consistent with the one employed in the current correlation method, suggesting that the calibration of the equation coefficients of the method to the new data set is the appropriate way to improve the capability of the method. Some modifications were made on the definition of the objective function which is to be minimized during the calibration process, and a weight calculated as the value of EFPY2 was assigned to each of the surveillance data so that the data obtained after long-term operation are considered with larger significance in the mathematical process. The revised correlation method allows us to predict the Japanese surveillance data with better accuracy. Analysis of the prediction residuals suggests the margins to be 22degC for as-calculated values and 18degC for the values with heat-specific adjustment. (author)