Neutron irradiation embrittlement of reactor pressure vessel (RPV) steels is one of the key aging phenomena that need to be properly managed and considered in the plant operations. The current embrittlement correlation method in Japan, JEAC 4201-2007, was developed in 2007 using the surveillance database as of the end of year 2006. Since then, some new surveillance data were compiled, and some of the data from the materials irradiated to high fluences of 6×1019 – 1×1020 n/cm2, E>1MeV showed larger amount of embrittlement than the predictions of JEAC4201-2007 method. The microstructural changes in such materials were examined to identify the embrittlement mechanism of RPV steels at high fluences, and the modification of the JEAC4201-2007 was performed. This paper describes the summary of the microstructural characterization, and the details of the revision of the JEAC4201-2007. The revised embrittlement correlation method work well to the current Japanese surveillance data together with newly determined margins.

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