Mechanistic modeling of transition temperature shift of Japanese RPV materials

A new correlation method to predict neutron irradiation embrittlement of reactor pressure vessel (RPV) materials of Japanese nuclear power plants is developed based on the understandings of the embrittlement mechanisms. A set of rate equations is constructed to describe the microstructural changes in the RPV materials during irradiation. Formation of copper-enriched clusters (CEC) and matrix damage (MD) are considered as the two primary causes of the embrittlement. Not only the effects of chemical compositions, such as copper and nickel, and neutron fluence, but also the effects of irradiation temperature as well as neutron flux are formulated in the rate equations to describe the evolution of CEC and MD. Transition temperature shifts corresponding to the microstructural changes are calculated using the predicted number densities of the CEC and MD. Coefficients of the rate equations are optimized using the Japanese surveillance database with a specific attention to reproduce the embrittlement trend of each material of the Japanese RPVs. The standard deviation of 12.1 C of the current Japanese correlation method, JEAC 4201, is reduced down to 10.6 C in the proposed new correlation method. Possibility of adjusting the uncertainty in the initial transition temperatures is discussed. (orig.)