Pressure vessel steel embrittlement management is needed to predict the tearing resistance at end of life, but also to update these predictions taking into account the evolution of the principal parameters, such as fuel management, thermohydraulics of the primary coolant circuit, and lifetime extension capability.
French 900-MW units are characterized by a high neutron fluence on the inner face of the pressure vessel at the predicted end of life. Electricité de France's (EDF) purpose is to establish the appropriate knowledge and software to predict the pressure vessel embrittlement during operation.
This work is in progress in two ways. First, appropriate existing software is being used to calculate the consequences in terms of neutron flux, on the inner face of the pressure vessel, of various operating conditions and fuel patterns, and to be able to select the best of these. Second, specific software is being developed to calculate the maximum neutron flux on the inner face of the pressure vessel, according to the contribution of the major fuel elements so we can appreciate the fluence level of each refueling pattern and choose the one that is less severe. These results have shown that a benefit of about 30% can be obtained by optimized patterns with high discharge burn-up.
Neutron fluence management can be successfully integrated in the core refueling calculation procedure, but further progress is necessary for complete validation and background to optimize the steel surveillance program.