EFFECTS OF NEUTRON IRRADIATION ON THE FRACTURE TOUGHNESS OF RPV MATERIALS: PREDICTION OF MATERIAL PROPERTY CHANGES FOR IRRADIATED EURO REFERENCE MATERIAL 'A' AND OTHER RPV MATERIALS

摘要

A micromechanistic model is used to estimate the irradiation-induced change in the Master Curve reference temperature for cleavage fracture as a function of the associated change in material yield stress relative to the yield stress in the unirradiated condition. The model is shown to predict well the behaviour of Euro Reference Material A (quenched and tempered 22NiMoCr37 ring forging) irradiated at temperatures of T = 285°C and T= 150°C with neutron fluences of 4.3E+19 n/cm~2 (E_n > 1MeV) and 3.1E+19 n/cm~2 (E_n > 1MeV) respectively. Further validation of the model is provided with reference to published data for a range of irradiated RPV plate, forging and weld materials, where measured and predicted values of the change in the Master Curve reference temperature are successfully compared. For the LWR materials and irradiation conditions considered, the fitted parameters of the model are consistent with the view that the primary effect of neutron irradiation is to increase the friction stress for plastic flow of crack-tip material, whereby the irradiation-induced change in yield stress may be associated with a change in a non-hardening, athermal term e.g. as described in the Zerelli-Armstrong constitutive equation. The model predictions compare well with trend curves due to Sokolov and Nanstad, and Wallin and Laukkanen. A particular advantage of the model, compared with these more general formulations, is that it is potentially better suited to a more detailed analysis and interpolation of RPV material datasets covering a range of irradiation conditions where flow properties are reasonably well characterised.