The evolution of the microstructure of the French Chooz A pressure vessel steel (characterized by a low copper content (<0.1 at.% Cu)) has been studied during annealing at 450°C. The irradiated material (fluence: 0.5 to 16×1023 n.m-2, flux : 2×1015 n.m-2.s-1, temperature : 275°C) shows a quick recovery of hardness after 20 h of aging. The intensity of the SANS scattered signal as well, as the thermoelectric power one, decreases with annealing time. In addition, 3D images, at the atomic scale, given by the Tomographic Atom Probe show that during the first hours of annealing, the neutron-induced Cu-Ni-Mn-Si clusters formed during the in-service irradiation are “dissolved” while copper precipitates are formed. Their low number density is in agreement with the full recovery of the hardness.
These results are of primary importance for prediction of recovery of an irradiation embrittled steel and also for prediction of its behavior during reirradiation. This work also shows that the nondestructive technique, thermoelectric power, may be used to “follow” the microstructural evolution of materials during annealing treatments.