SANS response of VVER440-type weld material after neutron irradiation, post-irradiation annealing and reirradiation

We present the first small angle neutron scattering (SANS) study of neutron irradiated, annealed and reirradiated VVER440-type reactor pressure vessel weld material. The SANS results are analyzed in terms of the size distribution of irradiation-induced defect/solute atom clusters and of the ratio of total and nuclear scattering intensity in a saturation magnetic field (A-ratio). The measured A-ratio is compared with calculations performed for the cluster composition reported by Pareige et al. [Phil. Mag. 85 (2005) 429-441] for a similar weld material investigated by means of three-dimensional atom probe tomography. The observed deviation between both estimates and possible reasons for the discrepancy are discussed. Special emphasis is placed on the differences between the materials response to the original irradiation and to reirradiation after annealing. We have observed that reirradiation-induced clusters are different in composition and their formation saturates at a lower volume fraction than in the case of the original irradiation.