Small-angle neutron scattering study of post-irradiation annealed neutron irradiated pressure vessel steels

Neutron irradiated reactor pressure vessel steels of two different Cu contents exposed to isochronal post-irradiation annealing treatments at stepwise increasing levels of temperature have been investigated. Results of small-angle neutron scattering experiments are reported and compared with the respective unirradiated and as-irradiated conditions. Volume distributions of scatterers and ratios of total-to-nuclear scattering intensities (A-ratio) were calculated from the measured differential macroscopic scattering cross-sections. Finally, total volume fractions of scatterers, peak radii of the volume distribution and partial information on the composition of scatterers were extracted and discussed as a function of Cu level and annealing temperature. Our measurements indicate that irradiation-induced clusters dissolve completely for the lower Cu level of 0.15 wt% but dissolve only partly prior to the onset of coarsening for the higher Cu level of 0.29 wt%. An Arrhenius type behaviour and a remarkable reversibility of the irradiation-induced cluster distribution are observed in the complete dissolution regime.