Small-angle neutron scattering of low-Cu RPV steels neutron-irradiated at 255 °C and post-irradiation annealed at 290 °C

•Three low-Cu pressure vessel steel were irradiated at 255 °C and post-irradiation annealed at 290 °C.•By means of SANS it was found that the irradiation-induced features do not dissolve upon annealing.•In two cases an increase in scattering is found.•A cluster–matrix interface model rationalizing the measured intensity increase is proposed.•The influence of residual elements on the observed annealing effect is discussed.

Fast neutron irradiation of low-Cu reactor pressure vessel (RPV) steels gives rise to the formation of solute clusters and a degradation of mechanical properties. A remarkable acceleration of cluster formation was identified in a previous investigation of low-Cu steels irradiated at 255 °C. The thermal stability of the observed features at a typical RPV operation temperature (290 °C) is of particular interest. Small-angle neutron scattering experiments for three low-Cu materials irradiated at 255 °C and exposed to post-irradiation annealing at 290 °C are reported. The results indicate stability of the irradiation-induced features at a temperature of 290 °C for all three materials. In two cases, even an increase in scattering intensity upon post-irradiation annealing has been observed. The effect of the annealing treatment on the nature of clusters is discussed and a model of the cluster–matrix interface is introduced to rationalize the observed increase in scattering intensity. The role of the residual elements is discussed.