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.