Defect particles with typical mean radii of 7 to 9 Å were detected by SANS in neutron-irradiated (E > 1 MeV) steel specimens cut out of the same block and subjected to four various fluences in the range (0.5 to 5.0) × 10¹⁹ n/cm². In addition to the as-irradiated condition, the effect of a heat treatment after irradiation and, in another series of experiments, the consequences of an anneal in between the two halves of the irradiation period were studied.

The forged RPV material contains 0.173 wt% Cu and 1.47 wt% Mn; the irradiations at the given fluences were performed at 290°C in a research reactor. The SANS results show that: 1. In the as-irradiated condition themean particle radius is ˜9 Å and the size distribution functions are reasonably sharp (ΔR/¯R = 0.25). For the smaller doses the mean radius is somewhat smaller (6 to 8 Å). The average copper content of the particles is _c = 50 to 70 at% calculated on the base of a copper-vacancy agglomerate model, and it varies but slightly with the fluence. For the volume fraction we obtain f= 0.15 to 0.18% except for the lowest fluence. 2. Post irradiation annealing (420°C/24 h) decreases the particle volume fraction as expected, and it also broadens the size distribution. The scattering contrasts show the significantly higher copper content in the damage particles (c = 85 to 90 at%) than in the as-irradiated material. 3. The irradiated-annealed (450°C/168 h)-reirradiated samples show a qualitatively different scattering pattern. One-peaked size distributions, even if very broad (ΔR/¯R ˜ 0.45), give at best a moderately good fit only to the small fluence data, and two characteristic particle sizes could be determined for the higher fluence samples (¯R₁ = 7 to 9 Å and ¯R₂ = 20 to 30 Å)with f= 0,07 to 0.14%.