Atom probe tomography and small-angle neutron scattering applied to reactor pressure vessel weld material from decommissioned nuclear power plant

Trepans of weld metal were taken from the decommissioned reactor pressure vessel of Greifswald Unit 4. As-irradiated material from two through-thickness positions (Layers 4 and 6) of the weld was characterized by SANS and APT. For SANS, irradiated and annealed (475°C/152 h) material was taken as reference. We have found that (1) Cu and Si levels are higher in both matrix and clusters of Layer 6. This causes more and larger clusters to form in Layer 6 resulting in higher hardness and BDT temperature, although neutron exposure was lower for Layer 6. (2) The mean cluster composition and concentration deduced from APT are in considerable disagreement with SANS data. A possible overestimation of Fe in clusters and the presence of vacancies may solve the discrepancy.