Atom probe characterization of copper solubility in the Midland weld after neutron irradiation and thermal annealing

An atom probe field ion microscopy characterization has been performed to determine the copper matrix concentration in a submerged arc beltline weld of the Midland Unit 1 pressurized water reactor after four conditions: unirradiated, unirradiated and annealed for 168 h at 454°C, neutron-irradiated in a test reactor to a fluence of 1.1×10²³ n m⁻² (E>1 MeV) at a temperature of 288°C, and neutron-irradiated and annealed for 168 h at 454°C. Atom probe analysis of the unirradiated material revealed a substantial depletion of the copper in the matrix to 0.119±0.007 at.% Cu from the bulk value of between 0.18 and 0.28 at.% Cu. Annealing the unirradiated material produced intragranular copper-enriched precipitates and reduced the matrix copper level by 25% to 0.088±0.012 at.% Cu. Neutron irradiation also produced copper-enriched precipitates and reduced the matrix copper level by almost 50% over the stress relieved material to 0.058±0.008 at.% Cu. Annealing the neutron-irradiated material reduced the matrix copper level further to 0.050±0.010 at.% Cu. These results indicate that the annealing treatment coarsens the copper-enriched precipitates produced during neutron irradiation with a slight decrease in the matrix copper content.