APFIM characterization of a high phosphorus Russian RPV weld

A microstructural characterization of a high phosphorus (0.035 wt% P) weld from the pressure vessel of a Russian VVER nuclear reactor has been performed. The microstructure of these steels consists of intragranular and intergranular vanadium carbonitride precipitates of average composition 51.3 ± 0.9 at% V, 18.8 ± 0.7 at% C, 22.1 ± 0.7 at% N, 4.9 ± 0.4 at% Cr, 2.4 ± 0.3 at% Mo, 0.36 ± 0.05 at% Fe, 0.07 ± 0.05 at% B and 0.03 ± 0.03 at% P. The lath and grain boundaries were also coated with a thin film of molybdenum carbonitride precipitates. The phosphorus coverage at the boundaries in the unirradiated material was ∼ 13% of a monolayer in agreement with predictions from the McLean model of equilibrium segregation. After neutron irradiation to a fluence of 1.15 × 1020 n cm−2, the phosphorus coverage had increased significantly to up to ∼ 60% of a monolayer. This result indicates that neutron irradiation significantly enhanced the phosphorus segregation process. Phosphorus and copper clusters were also observed in the matrix of the neutron-irradiated material.