Isochronal Resistivity Study of Defect Recovery in High-Neutron-Fluence Irradiated Molybdenum

An isochronal resistivity study from ∼70° to 1200°C has been completed on molybdenum irradiated at ∼70°C to fast neutron (En≥1 MeV) fluences from 1.1×1019 n/cm2 to 1.5×1021 n/cm2. For all but the highest fluence specimen, three major recovery regions are observed at about 0.15 Tm (Stage III), 0.22 Tm, and 0.31 Tm (Stage IV). It is considered that the lowest temperature region results from the recovery of self interstitials, and that the highest temperature region represents excess-monovacancy recovery if there is an excess, with possible contributions from the recovery of defect clusters. The intermediate region could contain contributions from recovering divacancies. An anomalous saturation effect occurs at ∼5×1019 to 1020 n/cm2 (En≥1 MeV) with a drop-off in the radiation-induced resistivity increment fraction at high fluences. This drop off in resistivity appears to result from the saturation and complete irradiation annealing of the Stage III defect due to its high mobility at the relatively high homologous-irradiation temperature (0.12 Tm) leaving essentially only a large recovery in the region of Stage IV after irradiation to the highest fluence. A nonannealing residual resistivity increment observed for the highest fluence specimen is attributed to the presence of Tc (and possible Ru) formed via thermal neutron (n, γ) reactions.