High-Temperature Recovery of Tungsten after Neutron Irradiation

The recovery of neutron-irradiation-induced defects in tungsten has been investigated via isochronal resistivity studies after irradiations from 8.5×${10}^{17}$ to 1.5×${10}^{21}$ $n$ ${\mathrm{cm}}^{-2\mathrm{}}$ ($E_n>1\mathrm{}$ MeV). These studies have shown that after irradiation at reactor ambient temperature (∼70°C), recovery occurs in three principal recovery regions. The first region (stage III) occurs between 100 and 450°C, the second between 450 and 650°C, and the third (stage IV) between 650 and 1000°C. The temperature of maximum recovery of the stage-III region decreased from 360 to 270°C with increasing fluence. The temperature of the maximum recovery of the second region also appears to decrease slightly with increasing fluence. The temperature of maximum recovery of stage IV is independent of the neutron fluence over the fluence range utilized in this investigation. A residual resistivity, remaining after completion of the isochronal anneals, shows a linear (slope of 1) dependence on the thermal-neutron fluence. This is believed due to the formation of rhenium, as a result of thermal-neutron ($n, \gamma$) reactions in tungsten. The shifts of the peak temperatures of the lower-temperature recovery regions are believed to result primarily from the weakening of the lattice by rhenium at low temperatures and high thermal-neutron fluences. The shift of the stage-III recovery region may also contain effects due to interactions between overlapping damage regions. Above about ${10}^{20}$ $n$ ${\mathrm{cm}}^{-2\mathrm{}}$ ($E_n>1\mathrm{}$ MeV), a deviation from a linear dependence of the log of the resistivity on the log of the fast neutron fluence supports the approach of saturation.