Damage observed in Mo irradiated with 14 MeV neutrons at RTNS-II

The damage in pure molybdenum irradiated at RTNS-II with 14 MeV neutrons was examined by electron microscopy. Annealed specimens were irradiated between 60 C and 450 C at fluences from 4 x 1019 to 7 x 1022 n/m2. Small defects clusters were observed in the irradiated specimens. The clusters, which become larger with increasing irradiation temperature and fluence, were determined to be interstitial loops from correlation of results from bulk and thin specimens. Groups of small loops were formed for high fluences at high temperatures. Voids were not observed in the specimens. The damage structures formed by fusion-neutron irradiation differed considerably from those formed by fission-neutron irradiation in a previous study. Whereas small interstitial loops are formed for fusion-neutron irradiation, large interstitial loops with groups of small vacancy loops inside them are formed for fission-neutron irradiation. This difference in damage structure is attributed to the change in deposited energy density. (orig.)