SUBSTRUCTURE OF IRRADIATED MOLYBDENUM.

Molybdenum specimens, irradiated at reactor ambient temperatures in the ORR to a fast neutron fluence of similar 1 multiplied by 10**2**0n cm** minus **2 and at six different temperatures in the range 425-1000 C in the EBR-II to a fast neutron fluence of similar 1 multiplied by 10**2**2n cm** minus **2, were examined using a 200 kV transmission electron microscope. Post-irradiation annealing and annealing plus tensile testing was performed on some of the specimens prior to the TEM evaluations. In the unstressed regions, the cluster defects were found to be mainly black spots and small loops up to an annealing temperature of 830 C. These loops were found to grow in size and decrease in density at temperatures above 830 C. In the fracture region, the loop density was found to be higher and the loop size smaller. The loop density and size reached a limiting value in the samples annealed at temperatures between 1025 and 1200 C. The vacancy/interstitial nature of the loops at a post-irradiation anneal of 950 C was also analyzed. The loops were found to be mainly of the vacancy type.