Clusters of small loops were observed, using transmission electron microscopy, in fission neutron-irradiated material at high temperatures. The groups (rafts) of small dislocation loops were observed to one side of dislocations, or within large interstitial loops. The small loops in a raft were aligned in rows. The Burgers vectors of the small loops were the same as that of the dislocation or large loop. The small loops in the rafts were of vacancy type. It was also observed that dislocation dipoles were chopped into small loops. It was deduced that formation of the rafts occurred when interstitial atoms aggregated and grew into large convoluted loops. The dislocation dipoles on the convoluted loops were then chopped off to form rows of small loops. Because of this mechanism, the small loops in the rafts were destined to be opposite in type to the large interstitial loops.

Formation Mechanism of Clustered Small Loops (Rafts) in Fission-Neutron Irradiated Mo at High Temperatures. K.Yamakawa, Y.Shimomura: Journal of Nuclear Materials, 1999, 271-272, 41-5