Material which had been irradiated with neutrons to low doses was examined by using transmission electron microscopy. It was noted that a lower number density of defect clusters was observed at 473 than at 573K. The development of dislocation loops and voids occurred at 623K, and the formation of irregularly-shaped dislocation loops occurred at 673K. The accumulation of a large number of invisible vacancy-type defect clusters, during irradiation at 473K, was strongly suggested by the results of experiments in which the temperature was cycled.

Invisible and Visible Point Defect Clusters in Neutron Irradiated Iron. M.Horiki, T.Yoshiie, M.Iseki, M.Kiritani: Journal of Nuclear Materials, 1999, 271-272, 256-60