Irradiations at temperatures in the approximate range 0.3 to 0.5Tm to intermediate and high neutron doses, typical of those encountered in the cores of fast breeder reactors and projected for the first wall and breeder structural components in a DEMO D-T tokamak fusion system, produce significant density decreases and volume swelling of some austenitic steels [1] and other non-fissile metals and alloys. The swelling is associated with the formation of intragranular voids and dislocation loops.