Comparison is made of data on mechanical properties of high-purity iron single crystals irradiated with fission neutrons at the Japan Materials Testing Reactor (JMTR) and 14-MeV D-T neutrons at the Rotating Target Neutron Source (RTNS-II) at low fluences below 10²² n/m². Both irradiations below 363 K led to irradiation hardening and the suppression of a hump appearing on the temperature dependence of yield stress of unirradiated high-purity iron, which suggests that radiation-induced defects formed at low temperatures significantly influence thermally activated deformation of iron.

In the isochronal annealing experiments of yield stress, the RTNS-II-irradiated irons showed a high annealing resistance accompanied by three recovery stages: stage I (RT to 450 K), stage II (650 to 750 K) and stage III (900 to 950 K). Recovery stages of the JMTR-irradiated irons were observed in the temperature ranges of 500 to 600 K (state Ij) and 700 to 780 K (stage IIj), followed by no radiation effect above 800 K. These recovery stages, except for stage III, were explained from a series of annealing of interstitial clusters with size distribution. Defects responsible for stage III were associated with complex vacancy type defect-nuclear reaction products (helium or hydrogen).