Irradiation of A-302-B steel has been performed for fast-neutron fluences up to 1021 n/cm2 (E > 500 keV). Charpy impact and hardness results point to a parallel evolution of these mechanical property values. On a semilogarithmic graph an increase in slope by a factor of 4 is observed between 3 and 5 × 1019 n/cm2. At 8.5 × 1020 n/cm2, increases in ductile-brittle transition temperature and in hardness were 305°C and 160 kg/mm2, respectively. Subsequent annealing treatments between 400 and 500°C indicate that major recovery can be attained. An explanation of the embrittlement and recovery evolution, based on the Davidenkov criterion, is developed.