Neutron-induced swelling and embrittlement of pure iron and pure nickel irradiated in the BN-350 and BOR-60 fast reactors

Pure iron and nickel were irradiated in the range of 2–15 × 10−7 dpa/s at 345–650 °C to very high neutron exposures in two fast reactors, BOR-60 and BN-350, to study void swelling and changes in mechanical properties of these two metals. Both nickel and iron swell in this temperature range with the maximum swelling rate at ∼500 °C in nickel, but possibly at ⩽350 °C for iron. It also appears that the swelling rate in nickel and possibly in iron may be dependent on the dpa rate, increasing with decreasing dpa rate. The evolution of mechanical properties of the two metals is quite different. The differences reflect the fact that b.c.c. iron is subject to a low-temperature embrittlement arising from a shift in ductile–brittle transition temperature, while f.c.c. nickel is not. Nickel, however, exhibits high temperature embrittlement, thought to arise from the collection of transmutant helium gas at the grain boundaries. Iron is not strongly affected by transmutation since it generates much less helium during equivalent irradiation.