Many stainless steels and nickel base alloys which are used as cladding materials in nuclear reactors exhibit embrittlement in high temperature tests following neutron irradiation. An electron microscope investigation has been carried out on three 20Cr-25Ni austenitic stainless steels irradiated at 650 and 750 C in a thermal neutron flux and tension tested at 750 C., and on an M316L austenitic stainless steel irradiated at 300 C in a fast neutron flux.

It is observed that heterogeneous precipitation of bubbles occurs during irradiation at 650 C and that during irradiation at 750 C homogeneous precipitation occurs as well. This behavior and the effects of postirradiation annealing are discussed in terms of the helium concentration and the vacancy concentration. It is shown that during high temperature straining, growth of grain boundary bubbles occurs, and that the bubble spacing and the shape of the grain boundary cracks obtained support the theory that embrittlement is due to the linking up of bubbles which have exceeded the critical size for continuous growth.