The influence of helium on microstructural evolution: Implications for DT fusion reactors

The influence of helium on the microstructural evolution of irradiated metals is reviewed. The review encompasses data from past work involving charged particle irradiations and neutron irradiations in fission reactors, but emphasizes more recent results. This latter data was obtained from experiments in which either the reactor neutron spectrum or the isotopic content of the nickel in the irradiated alloys was tailored to yield a ratio of helium to displacement production (He/dpa ratio) that is near the value that will be obtained in a DT fusion reactor. Both the absolute level of helium present and the He/dpa ratio are shown to be important parameters. All major components of the irradiated microstructure: cavities, precipitates, and dislocations are shown to be sensitive to helium as a result of either its direct influence on cavity formation, or its indirect effect on point defect and solute partitioning. The results emphasize the importance of careful experimental and theoretical analysis if data from fission reactor experiments are to be used in fusion reactor design. In particular, the effects of helium appear to be greatest during the swelling incubation period, the time of most practical interest for fusion reactor designers. (orig.)