Evaluation of transmutation and low induced radioactivity and requirements for candidate structural materials

The effects of fusion neutron spectrum and fluence on the evaluation of transmutation and induced radioactivity of candidate reduced and/or low activation materials such as 9Cr-2W ferritic steel, V-5Cr-5Ti and SiCf/SiC composite have been examined by simulation calculations using structural models based on ITER design. The transmutations from W to Re and Os in the ferritic steel and from V to Cr in the V alloy were predicted to considerably occur depending on thermal neutron fluxes mainly determined by the composition of blanket/shields while induced radioactivity decay behaviours are not appreciably affected by the spectrum change of fusion neutrons. The acceptable concentrations of impurities such as Mo, Nb and To for waste disposal and hands-on recycling have been more restricted with increasing fluence because of the accumulation of nuclides controlling the radioactivity. In the effort of manufacturing high-purity materials, fused-salt electrolysis for vanadium and chemical vapor infiltration for SiCf/SiC composite have been shown to be potential processes to obtain purified materials satisfying the low activation criteria.