Characterization of nuclear transmutations in materials irradiation test facilities

Abstract

This study presents a comparison of nuclear transmutation rates for candidate fusion first wall/blanket structural materials in available fission test reactors with those produced in a typical fusion spectrum. The materials analyzed in this study include a vanadium alloy (V4Cr4Ti), a reduced activation martensitic steel (Fe9Cr2WVTa), a high conductivity copper alloy (CuCrZr), and the SiC compound. The fission irradiation facilities considered include the EBR-II (Experimental Breeder Reactor) fast reactor, and two high flux mixed spectrum reactors: HFIR (High Flux Irradiation Reactor) and SM-3 (Russian reactor). The estimated transmutation and displacement per atom (DPA) rates in these test reactors are compared with the calculated transmutation and DPA rates characteristic of a D-T fusion first wall spectrum. In general, past work has shown that the displacement damage produced in these fission reactors can be correlated to displacement damage in a fusion spectrum; however, the generation of helium and hydrogen through threshold reactions (( n,xα ) and ( n,xp )) are much higher in a fusion spectrum. As shown in this study, the compositional changes for several candidate structural materials exposed to a fast fission reactor spectrum are very low, similar to those for a characteristic fusion spectrum. However, the relatively high thermalized spectrum of a mixed spectrum reactor produces transmutation rates quite different from those predicted for a fusion reactor, resulting in substantial differences in the final composition of several candidate alloys after relatively short irradiation time. As examples, the transmutation rates of W, Ta, V, Cu, among others, differ considerably when the irradiation is performed under a mixed spectrum reactor spectrum and under a fusion first wall spectrum. The out-of-core positions in mixed spectrum reactors can be partially shielded against low energy neutrons, e.g. by hafnium, to reduce the transmutation rates, but the displacement rates at these positions are much lower than those for the core positions. Fast reactors (EBR-II) provide the only possibility for obtaining high damage rates without producing significant compositional effects in vanadium alloys, ferritic steels and copper alloys.