A study of time-dependent, low energy (2 × 10-4 eV<E<0.2eV) neutron spectra and tritium breeding in 3He + C blanket of a fusion reactor

Recently, Sumita et al. (Fusion Eng. Des., 18 (1991) 355–360) have shown that useful information can be obtained about tritium production reaction rates by studying the time-dependent neutron spectra following a pulse of 14 MeV neutrons. In this paper, we report the time-dependent tritium production rate (TPR), total (time-integrated) tritium breeding ratio (TBR) and neutron spectra following a thermal pulse in finite and infinite 3He + C assemblies with two different atomic densities of 3He and different values of η (the ratio of cold to epicold neutrons in the injected pulse). These results have been obtained by solving the multigroup diffusion equation as an eignvalue problem. infinite assembly with a natural atomic density of 3He in graphite, TPR remains essentially constant up to about 1 μs for all the values of η considered here. It shows a gradual decrease in the range 1–10 μs and at later times the fall is rather steep. As expected, the TPR value increases as η increases, but remains less than unity. For the higher atomic density, our results exhibit the same qualitative features. However, the TPR value remains constant only up to 100 ns after the injection of the pulse, and the constant value of TPR is greater than unity. The time-dependent neutron spectra show significant changes only in the sub-Bragg region. lculations also show that the TBR value increases with assembly size for a given value of η and/or atomic density of 3He in graphite. From these results, we may conclude that 3He is a viable substitute for lithium for tritium breeding.