The fusion-fission hybrid reactor is a promising technology that is likely to assume more and more importance in the global energy scenario in the coming years. Although this kind of nuclear system dates back to the earliest times of the fusion projects (when it was recognized that using fusion neutrons to “support” nuclear fission fuel cycle could widely increase the exploitation of the fusion plants), it appears to receive relatively limited attention since the mid-1980s. Notwithstanding, hybrid fusion fission systems have been already studied for some decades, in the most prominent laboratories and a relatively large bibliography was produced. Obviously much more papers on this topic have been published in more recent years. The fusion-fission hybrid concept can use both the nuclear fusion and fission processes: in a typical application, neutrons from fusion reactions can be used to sustain the fission chain of a sub-critical system. This is the basis of the hybrid reactor concept: neutron generation is not produced just in neutron-induced fissions, but also as a “by-product” of the fusion reactions inside the nuclear fusion reactor “core” (i.e., respectively, the void chamber for MCF or the fuel particles for ICF). This method allows to have an intrinsically safe facility (with a higher efficiency than a fusion reactor itself and a harder neutron energy spectrum than a fission reactor) which could be suitable for nuclear waste transmutation, too. In the last years, many initiatives on nuclear waste transmutation were proposed in order to reduce the long-term radiotoxicity of the wastes by eliminating a high fraction of the TRU from the SNF before their final disposal. In this frame, as already anticipated, hybrid fusion-fission systems could have an additional degree of freedom because of the independent source: this means that the neutron spectrum can be (reasonably) tailored for the transmutation purposes. In the present study a special focus has been devoted to the transmutation of SNF from fission reactors loaded in a fusion system, operated under the hypothesis to take into account the behaviour of a planned “real” (i.e. pulsed) MCF (ITER-like) plant.
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