Fast vs. thermal spectrum systems It is generally acknowledged that a fast neutron spectrum is more advantageous than a thermal spectrum with regards to TRU transmutation [Wiese H.W., 1998] simply because of the fact that fast neutrons are capable of inducing fission in almost all actinides while thermal neutrons can induce fission in only about half of them. This feature of the fast spectrum leads to better neutron economy. In the thermal spectrum system, neutrons are required first to convert some of the actinides to fissile nuclides by neutron capture and only then destroy them by fission. Moreover, the capture to fission cross-section ratio is comparable in magnitude for most of the nuclides in the fast spectrum which prevents buildup of higher MA isotopes (e.g. Cm) [Salvatores M., 2002]. The effectiveness of TRU transmutation in thermal reactors can be argued by the fact that MAs have large absorption cross sections in thermal spectra. In such spectra, the conversion of MAs by neutron capture to fissile nuclides can be very rapid. The high fission cross sections of the fissile isotopes in thermal spectra in turn allow their fast and effective destruction. In addition, the absolute magnitude of the thermal spectrum cross sections for neutron absorption is 200-300 times larger than those for fast neutrons. Thus, at a given power level a thermal spectrum system requires a significantly smaller actinide inventory, even though fast systems operate at higher neutron flux levels than thermal systems. This ultimately implies that thermal spectrum systems will discharge a smaller amount of minor actinides for reprocessing and, therefore, potentially reduce reprocessing costs. However, this is to be evaluated against particular designs of both the manufacturing and reprocessing facilities. For example, pyrochemical reprocessing and advanced fuel fabrication techniques [Wade D.C. et al., 1988] or on-line fuel reprocessing for the molten salt fuel systems [Vergnes J. et al., 2002] may offer significant benefits with respect to the economics of waste transmutation. In the case of LWRs, the introduction of TRU into the core may reduce the requirements for burnable absorbers. The neutrons captured in even-even TRU nuclides which cannot be directly fissioned by thermal neutrons are not lost as in the case of burnable poison but transmuted to useful fissile nuclides. Careful choice of TRU amount and elemental composition can reduce the reactivity swing of the core to an extent where burnable absorbers would no longer be needed. The possibility of using TRU as a “fertile” poison in long-life reactor cores is demonstrated in [Peryoga Y. et al., 2002].
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