Neutronic considerations for direct enrichment and spent fuel regeneration in hybrids without reprocessing

Guidelines for the direct enrichment and spent nuclear fuel rejuvenation without reprocessing in the hybrid reactor blanket are presented and some underestimated effects occurring in the hybrid reactor blanket are investigated. It is demonstrated that the neglecting of resonance self-shielding in fertile materials leads to significant overestimation of the fissile production. The possibilities of effective fission suppression, difficult at higher fissile concentrations but necessary for the protection of bred fuel and for the power minimization are indicated. The unnegligible gain for the delayed fissile build up and thus the reduction of actual enrichment in the blanket due to intermediate nuclides in the breeding cycle are evaluated. The fast neutron attenuation and slowing-down should result of collisions with heavy nuclei (e.g. Pb) or low absorption and high (n, 2n) reaction cross-sections, while the use of a moderator is not recommended. Thus, the sufficient neutron multiplication without the undesirable excessive neutron energy decrease and dispersion during the slowing-down can be obtained. The hybrid performance (1350 kg/GW(th)yr of U 233 at 3% enrichment) obtained on the basis of the above indications suggests reconsideration of the hybrid as a nuclear fuel enricher-regenerator without reprocessing. (orig.)