Neutronic and nonproliferation characteristics of (PuO_2-UO_2) and (PuO_2-ThO_2) as fast reactor fuels

摘要

Computational core physics analysis carried out for a typical fast breeder reactor (FBR) core design is presented through two case studies; one using only (PuO_2-UO_2) MOX fuel and another that replaces ～41 % of (PuO_2-UO_2) MOX with (PuO_2-ThO_2) MOX while conserving the total Pu content. The basic computational framework employed uses the MONTEBURNS2 PERL script to couple the neutronic code, MCNP5 with the depletion/burn-up code, ORIGEN2.2. The parameters computed and compared are: net neutron multiplication factor (k_(eff)); regionally averaged neutron spectrum; neutron flux; thermal power distribution; breeding ratio; fuel burn-up; fissile material build-up/depletion-~(232)U build-up; and fuel temperature dependent Doppler Effect, coolant temperature dependent sodium expansion coefficient, and nonproliferation characteristics such as dose rates, spontaneous fission and gamma emissions. The analyses of the case studies indicate that the core physics characteristics, except K_(eff). are only marginally different in their magnitudes between the two cases, if not equal. The first case study shows that diversion of either 8 radial blanket sub-assemblies (weapon grade Pu) or 1 spent fuel sub-assembly (reactor grade Pu) discharged from an equilibrium core is sufficient to derive a significant quantity (SO.). The second case study shows that a considerable improvement in proliferation resistance can be achieved with the peripheral loading of (PuO_2-ThO_2) MOX pins in all the fuel sub-assemblies of a fast reactor, which should aid in nuclear material safeguards. The comparison of transuranic (TRU) generation for both the cases showed that about 60% reduction in neptunium production has been achieved for the new proposed partially ThO_2-PuO_2 loaded FBR design, whereas other higher TRUs like americium, curium, etc. did not show significant reduction.