Analysis of Pu-Only Partitioning Strategies in LMFBR Fuel Cycles

Abstract

Sodium cooled Fast Reactors (SFR) have been under consideration for production of electricity, fissile material production, and for destruction of transuranics for decades. The neutron economy of a SFR can be operated in one of two ways. One possibility is to operate the reactor in a transuranic burner mode which has been the focus of active R&D in the last 15 years. However, prior to that the focus was on breeding transuranics. This later mode of managing the neutron economy relies on ensuring the maximum fuel utilization possible in such a way as to maximize the amount of plutonium produced per unit of fission energy in the reactor core. The goal of maximizing plutonium production in this study is as fissile feed stock for the production of MOX fuel to be used in Light Water Reactors (LWR). Throughout the l970’s, this fuel cycle scenario was the focus of much research by the Atomic Energy Commission in the event that uranium supplies would be scarce. To date, there has been sufficient uranium to supply the once through nuclear fuel cycle. However, interest in a synergistic relationship Liquid Metal Fast Breeder Reactors (LMFBR) and a consumer LWR fleet persists, prompting this study. This study considered LMFBR concepts with varying additions of axial and radial reflectors. Three scenarios were considered in collaboration with a companion study on the LWR-MOX designs based on plutonium nuclide vectors produced by this study. The first scenario is a LMFBR providing fissile material to make MOX fuel where the MOX part of the fuel cycle is operated in a once-through-then-out mode. The second scenario is the same as the first but with the MOX part of the fuel cycle multi-recycling its own plutonium with LMFBR being used for the make-up feed. In these first two scenarios, plutonium partitioning from the minor actinides (MA) was assumed. Also, the plutonium management strategy of the LMFBR ensured that only the high fissile purity plutonium bred from blankets was sold to the MOX LWRs. The third scenario considered a LMFBR fuel cycle in an expansionary mode where excess bred transuranic material is accumulated for spinning off additional LMFBR cores. In this latter scenario, no plutonium partitioning was considered. After every cycle, transuranic from both driver and blankets is sold to the MOX LWRs. The MA production from LMFBR operated in a Pu-only fuel cycle is roughly only 1% that of the transuranic production rate. This is in contrast to LWR fuel cycles where the MA content in TRU is closer to 10% or more. If such a LMFBR were operated to provide fissile material to a fleet of MOX reactors, then 1 GWe of LMFBR could support between approximately 0.11 and 0.43 GWe of LWR-MOX reactors for a LMFBR conversion ratio between 1.1 and 1.5, if the MOX reactors were operated in a once-through-then out mode. If the plutonium is continuously recycled in the MOX reactors then the support ratio is approximately 1 GWe of LMFBR for between 0.13 and 0.65 GWe of LWR-MOX reactors depending on the LMFBR conversion ratio. Also, it was found that if the LMFBR fleet were operated in a purely expansionary mode, the smallest doubling time achievable would be seven years.