Preliminary report on blending strategies for inert-matrix fuel recycling in LWRs.

Abstract

Various recycle strategies have been proposed to manage the inventory of transuranics in commercial spent nuclear fuel (CSNF), with a particular goal of increasing the loading capacity of spent fuel and reprocessing wastes in the Yucca Mountain repository. Transuranic recycling in commercial LWRs can be seen as a viable means of slowing the accumulation of transuranics in the nationwide CSNF stockpile. Furthermore, this type of approach is an important first step in demonstrating the benefits of a nuclear fuel cycle which incorporates recycling, such as envisioned for Generation-IV reactor systems under development. Recycling strategies of this sort are not proposed as an attempt to eliminate the need of a geologic nuclear waste repository, but as a means to enhance the usefulness of the repository currently under construction in the U.S., perhaps circumventing the need for a second facility. A US-DOE Secretarial recommendation on the need for the construction of a second geologic repository is required by 2010. The Advanced Fuel Cycle Initiative (AFCI) has supported a breadth of work to evaluate the ideal transuranic separation and recycle strategy. Previous AFCI studies of LWR-based transmutation have considered the benefits of homogeneously recycling plutonium, plutonium and neptunium, and all transuranic (TRU) species. A study of a wide range of hypothetical separation schemes (Pu, Pu+Np, Pu+Np+Am, etc.) with multi-recycling has also been performed, focusing on the proliferation resistance of the various fuel cycles and fuel handling issues. The direct recycle of the recovered TRU from spent inert-matrix fuel (IMF) into new IMF was found to be quite limited due to the rapid burndown of the fissile plutonium. The IMF is very effective at destroying the fissile fraction of the TRU with destruction rates in excess of 80% of the fissile material without recycling the IMF. Blending strategies have been proposed to mitigate the rapid burndown of the fissile plutonium by mixing high fissile feed from new sources (e.g., spent UO{sub 2} pins) with the low fissile material recovered from the recycled transmutation fuel. The blending of the fuels is anticipated to aid the multi-recycle of the transuranics. A systematic study of blending strategies (for both IMF and MOX) has been initiated and is currently ongoing. This work extends the previous study that considered separation strategies for plutonium, neptunium, and americium recycling in MOX, CORAIL, and IMF{sub 6} by considering blending schemes and approach to continuous recycle. Plutonium and americium are recycled in order to reduce the intermediate term (100 to 1500 years after spent fuel irradiation) decay heat of the disposed waste which accounts for the bulk of the repository heating. Since the long-term released dose from the repository is dominated by neptunium, it is sensible to consume it by transmutation in a reactor, as well. Curium accounts for {approx}0.6% of the TRU mass in spent UO{sub 2} fuel ({approx}0.008% of the heavy metal), but does constitute significantly higher fractions in spent transmutation fuels. This initial evaluation will focus on blending strategies for the multirecycling of Pu+Np+Am. The impact of curium recycle will be investigated as part of the systematic study of blending strategies. The initial study focuses on understanding a simple strategy for IMF recycle and blending. More complex strategies (i.e., heterogeneous assemblies) will be evaluated later in the year, including enriched uranium support options. Currently, a preliminary study of a serial blending strategy has been performed in order to evaluate the impact of blending on the performance of the IMF recycle and to evaluate the potential for continuous or infinite recycle. The continuous recycle of Pu+Np+Am in IMF would allow for complete destruction of all heat contributing actinides in the same LWRs that originally produced them. The only transuranics sent to the repository would be those lost in reprocessing and curium if it is not eventually recycled.