A strategy for transition from a uranium fueled, open cycle SFR to a transuranic fueled, closed cycle sodium cooled fast reactor

Abstract

Reactors utilizing a highly energetic neutron spectrum, often termed fast reactors, offer large fuel utilization improvements over the thermal reactors currently used for nuclear energy generation. Conventional fast reactor deployment has been hindered by the perceived need to use plutonium as fuel, coupling the commercial introduction of fast reactors to the deployment of large-scale thermal reactor used fuel reprocessing. However, the future of used fuel treatment in the United States is highly uncertain, creating a bottleneck for the introduction of fast reactor technology. A strategy centered around using uranium-fueled fast reactor cores in a once-through mode-a uranium startup fast reactor (USFR)-decouples fast reactor commercialization from fuel reprocessing and enables transition to a recycle mode once the technology becomes available and economic. The present work investigates the optimal strategy for recycling spent fuel from once-through sodium cooled fast reactors (SFRs), by analyzing the performance of various designs. A range of acceptable transitions are described and their economic, breeding, nonproliferation, and safety performance are characterized. A key finding is that the burnups of all cores were limited by the allowable fluence to the cladding rather than by the core reactivity. The carbide cores achieve fluence-limited burnups 15-25% greater than the comparable metal cores, though the metal cores can be optimized via decrementing the fuel volume fraction to reach fluence-limited burnups within 10% of the carbide cores. The removal of minor actinides from the recycled fuel has a minimal impact on the achievable burnups of both types of fuels, decreasing the fluence-limited burnup by less than half a percent in all cases. Similarly, long-term storage of the USFR fuel had minimal impact on the achievable burnups of all cores, decreasing the fluencelimited burnup by no more than 2% in all cases. Levelized fuel costs were in the range of 5.98 mills/kWh to 7.27 mills/kWh for the carbide cores, and 6.81 mills/kWh to 7.57 mills/kWh for the optimized metal cores, which is competitive with fuel costs of current LWRs and once-through SFRs. The metal and carbide multicore cores, made using slightly more than one once-through SFR core, functioned as slight fissile burners with fissile inventory ratios (FIRs) near 0.9. The uranium+ cores, made using one oncethrough SFR core plus natural uranium makeup, functioned in a fissile self-sustaining mode with FIRs near unity. All cores discharged fuel that was less attractive for weapon use than that of an LWR. The carbide cores had maximum sodium void worths in the range of $2.81-$2.86, approximately half the worth of the metal cores, which were in the range of $4.97-$5.14. Carbide and metal multicore cores possessed initial reactivities in the range of 15,000 pcm, requiring either multi-batch staggered reloading or control system modifications to achieve acceptable shutdown margins. The uranium+ carbide and metal cores achieved acceptable shutdown margin with the nominal control configuration and the single-batch reloading scheme. The overall conclusion is that USFR spent fuel is readily usable for recycle.