The proliferation resistance of the nuclear fuel cycle would be increased if one could eliminate the need for both uranium enrichment and spent fuel reprocessing. Heavy-water and graphite moderated critical reactors can extract energy from natural uranium but offer a very low uranium utilization (low discharge burnup). The objective of the present study is to explore the feasibility of achieving high fuel utilization without resorting to enrichment and reprocessing using spallation neutron source driven subcritical reactors. Three different high burnup once through subcritical nuclear systems are investigated: a fluoride salt cooled high temperature reactor (FHR) with pebble fuel, a helium cooled core with sphere pack fuel based on General Atomics' EM2 reactor concept, and a sodium cooled fast reactor that is loaded with fuel discharged from a high burnup Breed-and-Burn (B&B) fast reactor that is fed with depleted uranium, after removing the gaseous fission products and inserting the voided fuel rods into a new clad (without removing the old one).

The pebble fuel design and fuel cycle for the FHR concept was optimized for maximum electric power multiplication using natural thorium fuelled subcritical core. The maximum attainable power multiplication was not high enough to merit future studies.

The optimal discharge burnup of the fuel in the EM2 type subcritical core was found to be approximately 30% FIMA and the corresponding power multiplication was found higher than in the FHR but still not high enough for practical applications.

Significantly better performance was obtained from the sodium-cooled source-driven core that is fed with metallic U-TRU-Zr fuel discharged at 20% FIMA from a critical B&B fast reactor that underwent recladding. The maximum attainable power multiplication was found to be close to 10 while fissioning an additional 20% of the loaded heavy metal.