Impact of fuel choices on spent fuel characteristics for once-through heavy metal cooled reactors

Recently, heavy metal cooled reactors have been studied, either as potential candidates for burning of actinides from LWR spent fuel or as a promising power generation source for long term deployment. This study investigates a lead-bismuth- cooled core in a once-through fuel cycle that does not require expensive fuel reprocessing and strives for long core life while maintaining low reactivity swing. Contrary to earlier studies, our systematic scoping study, while for cores with acceptable operating and safety characteristics, is focused primarily on the spent fuel inventory, toxicity and proliferation resistance. Oxide, carbide, nitride and metallic fuels with U-238 and Th-232 as fertile materials and U-235 and Pu drivers have been investigated in a systematic manner. Metallic fuels were found to provide the best conversion ratio, followed by carbide fuels, nitride fuels and finally the oxides. Within one chemical form, Pu fuels exhibited the smallest reactivity decrease with burnup, U235-driven fuels were the next best and the fuels containing Th showed the largest decrease. Fuels driven by U-235 and having U-238 as fertile material were found to discharge fuel with the smallest waste inventory, decay heat, radioactivity and ingestion hazard. The Pu based fuels show significantly higher ingestion hazard and curie inventory. Fuels with fertile Th-232 produce less plutonium, nevertheless, their fuel toxicity is slightly higher than that of the U-238 fuels due to the activity of polonium, radon, and radium from the thorium decay chain. UZr and PuUZr fuels have been identified as the most promising alternatives for long life core concept design. They show the best potential to achieve small reactivity swing over the long core life and at the same time permit a relatively large core having a negative coolant density reactivity coefficient. Moreover, the UZr discharged fuel exhibits the smallest radiotoxicity, waste inventory, decay heat and radioactivity of all the fuels investigated. The key drawback is proliferation concern due to the high content of bred-in fissile material with an isotopic composition comparable to weapons grade plutonium. This is a key challenge to be overcome for this fuel type and all uranium driven fuels in hard spectrum reactors. On the other hand, the Pu-driven fuels, among them UPuZr, exhibit the largest radiotoxicity, waste inventory, decay heat and radioactivity, but less proliferation threat. (author)