Uranium extraction from TRISO-coated fuel particles using supercritical CO2 containing tri-n-butyl phosphate

Abstract

High-temperature gas-cooled reactors (HTGRs) are advanced nuclear systems that will receive heavy use in the future. It is important to develop spent nuclear fuel reprocessing technologies for HTGR. A new method for recovering uranium from tristructural-isotropic (TRISO-) coated fuel particles with supercritical CO2 containing tri-n-butyl phosphate (TBP) as a complexing agent was investigated. TRISO-coated fuel particles from HTGR fuel elements were first crushed to expose UO2 pellet fuel kernels. The crushed TRISO-coated fuel particles were then treated under O2 stream at 750°C, resulting in a mixture of U3O8 powder and SiC shells. The conversion of U3O8 into solid uranyl nitrate by its reaction with liquid N2O4 in the presence of a small amount of water was carried out. Complete conversion was achieved after 60min of reaction at 80°C, whereas the SiC shells were not converted by N2O4. Uranyl nitrate in the converted mixture was extracted with supercritical CO2 containing TBP. The cumulative extraction efficiency was above 98% after 20min of online extraction at 50°C and 25MPa, whereas the SiC shells were not extracted by TBP. The results suggest an attractive strategy for reprocessing spent nuclear fuel from HTGR to minimize the generation of secondary radioactive waste.