Study on the technique of spent fuel reprocessing with supercritical fluid direct extraction method (Super-DIREX method)

The spent fuel reprocessing method with supercritical fluid has been developed. Uranium and plutonium elements can be extracted directly from spent fuel oxides by supercritical carbon dioxide (CO₂) containing nitric acid and tributylphosphate (HNO₃-TBP) complex at 313-333K and 10-20 MPa. It is called Super-DIREX method which stands for Supercritical fluid Direct Extraction method. In Super-DIREX method, the process from dissolution to extraction(co-decontamination) in PUREX is consolidated to one component of direct extraction column', so that the cost of the spent fuel reprocessing plant is expected to be reduced from purex process. In order to evaluate the ability of extraction, basic tests have been carried out using unirradiated uranium oxide with artificial fission products (FPs) oxide. The concentration of uranium extracted in supercritical fluid and the decontamination factor (DF) of the FPs were evaluated. The test results revealed that it would be possible to extract U selectively from the U-oxide. The DFs of almost all FPs were more than 10². The concentration of uranium extracted is approx. 4.5 g/L-scCO₂ under 313K of temperature and 12 MPa of pressure. An increase of pressure may improve the concentration of uranium extracted. (author)