Fig. 1-2 Apparatus for extraction experiments using supercritical CO2 fluid In the extraction vessel, uranium forms a complex with TBP, which is soluble into a supercritical CO2 fluid and is carried to the trap. Thus uranium is collected in the trap in a form of complex.

Fig. 1-3 Phase diagram Carbon dioxide is a gas under ordinary conditions but becomes a supercritical fluid at above 31 degrees cent. and 73 atm.

When both temperature and pressure of a gas are raised above its critical values (Tc and Pc), it becomes a supercritical fluid, SF (Fig. 1-2). The fluid has various attractive properties of solvent power, diffusibility, mass transfer and others, compared to the ordinal simple state, of gas or liquid. Thus, SF can provide various research subjects not only scientifically but also technologically. Supercritical fluid extraction (SFE) offers several advantages over conventional extraction, such as liquid-to-liquid and solid-to-liquid extraction. Here particular attention is placed on the recovery of uranium and transuranium elements from various waste solutions using carbon dioxide because of its moderate critical constants (Tc=31 degrees cent., Pc=73 atm), inertness, and availability in purified form. The process prevailing at present for the uranium extraction is the tri-n-butylphosphate extraction (Purex process), where a large amount of waste of used organic solvents is generated. This is one of the significant drawbacks of the process. The experiments on SFE, however, clearly demonstrate the advantage of easy recovery of the gas simply by reducing its pressure to atmospheric pressure after the extraction process (Fig. 1-3). In addition to this, satisfactory extractability has been confirmed. Because the special advantage of SFE is applicable to the separation process handling of large amounts of radioactive materials, extensive research on the wide spectrum concerning SFE are progressing toward its incorporation to the reprocessing of spent fuels.

Reference S. Iso et al., Extraction of Uranium (IV) from Nitric Acid Solution into Supercritical Carbon Dioxide Containing Tri-n-butylphosphate, Chem. Lett., 1995 (5), 365 (1995).

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