The fluorescence lifetimes of Eu(III) and Cm(III) doped into crystalline lanthanum hydrate compounds were measured to obtain the calibration between the decay constant k obi (the reciprocal of the excited-state lifetime) and the number of water molecules «H 2 o in the first coordination sphere. The n Hl0 of Cm(III) in various aqueous solutions were calculated using the calibration relation obtained and the lifetime in the literature. All of the nH2o values so calculated were chemically reasonable.

Chemical species of U(VI) in 0.1 Μ Perchlorate solutions in equilibrium with 0.03% C0 2 partial pressure were investigated by time-resolved laser-induced fluorescence spectroscopy. The emission spectra of U(VI) solutions (pH = 3.8 to 7.0) in solidliquid equilibrium with schoepite U0 3 · 2H 2 0 were interpreted by the summation of the emission spectra of the individual components, UO! + (aq), (U0 2 ) 2 (0H)r(aq), (UO 2 ) 3 (0H)i(aq) and U0 2 C0 3 (aq). Numerical evaluation of the emission spectra using the single component of the species gave a satisfactory agreement with the distribution of the species calculated from the formation constants of U(VI)-hydroxides and -carbonate complexes. Emission spectra of the higher carbonate species U0 2 (C0 3 )1" and U0 2 (C0 3 )^ could not be observed.

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Extraction behaviors of U(VI) and selected fission product elements from HN0 3 solution into a supercritical C0 2 fluid containing tributylphosphate (TBP) were studied under equilibrium and dynamic extraction conditions. Uranium(VI) was found to be extracted in the supercritical C0 2 phase as a form of U0 2 (N0 3 ) 2 (TBP) 2 with a distribution ratio of, e.g., 2.2±0.3 for the extraction of 2 X10" 4 to 2 X 10" 2 M U(VI) from 3 M HN0 3 into supercritical C0 2 -0.3 M TBP mixture at 60°C and 15 MPa. The distribution ratio of U(VI) increased with an increase of HN0 3 concentration (0.1-6 M) or TBP (0.1-0.3 M), along with a decrease of pressure (10-40 MPa) at 60 °C, and with an increase of the temperature (40-80°C) at 15 MPa. A salting-out effect enhancing the distribution of U(VI) was observed by addition of a fairly high concentration of LiN0 3 to the aqueous phase. Results of dynamic extraction using a continuous flow of supercritical C0 2 -TBP through the aqueous nitric acid solution showed that >98% of U(VI) could be exctracted from 3 M HN0 3 +3 M LÍN0 3 solution into supercritical C0 2 -0.08 M TBP (60°C, 15 MPa). Main fission product elements such as lanthanides, Cs, Sr, Ba, Zr, Mo, and Pd were hardly extracted into the supercritical C0 2 phase under this condition, which suggests that the supercritical C0 2 extraction is a feasible method for the separation and recovery of uranium from these elements. Since rapid and complete removal of the C0 2 medium from the extracts is attained by gasification of C0 2 , the amount of the organic solvent waste generated in the extraction process can be largely minimized. Advantages of the supercritical C0 2 extraction in its application to the separation process in the field of nuclear technology are discussed.