Actinide Speciation in Spent Fuel Leaching Studies
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ACTINIDE SPECIATION IN SPENT FUEL LEACHING STUDIES GREGORY R. CHOPPIN, Department of Chemistry, Florida State University, Tallahassee, Florida 32306 ABSTRACT The uranium and plutonium chemistry relevant for spent fuel leach studies is discussed. Model calculations are evaluated by comparison with the results of spent U02 fuel leaching obtained in the Swedish SKB program. The thermodynamic data were found to agree sufficiently with the measured solution concentrations that it can be assumed that in oxic natural waters, leachate uranium from spent fuel would be uranyl carbonate complexes in the solution (or hydroxo complexes in the absence of carbonate) while schoepite would be the solubility limiting solid. For plutonium, Pu02+ would be the solution species and Pu(OH)4 the solubility limiting solid. INTRODUCTION In some national plans for permanent disposal of nuclear wastes, spent nuclear fuel, properly packaged, would be the primary disposal form. For such a disposal mode, the limited solubility of the uranium dioxide fuel would serve as one of the main barriers to the release of radioactivity to the environment. To validate the value of U02 as a major barrier, the dissolution rate of fuel samples of various burnups have been measured under a range of experimental conditions [1-8]. A comparison (8] of experimental solubilities of uranium and plutonium with calculated values based on several standard compilations of actinide thermodynamic data [9-11] found good agreement for acidic (pH
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5) solutions but increasing differences in neutral and basic systems. The measured concentrations in the neutral oxic solutions were greater than the calculated. There have been many studies on the speciation of actinides in neutral systems in the last several years. Therefore, it seemed useful to reconsider these leaching data and to attempt to use the later data to better model the speciation in such systems. To keep this paper a reasonable length, oxic solutions only are considered. URANIUM CHEMISTRY a.
Redox
Uranium has a strong tendency to exist in the dioxo uranium(VI) form, U02 2 +, in oxic solutions. Only when the Eh of the solution is ca. 0.2 V or less does it convert to the tetravalent oxidation state. Consequently, in systems in contact with air, the dissolved species can be predicted with confidence to be the uranyl(VI) species. b.
Complexation
Uranyl(VI) cations react with a wide variety of inorganic and organic anions. Although the uranyl cation is dipositive Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
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2 U02 +), the anions which complex within the primary coordii.e., nation sphere (inner sphere complexes) sense the partial charge The sequence of the uranium atom which is approximately +3.2 [12]. different oxidation of complexing strength of the uranium in its states is:
U4+ > U0 2 2 + > U3 + > U0 2 + two states are known but are only of interest under (The latter exceptional conditions). In natural waters, uranyl(VI) cations can be complexed significantly by fluoride, phosphate,
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