Bulk Solubility and Speciation of Plutonium(VI) in Phosphate-Containing Solutions
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BULK SOLUBILITY AND SPECIATION OF PLUTONIUM(VI) IN PHOSPHATE-CONTAINING SOLUTIONS H. T. WEGER, S. OKAJIMA, J. C. CUNNANE AND D.T. REED Argonne National Laboratory, 9700 South Cass Ave, Argonne IL 60439 ABSTRACT The solubility and speciation of Pu(VI) with phosphate is being investigated to determine the ability of phosphate to act as an actinide getter. In the initial studies performed, solubility was approached from oversaturation at an initial pH = 4, 10 and 13.4. Absorption spectra were recorded, the solution filtered, and the filtrate analyzed for Pu content. Absorption spectra were obtained at varying phosphate concentrations, and at pH of 2.7 to 11.9. The effect of complexation on the 833 nm Pu(VI) band was characterized. Evidence for three phosphate complexes was obtained for pH < 10, which have absorption bands at 842, 846 and 849 nm. Evidence for colloid formation was observed, but is not conclusive. The possible presence of colloids prevented an accurate determination of true solubility. A concentration of 10-5 to 10-6 M Pu(VI) was measured in solutions at pH < 10 that was filtered with a 50 nm filter. Pu(VI)-phosphate complexes predominated at pH < 11.6. At higher pH, however, only hydrolyzed Pu(VI) was detected. At pH = 12, the concentration of Pu(VI) was as high as 10-4 M.
BACKGROUND
An understanding of the solubility and migration behavior of actinides is needed to help determine the suitability of various nuclear waste management options under consideration by the U.S. Department of Energy. These options include the suitability of the potential Yucca Mountain site as a high-level nuclear waste repository, a land-fill disposal strategy, and LLRW/TRU waste sites currently being evaluated. Solubility limits are needed to set the upper limit of actinide concentration should groundwater come in contact with the wasteform. These data will help determine the source term concentration of actinides to help predict the long-term performance of the waste disposal option. A potential consideration in the design of either the waste package or the wasteform is the placement of actinide getters, in this case, species that result in the formation of insoluble radionuclide phases in close proximity to the wasteform. This will lower the overall solubility of the radionuclide, and improve overall site performance. This getter material can be placed in either the waste container or the backfill surrounding the container. Some materials under consideration are phosphate-bearing cements, ceramics, and minerals. These can provide a durable source of phosphate that may help decrease migration of actinides in the near field. It is generally conceded that the solubility of phosphate phases is exceedingly low. Although phosphate is a relatively common constituent of groundwater, there is surprisingly little data on its interaction with actinide species. Only three papers were found concerning Pu(VI) in phosphate solutions t-3 . The lowest concentration reported in the undersaturation Mat. Res. Soc. Symp. Proc. Vol. 294. ©199
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