Radionuclide speciation plutonium

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

For the purposes of this study, five actinide elements are considered Am(lll), Np(V), Pu(V), Th(IV), and U(VI). All five of these radionuclides are an important part of the radionuclide inventory intended for Yucca Mountain (Kerrisk, 1985 Oversby, 1987). With the exception of Pu, all are considered to be strongly dominated by a single oxidation state in the oxidizing groundwaters assumed for Yucca Mountain. Although the redox chemistry of Pu is very complex, U.S. Department of Energy research on radionuclide speciation and solubility indicates that Pu(V) is the dominant oxidation state for Pu in water from wells J-13 and UE-25 p l (Nitsche et al., 1993 Triay et al., 1997). Plutonium also may be present in the +4 and +6 oxidation states, but only Pu(V) is considered here. 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

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