Radionuclide speciation

Moulin, V., and Moulin, C. (2001). Radionuclide speciation in the environment a review. Radiochim. Acta 89,773-778. 

It is clear from previous comments that radionuclide speciation studies must consider not only oxidation states and specific inorganic forms but also complex species arising through association with natural organic matter and the possibility of different physical states. The relative importance of these various physicochemical forms will, in practice, be dictated by a combination of the basic elemental characteristics of the radionuclide and the type of environment into which it is placed. Thus in seawater, colloidal organic complex species are likely to be far less dominant than for the same radionuclide in a very low ionic strength freshwater. In the case of soil and sediment interstitial waters or groundwaters,... 

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... 

An approach such as that suggested for the re-examination of the molecular weight distribution of dissolved organic carbon in seawater (Carlson et al., 1985) has been successfully adapted to evaluate the performance of ultrafiltration in the study of radionuclide speciation ... 

For experimental studies, a chemical thermodynamic modelling approach could theoretically reduce unnecessary experimental effort and hence the overall cost of a research project. Once experiments are underway, the computer simulation should also offer valuable assistance in the interpretation of results. Modelling techniques with particular reference to radionuclide speciation have been discussed by Cross and Day (1986) who pointed out that models are only as good as the thermodynamic data upon which they are based. For example, formation constants (a prerequisite for chemical modelling) are invariably generated in idealised laboratory conditions and their use seldom reflects the natural environment... 

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. 

Wanner, H. 1985. Modelling radionuclide speciation and solubility limits in the near-field of a deep repository. Scientific Basis for Nuclear Waste Management, IX, 509-516. 

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