Solubility of radionuclides

The chapters of this volume are organized into sections that cover the chemical aspects that are important to understanding the behavior of disposed radioactive wastes. These aspects include radionuclide sorption and desorption, solubility of radionuclide compounds, chemical species of radionuclides in natural waters, hydrothermal geochemical reactions, measurements of radionuclide migration, solid state chemistry of wastes, and waste-form leaching behavior. The information in each of these sections is necessary to predict the transport of radionuclides from wastes via natural waters and thus to predict the safety of the disposed waste. 

In addition to describing sorption and desorption, isotherms can be used to estimate the solubilities of radionuclides in the groundwater-radionuclide-geologic solid system. For radionuclides that form slightly soluble compounds (e.g., SrC03, Pu02 H20) in these systems, isotherms can define the approximate concentrations above which precipitation, rather than sorption, dominates removal from solution. 

Table II. Solubilities of Radionuclides in Grande Ronde Groundwater at Eh = -.0.3 V...

There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

The fete of radionuclides in the marine environment is similar to that of the stable isotopes, being dependent on chemical speciation, including redox state, solubility, and tendency to form complex ions. For example, Pu and Am are particle reac-... 

Brookhaven National Laboratory s (BNL s) biochemical recovery of radionuclides and heavy metals is a patented biochemical recovery process for the removal of metals and radionuclides from contaminated minerals, soil, and waste sites. In this process, citric acid, a naturally occurring organic complexing agent, is used to extract metals and radionuclides from solid wastes by the formation of water-soluble, metal-citrate complexes. The complex-rich extract is then subjected to microbiological biodegradation that removes most of the extracted heavy metals. 

One disadvantage of borosilicate glass is the low solubility of sulphates, molybdates, chromates, and halogenides, which may cause separation of metastable phases (Fig. 1) at relatively low contents of these components (1-3 wt%, dependent on glass composition Camara et al. 1980 Kawamoto et al. 1981 Stefanovsky 1989 Stefanovsky Lifanov 1989). At higher concentrations, yellow phase formation may occur (Morris Chidley 1976 Stefanovsky Lifanov 1988 Lutze 1988). The yellow phase, consisting of alkali and alkaline earth molybdates, sulphates, chromates, and halogenides, concentrates Cs and Sr radionuclides, and its presence increases leach rates of these radionuclides. 

Accuracy. Quantitative estimates of radionuclide solubility and speciation need reliable thermodynamic data, that is,... 

Waste Form. The leach rates of radionuclides by groundwater are slow both for vitrified HLW and for SUF. A glass lifetime of at least 3000 years is expected, assuming unlimited supply of water (24). If the limited solubility of glass in groundwater is considered as well as the low water flow the lifetime would be several orders of magnitude larger. 

Second, generic and site-specific assessments of near-surface disposal facilities for radioactive waste have shown that allowable doses to hypothetical inadvertent intruders usually are more restrictive in determining acceptable disposals than allowable doses to individuals beyond the boundary of the disposal site. This conclusion is based on predictions that concentrations of radionuclides in the environment (e.g., ground-water) at locations beyond the site boundary usually should be far less than the concentrations at the disposal site to which an inadvertent intruder could be exposed, owing to such factors as the limited solubility of some radionuclides, the partitioning of radionuclides between liquid and solid phases, and the dilution in transport of radionuclides in water or air beyond the site boundary. More people are likely to be exposed beyond the site boundary than on the disposal site, but acceptable disposals of radioactive waste in near-surface facilities have been based on assessments of dose to individuals, rather than populations. 

Solubility constraints define the maximum concentrations of radionuclides at the point of release from the waste. In the second section, radionuclide solubilities in natural waters are reported as measured values and estimated values from thermodynamic data. In addition, information is given concerning the chemical species of radionuclides that could be present in natural waters. 

Field measurements of radionuclide migration can be used to help substantiate laboratory measurements of sorption, solubility, and identification of important chemical species. The fourth section describes three field investigations that provide information on the effects of organics, colloids and environmental conditions (Eh, pH, and temperature) on radionuclide transport. The chemical species of radionuclides that are mobile under specific field conditions are identified. 

This volume covers ongoing research and, thus, leaves many questions unanswered and many problems unsolved. The geochemistry of disposed radioactive wastes involves many complex issues that will require years of additional research to resolve. High-priority problems include integration of geochemical data with computer models of chemical interaction and transport, definition of environmental conditions that affect the behavior of radionuclides at specific disposal sites, evaluation of complex formation of dissolved radionuclides with inorganic and organic complexants, and determination of radionuclide solubilities in natural waters. 

Effects of Groundwater Composition and Eh. Radionuclide sorption on geologic solids is dependent on the chemical composition of the groundwater solution and the redox potential (Eh) of the solid-groundwater system. Aquifers at various depths in the Columbia Plateau formation have -been observed to have significant differences in composition. To accurately model radionuclide migration, it is necessary to understand the effects of chemical components and Eh on sorption and solubility of key radionuclides. An additional benefit of this work is to better understand the mechanisms of sorption and desorption of the radionuclides. 

Of the elements considered in this study (see Table II), nickel, palladium, antimony, and lead are particularly sensitive to the presence of reduced sulfur species (S2, HS") in the groundwater. For each of these radionuclides, if sulfur speciates under thermodynamic equilibrium conditions, solid sulfide phases will control their solubility at low Eh values. The implication of this fact is illustrated in Figure 1 by a bold, dashed line that corresponds to the solubility of nickel in the reference groundwater and a patterned zone representing the total range... 

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