Radionuclides solubility

Equilibrium thermodynamics is one of the pillars supporting the safety analyses of radioactive waste repositories. Thermodynamic constants are used for modelling reference porewaters, calculating radionuclide solubility limits, deriving case-specific sorption coefficients, and analysing experimental results. It is essential to use the same data base in all instances of the modelling chain in order to ensure internally consistent results. 

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

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. 

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. 

High-level radioactive defense waste solutions, originating from plutonium recovery and waste processing operations at the U.S. Department of Energy s Hanford Site, currently are stored in mild steel-lined concrete tanks located in thick sedimentary beds of sand and gravel. Statistically designed experiments were used to identify the effects of 12 major chemical components of Hanford waste solution on radionuclide solubility and sorption. 

Of these parameters, radionuclide solubilities that are addressed in this paper represent an important control on the concentration of specific radionuclides in groundwater available for transport to the accessible environment. 

For these reasons, it is useful to evaluate the effects of forming or not forming aqueous sulfide complexing agents and sulfide precipitates on radionuclide solubilities and aqueous speciation. Therefore, calculations were performed for key radionuclides for two cases (1) reduction of sulfate to sulfide and bisulfide was allowed to occur as Eh was lowered and (2) sulfate reduction as a function of Eh was prohibited so that sulfate was the only sulfur complexing agent available at all Eh values. 

Few direct, experimental determinations of radionuclide solubilities applicable to this study are available in the literature. However, solubility estimates for some radionuclides can be extracted from the results of conventional sorption experiments (26). In addition, natural levels of some elements listed in Table II that are observed in Hanford groundwaters provide crude estimates of minimum solubility limits for them. Finally, measured solubility results for americium in aqueous solutions have been reported (27). 

Jacobs, G. K. Apted, M. J. "Eh-pH Conditions for Groundwater at the Hanford Site, Washington Implications for Radionuclide Solubility in a Nuclear Waste Repository Located in Basalt, Trans. Am. Geophys. Un.1981,62,1065. 

In long-term tests ( 1 yr), the amount of 38pu> Sr, and Cs released are affected more by the radionuclide solubility in the leachant and layer formation on the glass than by the surface area of the glass. 

Results of Radionuclide Solubility, Speciation, and Sorption Studies... 

Solubility and speciation. Minimum requirements for reliable thermodynamic solubility studies include (i) solution equilibrium conditions (ii) effective and complete phase separation (iii) well-defined solid phases and (iv) knowledge of the speciation/oxidation state of the soluble species at equilibrium. Ideally, radionuclide solubilities should be measured in both oversaturation experiments, in which radionuclides are added to a solution untU a solid precipitates, and undersaturation experiments, in which a radionuchde solid is dissolved in aqueous media. Due to the difference in solubilities of crystalline versus amorphous solids and different kinetics of dissolution, precipitation, and recrystalhzation, the results of these two types of experiments rarely agree. In some experiments, the maximum concentrahon of the radionuchde source term in specific water is of interest, so the sohd that is used may be SF or nuclear waste glass rather than a pure radionuclide solid phase. 

TABLE 22.10. Radionuclide solubilities and limiting phase in reducing and oxidizing Finnsjd fresh water (SKB91)... 

An important consideration in preparing liquid samples is assurance of radionuclide solubility. Radionuclides subject to hydrolysis and radiocoiloidal behavior (see Section 4.2) require a suitably acidic medium. Other radionuclides may require specific reagents to prevent reactions that lead to insolubility or volatility, or a stable environment of temperature or darkness. 

Examples of impact assessments taken from the European Union Everest exercise give an idea of these variations for an hypothetical repository containing vitrified waste in a granite formation. Figure 1 summarizes, for the normal scenario, the variation of potential individual doses due to different radionuclides, when varying hydraulic parameters of the geosphere, retardation coefficients, matrix solubility, and radionuclide solubilities. 

The colloid problem is composed of a number of elements, including (i) the identification, isolation, and characterization of colloidal materials from target environments (ii) the development of predictive models for colloid release, transport, and deposition (iii) the characterization of interactions between radionuclides and colloids (sorptive processes for pseudocolloids, radionuclide solubility for intrin-... 

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