Modeling radionuclides

Sorption and desorption isotherms were obtained for sorption of radionuclides under oxidizing and reducing conditions. The Freundlich equation accurately describes most of these isotherms. Most radionuclides are apparently irreversibly sorbed on each of the geologic solids since the slopes of sorption and desorption isotherms for a given radionuclide are different. This hysteresis effect is very large and will cause a significant delay in radionuclide transport. It, therefore, should be included in modeling radionuclide transport to accurately assess the isolation capabilities of a repository in basalt. 

Sorption and Desorption Isotherms. To model radionuclide transport in groundwater through geologic media, it is necessary to mathematically describe sorption and desorption in terms of isotherms. The Freundlich isotherm was found to accurately describe sorption and desorption of all radionuclides studied in the interbed-groundwater systems, except when precipitation of the radionuclide occurred. 

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. 

Between the simplicity of the model and the complexity of the TLM, there are several other sorption models. These include various forms of isotherm equations (e.g., Langmuir and Freundlich isotherms) and models that include kinetic effects. The generalized two-layer model (Dzombak and Morel, 1990) (also referred to as the DLM) recently has been used to model radionuclide sorption by several research groups (Langmuir, 1997a Jenne, 1998 Davis, 2001). Constants used in this model are dependent upon the concentration of background electrolytes and... 

Neretnieks 1. and Rasmuson A. (1984) An approach to modeling radionuclide migration in a medium with strongly varying velocity and block sizes along the flow path. Water Resour. Res. 20(12), 1823-1836. 

The most commonly used adsorption model in contaminant transport calculations is the distribution coefficient, model. In large part this reflects the simplicity of including a value in transport calculations (cf. Freeze and Cherry 1979 Domenico and Schwartz 1990 Stumm 1992). Nevertheless, such applications should be limited to conditions where values can be expected to remain near constant during transport (cf. Reardon 1981). Alternatively, if a Kj can be confidently shown to be a maximal or minimal possible value, such calculations can provide bounding or conservative information on contaminant transport. The bounding minimum approach has become standard in the modeling radionuclide transport, for example (cf. Meijer 1992). 

A primary goal of adsorption measurements and models is to predict adsorption in a variety of possible water/rock systems for a range of conditions. For the actinides, such prediction is necessary to develop confidence that radionuclide releases from a nuclear waste repository will not exceed health standards at some time and distance from the repository. Most repository programs use the K t approach to describe and model radionuclide adsorption (cf. Meijer 1992 OECD 1993). In part, this surely reflects the simplicity of incorporating values in transport codes (Freeze and Cherry 1979 ... 

The results with both the GC and CA modeling approaches indicate that the surface complexation concept can be applied to model radionuclide adsorption by soils and sediments, and that models without an EDL can be useful for that purpose. The GC approach requires less information and laboratory data, and is probably more useful for immediate and practical applications. 

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. 

The analysis of the consequences of nuclear accidents began with physical concepts of core melt, discussed the mathematical and code models of radionuclide release and transport within the plant to its release into the environment, models for atmospheric transport and the calculation of health effects in humans. After the probabilities and consequences of the accidents have been determined, they must be assembled and the results studied and presented to convey the meanings. 

Powers, D. A, et al., I985, VANESA, A Mechanistic Model of Radionuclide Release and Aerosol Generation during Core Debris Interaction with Concrete, NUREG/CR-4308. 

Model of Pb, Bi, and Po degassing. For a purpose of clarity, it is considered here that the degassing reservoir has reached a chemical steady-state (i.e., radionuclide activities in the degassing reservoir are constant, that is d(Ik)iydt = 0 in Eqn. 4). This assumption usually is valid for very active basaltic systems like Stromboli, where erupted products display an almost constant chemical composition as shown above, and where the degassing reservoir is quickly and continuously replenished with deep undegassed magma. 

Model components. Models generally consider 3 populations of radionuclides ... 

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. 

Modeling of the transport of the long-lived nuclides, especially U, require knowledge of the input at the water table as a boundary condition for aquifer profiles. There are few studies of the characteristics of radionuclides in vadose zone waters or at the water table. Significant inputs are likely to occur to the aquifer due to elevated rates of weathering in soils, and this is likely to be dependent upon climatic parameters and has varied with time. Soils may also be a source of colloids and so provide an important control on colloidal transport near recharge regions. 

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