Migration radionuclide, dependence

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. 

The large number of measurements in various systems lead to the result that the interaction of radionuclides with solids is rather complex and depends on many parameters the species of the radionuclides in the solution, their properties and their dispersion, the components of the solid, the surface area of the particles, the nature of the sorption sites, the presence of organic substances and of microorganisms, and the interference or competition of other species. Therefore, an investigation of the behaviour of the radionuclides in the specific system of interest is unavoidable, if reliable information about their migration behaviour is required. 

Sorptive properties of mineral assemblages and soils. An important question for the prediction of radionuclide migration is whether sorption in the geomedia can be predicted from the properties of the constituent minerals. Attempts by researchers to use sorption models based on weighted radionuclide values of individual component minerals ( sorptive additivity ) have met with limited success (Meyer et al, 1984 Jacquier et al, 2001). Tripathi et al. (1993) used a competitive-additivity model based on surface-complexation theory to model the pH-dependent sorption of lead... 

The value of Kd depends on the experimental conditions, but it is widely believed that a Kjj determined at certain experimental conditions can be used to predict sorption for given adsorbent-adsorbate combination. The variability of Ku was emphasized by Krupka et al. [30], who recently collected Kd values related to migration of radionuclides in geosphere. A dozen of less recent compilations of Ko was quoted by Jenne [31]. 

Figure 16 and 17 show time history of temperature and pH inside buffer material (at the boundary between overpack and buffer material as shown in Figure 15), respectively. The pH inside buffer material highly depends on temperature and becomes constant at 1,000 years. Through this type of numerical experiments, we can predict near-field chemistry for radionuclides migration and overpack corrosion. 

Deposited cesium penetrates slowly from the soil surface into deeper soil layers [3] depending strongly on the soil type [2]. Sorption processes can further retard the Cs migration rate. The relative abundance of clay and mica minerals, particularly illite, results in the rapid and nearly irreversible cesium immobilization in the topsoil layer [4]. Meanwhile, cesium, as well as the other radionuclides that behave like cations, can be moved upward by plant uptake. This process depends on various factors plant... 

The future availability of natural resoturces is dependant on the limitation of radionuclide releases aiKl the non-radiological environmental protection. Furthermore, migration of radionuclides after closure of a disposal facility has to be taken into account as a long-term risk for contamination of natural resources (see principle 4). 

The retardation of radionuclide migration through the geosphere by sorption and ion exchange processes is dependent to a large extent upon the mineralogical character of the rock. Sorption depends upon mineral type and the radionuclide concerned, e.g. the following sequence of preference was observed for Sr sorption clays, zeolites, Fe-hydroxides > micas > hornblende, Fe-Ti oxides > feldspars > carbonates > quartz (Landstrom et al. 1982). 

---