Radionuclides migration

Chapter 6 was concerned, with determining the probability of various failures leading to insufficient core cooling of a nuclear reactor. This chapter describes how the accident effects are calculated as the accident progresses from radionuclide release, radionuclide migration within the plant, escape from retaining structures, atmospheric radionuclide transport and the public health effects. 

Ivanovich M (1991) Aspects of uranium-tliorium series disequilibrium applications to radionuclide migration studies. Radiochim Acta 52/53 257-268... 

YanchukV., Kolodnytsky M., Kovalchuk A., Levitsky V., Orlov O. Methods and tools of mathematical modeling of radionuclides migration in natural ecosystems Vol 1,2.-Zhytomyr ZIET, 2002. 

Colloid influence on the radionuclide migration from a nuclear waste repository... 

The reliable long-term safety assessment of a nuclear waste repository requires the quantification of all processes that may affect the isolation of the nuclear waste from the biosphere. The colloid-mediated radionuclide migration is discussed as a possible pathway for radionuclide release. As soon as groundwater has access to the nuclear waste, a complicated interactive network of physical and chemical reactions is initiated, and may lead to (1) radionuclide mobilization (2) radionuclide retardation by surface sorption and co-precipitation reactions and (3) radionuclide immobilization by mineralization reactions, that is, the inclusion of radionuclides into thermodynamically or kinetically stabilized solid host matrices. 

The potentially important role of colloidal species in the geochemical behaviour of the polyvalent actinides has nevertheless been stated by various authors (e.g., Kim 1991 Kersting et al. 1999). The present paper discusses the role of colloids on the release of radionuclides from a nuclear waste repositoiy with regard to the processes leading to (1) colloid generation and stability (2) radionuclide interaction with aquatic colloids and (3) colloid-borne radionuclide migration. 

Fig. 8. Radionuclide migration studied in a granitic shear zone at the Grimsel test site, Switzerland (injection flow rate 10 mL/min extraction flow rate 150 mL/min, dipole distance 2.3 m). Am(III), Pu(IV) and Th(IV) are co-eluted with the colloids grey vertical lines indicate maxima of breakthrough curves (Geckeis et al. 2003). In order to allow a direct comparison of breakthrough curves, the colloid and radionuclide concentrations (c in mg/mL) in the extracted water samples are normalized to the total injected mass of individual colloid or radionuclide tracers (mn in mg).

Mechanisms that may lead to the quasi irreversible binding of radionuclides to colloids belong to the key uncertainties of the assessment of the colloid problem. The kinetics of the dissociation of colloid-bound radionuclides are not yet understood. Radionuclide incorporation into stable colloids may enhance the colloid-mediated radionuclide release considerably. It is clear that only the investigation of the interaction mechanisms by spectroscopic methods is able to unravel the relevance of such processes. In order to allow the description of colloid-facilitated radionuclide migration, it is furthermore required to improve our understanding of the colloid interaction... 

Kim, J. 1., Delakowitz, B., Zeh, P., Klotz, D. Lazik, D. 1994. A column experiment for the study of colloidal radionuclide migration in Gorleben aquifer systems. Radiochimica Acta, 66/67, 165-171. 

Mori, A., Alexander, W. R. et al. 2003. The colloid and radionuclide retardation experiment at the grimsel test site Influence of bentonite colloids on radionuclide migration in a fractured rock. Colloids Surfaces, 217, 33-47. 

Walther, C., Bitea, C., Hauser, W., Kim, J. I. Scherbaum, F. J. 2002. Laser induced breakdown detection for the assessment of colloid mediated radionuclide migration. Nuclear Instruments and Methods in Physics Research Section B, 195, 374-388. 

Choppin, G. R. 19886. Humics and radionuclide migration. Radiochimica Acta, 44/45, 23-28. 

During the course of the experimental program, many phenomena will be investigated in detail. Most of these studies will address details of waste/salt interaction, radionuclide migration and salt stability. Results may affect the mode of long-term operation and design of the repository. 

One of the more important factors affecting the isolation of radioactive waste is the rate of release of the radioactivity from the solid waste form to the environment. The most probable mechanism for release and transport of radioactivity from a solid waste form is by leaching of radioactive isotopes with groundwater. The objective of leach-testing various waste forms is to evaluate the rate at which specific hazardous radionuclides migrate from waste if and when the waste form comes in contact with groundwater. In this paper, measurement of leach rates of radioactive waste by a method which incorporates neutron activation is described. 

In most mathematical analyses used to establish bounds for radionuclide migration rates through the abyssal red clays, the sorption properties of the sediment are generally represented mathematically by the sorption equilibrium distribution coefficients for each of the species involved. These coefficients are usually denoted by Kp. and are defined by... 

A number of methods have been proposed for predicting the migration of radionuclides through the soil. Most methods have used a concept of layers of soil, one upon another, with the radionuclide migrating from the top layer downward. 

Geckeis, H., Manh Thang, N. M., Bouby, M., and Kim, J. I. (2003). Aquatic colloids relevant to radionuclide migration Characterization by size fractionation and ICP-mass spectro-metric detection. Colloids Surf. A 217(1-3), 101-108. 

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. 

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. 

Geochemical models of sorption and desorption must be developed from this work and incorporated into transport models that predict radionuclide migration. A frequently used, simple sorption (or desorption) model is the empirical distribution coefficient, Kj. This quantity is simply the equilibrium concentration of sorbed radionuclide divided by the equilibrium concentration of radionuclide in solution. Values of Kd can be used to calculate a retardation factor, R, which is used in solute transport equations to predict radionuclide migration in groundwater. The calculations assume instantaneous sorption, a linear sorption isotherm, and single-valued adsorption-desorption isotherms. These assumptions have been shown to be erroneous for solute sorption in several groundwater-soil systems (1-2). A more accurate description of radionuclide sorption is an isothermal equation such as the Freundlich equation ... 

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