Radionuclide transport, barriers

This is not to say that all is doom and gloom. Although many of these tasks are formidable, the earth-science community believes them to be tractable and that a successful geologic repository for radioactive waste can be constructed. We only plead that our ignorance of earth s processes be considered in the development of a repository and that any repository constructed prior to the acquisition of the needed fundamental knowledge contain many independent natural and manmade barriers to radionuclide transport to compensate for our lack of knowledge. 

The efficiency of a rock formation as a transport barrier depends on fluid flow and on radionuclide retention in the rock due to a variety of physical and chemical processes. Open fissures or fractures in the rock provide pathways through which water and radionuclides may travel. Although most radionuclides have a strong tendency to sorb to mineral grains in the rock, tracers first have to diffuse from fractures into the rock matrix in order to access the extensive pool of sorption sites (Neretnieks, 1980). Diffusion in turn depends on mass transfer properties of the rock matrix and on the hydrodynamics of fracture networks, emphasizing the interaction between water flow, advective transport and retention processes. Although models for reactive transport in discrete fracture networks have been around for some time, it is only recently that a theoretical framework is available for systematic studies of the hydrodynamic impact on retention (e.g., Cvetkovic et al., 1999, 2002). 

Radioactive waste management is a quite mature field of application of basic geoscientific disciplines. As we will discuss in forthcoming sections, the long-term performance and henceforth the safety of radioactive waste disposal systems, deeply relies on the basic principles that control the release, mobility, and transport of the chemical elements in the geosphere. In the context of radioactive waste disposal, the waste matrix constitutes the innermost of the barriers that may control the release and ulterior transport of radionuclides through the ground-water systems. 

The transport process from the repository is schematically illustrated in Figure 3. Radionuclides and corrosion products would diffuse from the canister into the groundwater, and the migration will then be retarded first in the backfill barrier and after that in the bedrock. 

Blomqvist, G., "Leaching of French, British and Canadian Glass Containing High-level Radioactive Waste", KBS TR 08, 1977 Neretnieks, I., "Transport of Oxidants and Radionuclides Through a Clay Barrier", KBS TR 79, 1978... 

It is an attractive object of research to synthesize labelled compounds that are taking part in specific biochemical processes or able to pass specific barriers in the body, with the aim of detecting malfunctions and of localizing the origin of diseases. Complexes of short-lived no-carrier-added radionuclides and high yields of the syntheses are of special interest. In the case of short-lived radionuclides, such as fC, the synthesis must be fast and as far as possible automated. Labelled organic molecules can also be used to transport radionuclides to special places in the body for therapeutical application, i.e. as specific internal radiation sources. 

The PA modeller in charge of studying the near field behaviour would use a Monte Carlo assessment code in order to assess the transport of radionuclides through the engineered barriers to the surrounding rock. The PA modeller would like to have feedback from the engineering experts on the following key points ... 

The risk that, through a deterioration in their condition, the SNF containment barriers fail during recovery operations, releasing radionuclides either at the scene or during subsequent recovery and transportation to the land disposal site. 

We examine the problem of diffusion in a porous medium using a homogenization analysis (HA). Diffusion problems have important applications in environmental geosciences. We clarily the mechanism of diffusion, convective transport and adsorption in porous media at both the microscale and macroscale levels. Attention is particularly focused on diffusion processes in bentonite, which is an engineered geological barrier to be used to buffer the transport of radionuclides from deep geologic repositories. 

The repository shall be located at sufficient depth to protect adequately the emplaced waste from external events and processes, in a host rock having properties that adequately restrict the deterioration of physical barriers and the transport of radionuclides from the repository to the environment. 

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