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Geological repositories for disposal of high-level nuclear wastes generally rely on a multi-barrier system to isolate radioactive wastes from the biosphere. The multi-barrier system typically consists of the natural barrier system, which includes the repository host rock and its surrounding subsurface environment, and the engineered barrier system (EBS). The EBS represents the man-made, engineered materials placed within a repository, including the waste form, waste canisters, buffer materials, backfill, and seals. The EBS plays a significant role in the containment and long-term retardation of radionuclide release.
During the lifespan of a geologic repository, the performance of the EBS is affected by complex thermal, hydrogeological, mechanical, chemical and biological processes, such as heat release due to radionuclide decay, multiphase flow (including gas release due to canister corrosion), swelling of buffer materials, radionuclide diffusive transport, waste dissolution and chemical reactions. All these processes are related to each other. An in-depth understanding of these coupled processes is critical for the performance assessment (PA) for an EBS and the entire repository. Within the EBS work package of the Used Fuel Disposition (UFD) Campaign, LBNL’s research is currently focused on two relevant issues, namely the (1) thermal-hydraulic-mechanical-chemical (THMC) processes in buffer materials (bentonite), and the (2) diffusive transport in the EBS associated with clay host rock, with the long-term goal of developing a full understanding of (and verified modeling capabilities to simulate) the impact of coupled processes on radionuclide transport in different components of EBS, as well as the interaction between the near-field host rock (e.g., clay/shale) and EBS components—and how they affect radionuclide release.
LBNL’s specific research activities in the EBS area currently include:
The main component of the EBS used in a number of countries is bentonite or a bentonite-sand mixture. Bentonite is a clay with a high content of smectite minerals, having properties very suitable for isolating radioactive waste canisters. It has a very low hydraulic conductivity and very low anion diffusion capacity and low transport capacity of positive charged radionuclides. Thus it can serve as a good buffer around the waste canisters. Mechanically, its high swelling potential makes possible self sealing of openings within the EBS system and of gaps between the bentonite with the rock and with the radioactive waste canister. Its thermal conductivity is adequate to conduct heat away from the canister. Further it has good colloid and microbial filtration capability.
There are three phases in the evolution of the bentonite buffer in a radioactive waste repository. The first phase is the installation and initial water intake of the buffer over a period of 0 to 100 years dependent on repository design. The second phase is dominated by heat input from the waste canister with a significant time-varying temperature gradient, with the temperature intially increasing and then returning to ambient over a period of 100 to 1000 years. The third phase is the long-term period with tectonic or glacial processes under ambient or reduced temperature conditions from 1000 to a million years.
When a bentonite buffer is installed around waste canisters in a repository, it is initially partially saturated with water. The heat released from radioactive waste heats up the near field of the bentonite-rock system. Processes involved in the evolution of the bentonite buffer include thermally induced distribution of initial pore water in the clay during the early thermal phases. On the outer part of the bentonite buffer, water is uptaken from the rock-water interface, with potential swelling of the bentonite in this region. On the inner part of the bentonite buffer, next to the heat-releasing waste canister, moisture content decreases (desiccation), with potential shrinkage. Over time, the expansion of the bentonite as it is being hydrated may displace the position of the waste canister enveloped within it, as well as the position of the buffer-backfill interface. Chemically within the bentonite buffer, there may be the dissolution of buffer minerals and precipitation of chemical compounds.
The coupled THMC processes in Bentonite buffer have significant impacts on the integrity of EBS and performance of geological repository system. LBNL’s current focus of this task is on (1) developing and improving THMC modeling capabilities especially by incorporating the advanced constitutive models, (2) validating model development with test data, and (2) improving our understanding of the coupled processes and their impacts by modeling field tests and through scenario analyses. As an example, the figure shows simulation results for a generic repository system.
Model domain for an assumed bentonite back-filled horizontal emplacement drift at 500 m depth in clay host rock.
Simulated evolution of THM processes in buffer: (a) temperature, (b) liquid saturation, (c) fluid pressure, and (d) total radial stress .
Compacted bentonite has been proposed as backfill material in many of the European repositories, because of its very low permeability and its strong sorptive properties, both of which will limit the release of radionuclides. The low permeability of bentonite is largely due to the fact that it contains a high percentage of Na-montmorillonite, a clay that swells in water. The very low permeability of the compacted bentonite implies that transport of radionuclides away from the waste forms will be almost exclusively by molecular diffusion, with effective diffusivities far below that in water. Effective diffusivities for the compacted bentonite are very low as a result of its low porosity and the nanometer scale of the pores in the compacted clay. In much of the compacted bentonite, the pores are so small (<1 nanometer) that the electrical double layers balancing the charge of the bentonite (typically negative at circumneutral pH) overlap, thus potentially excluding anions altogether, or creating a deficiency in them with the diffuse double layer balancing the surface mineral charge.
Three types of water presence are recognized in compacted bentonite: (1) Interlayer water with only water and cations within the Tetrahedral-Octahedral-Tetrahedral (TOT) layers of the montmorillonite, (2) diffuse double layer containing cations and anions, but with an excess of ions (normally cations) to balance the charge of the clay surface, and (3) bulk or free pore water which is charge balanced. The proportions of each kind of water depend on the compaction of the bentonite, but also the ionic strength through its effect on the width of the diffuse double layer. In addition, the nature of cation affects the swelling and therefore the interlayer spacing.
A schematic showing clay-pore interface (modified from Leroy et al., 2007)
LBNL’s current focus of this task is on developing improved modeling approach for diffusive transport in bentonite based on the Donnan Equilibrium (between the diffusive double layer and bulk water) and explicit modeling of diffuse double layer. The modeling approach will be validated with test data. In addition, we will use our well-tested molecular dynamics (MD) simulation methodology to predict the activation energy of diffusion of Na+ and H2O in the 3-layer hydrate of Na-montmorillonite and determine the sensitivity of predicted EA values to a range of conditions and simulation parameters.
Compacted bentonite has been proposed as a backfill material for repositories due to its low permeability and strong sorptive properties. The transport of radioactive contaminants through bentonite layers is expected to be dominated by diffusion processes. Hence, laboratory diffusion testing is an important component for the design of waste containment barriers. As the basis of nuclear fuel, uranium is one of the primary elements to be considered in performance assessments for nuclear waste repositories. Furthermore, other radionuclides that are bound in the fuel matrix can only be released at the same rate as uranium dissolves and diffuses through the EBS. Hence, a fundamental understanding of uranium diffusion through bentonite is essential. In addition, in conceptual diffusion studies, uranium can serve as a useful ‘analog’ for other radioactive contaminants, such as Pu, Np, and Am.
The primary goal of this study is to (1) characterize U(VI) sorption and diffusion behavior in terms of chemical solution conditions (pH, salt composition, ionic strength, total carbonate concentration) and degree of clay compaction, and (2) provide experimental data for validating a reactive U(VI) diffusion model that is being developed in the Task “Modeling Reactive-Diffusive Transport in the EBS”. We conduct laboratory-scale experiments to evaluate U(VI) sorption and diffusion behavior in different chemical conditions. The synchrotron X-ray spectroscopic and electron-based imaging techniques are employed in these experiments.
Schematic for a test set-up
|Jens Birkholzer||NE/NW Program Head||Hydrogeology Department||510-486-7134||510-486-5686||JTBirkholzer@lbl.gov|
|Jonny Rutqvist||Staff Scientist||Hydrogeology Department||510-486-5432||510-486-5686||JRutqvist@lbl.gov|
|Carl Steefel||Staff Scientist||Geochemistry Department||510-486-7311||510-486-5686||CISteefel@lbl.gov|
|Ian Bourg||Staff Scientist||Geochemistry Departmentemail@example.com|
|James Davis||Staff Scientist||Geochemistry Department||510-486-2692||510-486-5686||JADavis@lbl.gov|
|Ruth Tinnacher||Staff Scientist||Geochemistry Department||510-486-8231||510-486-5686||RMTinnacher@lbl.gov|