Using COMSOL for the Transport Modelling of Some Special Cases in a Bentonite Buffer in a Final Repository for Spent Nuclear Fuel (original) (raw)
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COMSOL Conference …, 2008
The bentonite barrier is an essential part of safe nuclear waste repository in granitic bedrock. In this work COMSOL Multiphysics® is applied to THMC modelling meaning full coupling of thermal (T), hydrological (H), mechanical (M) and chemical (C) phenomena and processes taking place in a bentonite buffer. The system is studied in different geometries, which consists of an overall 3D layout and a 2D cross-section of bentonite buffer and open fracture. Altogether three main periods have identified in the modelling: 1) emplacement of canister, 2) saturation, and 3) long term safety. The main interest has been on diffusional transport of different species into and out of bentonite buffer.
Nuclear Technology
Finnish spent nuclear fuel disposal is planned to be based on the KBS-3V concept. Within this concept, the role of the bentonite buffer has been considered to be central. The aim was to model the evolution of a final repository during the thermal phase (heatgenerating period of spent fuel), when the bentonite is partially saturated at the beginning, and the rock matrix surrounding it is fully saturated. It is essential to study how temperature affects saturation and how both of these affect the chemistry of bentonite. In order to make the modelling more concrete, an experimental case was adopted: the Long Term Test of Buffer Materials (LOT) A2-parcel test at the Äspö Hard Rock Laboratory (HRL) in Sweden. In the A2-parcel the MX-80 bentonite was exposed to adverse (120-150 o C) temperature conditions and high-temperature gradients. The test parcel diameter was smaller than in the KBS-3V concept to speed up the saturation. Different kinds of thermodynamic and kinetic properties of minerals cause a redistribution of phases inside the bentonite. For example, according to laboratory tests, gypsum seems to dissolve and anhydrite seems to precipitate near the heater-bentonite interface. Also incoming groundwater affects the bentonite porewater and its properties. These changes may affect the mechanical properties of bentonite and it has to be clarified if these phenomena have to be taken into account in safety assessment. The applied model is a coupled thermo-hydro-chemical model, which means that all the mechanical alterations and effects are not considered. The purpose of the model was first to obtain similarity to the results compared to the experiment, and thus, the time frame was limited to 10 years (the LOT A-2 parcel test lasted approximately 6 years). The system is simplified to 1-D in order to reduce the computational work, which is significant mostly due to complex chemical calculations. TOUGH and TOUGHREACT was applied to model the reactive unsaturated transport processes in 1-D and the grid was pitched at uniform intervals. The results may be used to gain knowledge of the bentonite evolution during the thermal phase, and after a good match with experiment the modelling can be continued until the end of the thermal phase for thousands of years.
Applied Clay Science, 2014
The Swedish Nuclear Fuel and Waste Management Company (SKB) is conducting a series of long term tests at the Äspö Hard Rock Laboratory to assess the behaviour of the bentonite buffer under conditions similar to those expected in a KBS-3 repository for high level nuclear waste. The LOT A2 experiment consists of a vertical borehole with a central heater inside a copper tube surrounded by compacted bentonite. During four years, the temperature of the copper tube was maintained at 130°C, while the bentonite was progressively water saturated by the injection of groundwater. During this period, physical and hydro-geochemical data were collected. By using the code TOUGHREACT, a model was made to simulate the processes of solute transport which control the chemical and the mineralogical distribution observed in the bentonite at the end of the test. Additionally, a series of sensitivity analyses was performed to assess the influence of key parameters controlling the thermalhydro-geochemical processes. Numerical results indicate that, within the first year, the heated bentonite blocks are completely water saturated, which agree with the measured data. The simulated transport of chloride, the dissolution/precipitation of Ca sulphate minerals, and the cation redistribution in the montmorillonite interlayer also agree with data measured at the end of the experiment.
The bentonite barrier is an essential part of a safe spent fuel repository in granitic bedrock. In this work COMSOL Multiphysics®, TOUGHREACT and Numerrin are used and compared in modelling the saturation, cation-exchange and mass transport in compacted bentonite. Model comparison was not a straightforward task due different approaches in the model setup. Therefore, we tried to write down all the equations and via parameterisation of those equations to create model descriptions near each other.
Applied Geochemistry, 2010
The FEBEX experiment is a 1:1 simulation of a high level waste disposal facility in crystalline rock according to the Spanish radwaste disposal concept. This experiment has been performed in a gallery drilled in the underground laboratory Grimsel Test Site (Switzerland). Two boreholes parallel to the FEBEX drift were drilled 20 and 60 cm away from the granite-bentonite interface to provide data on potential bentonite-granite solutes transfer. Periodic sampling and analysis of the major ions showed: (a) the existence of solutes transfer from the bentonite porewater towards the granite groundwater, explaining the Cl À and Na + contents of the latter; (b) that the concentration of the natural tracers coming into the granite groundwater from the bentonite porewater increased over time. This bentonite-granite solutes transfer was modelled in order to predict the increase in the Cl À and Na + concentrations of the granite groundwater. The modelled results seem to confirm that the mechanism of solute migration in this scenario is that of diffusive transport. An effective diffusion coefficient of D e = 5 Â 10 À11 m 2 /s was that which best fitted the data obtained.
Rock Mechanics and Rock Engineering, 2013
This paper presents simulation results related to coupled thermal-hydraulic-mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy's used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBSrock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in horizontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geomechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host-rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our initial THM modeling results show strong THM-driven interactions between the bentonite buffer and the lowpermeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances Keywords Nuclear waste disposal Á Modeling Á Coupled processes Á Geomechanics Á Geochemistry Á Clay Á Shale Á Bentonite
Environmental Geology, 2008
In Task A of the international DECOVALEX-THMC project, five research teams study the influence of thermal-hydro-mechanical (THM) coupling on the safety of a hypothetical geological repository for spent fuel. In order to improve the analyses, the teams calibrated their bentonite models with results from laboratory experiments, including swelling pressure tests, water uptake tests, thermally gradient tests, and the CEA mock-up THM experiment. This paper describes the mathematical models used by the teams, and compares the results of their calibrations with the experimental data.
Mineralogical Magazine, 2012
A range of potential concepts for the geological disposal of high level wastes and spent fuel are being studied and considered in the UK. These include concepts that use bentonite as a buffer material around the waste containers. The bentonite will be required to fulfil certain safety functions, the most important being (1) to protect the waste containers from detrimental thermal, hydraulic, mechanical and chemical processes; and (2) to retard the release of radionuclides from any waste container that fails. The bentonite should have a low permeability and a high sorption capacity. These safety functions could be challenged by certain features, events and processes (FEPs) that may occur during the evolution of the disposal system. A consideration of how these FEPs may affect the safety functions can be used to identify and to prioritize the important areas for research on bentonite. We identify these important areas (which include hydration of compacted bentonite, illitization and e...