Hydrogeochemical evolution of the bentonite buffer in a KBS-3 repository for radioactive waste. Reactive transport modelling of the LOT A2 experiment (original) (raw)
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Nuclear Technology
After closure of an underground nuclear waste repository, the decay of radionuclides will raise temperature in the repository, and the bentonite buffer will resaturate by water inflow from the surrounding host rock. The perturbations from these thermal and hydrological processes are expected to dissipate within hundreds to a few thousand years. Here, we investigate coupled thermal-hydro-chemical processes and their effects on the short-term performance of a potential nuclear waste repository located in a clay formation. Using a simplified geometric configuration and abstracted hydraulic parameters of the clayey formation, we examine geochemical processes, coupled with thermo-hydrologic phenomena, and potential changes in porosity near the waste container during the early thermal period. The developed models were used for evaluating the mineral alterations and potential changes in porosity of the buffer, which can affect the repository performance. The results indicate that mineral alteration and associated changes in porosity induced by early thermal and hydrological processes are relatively small and are expected to not significantly affect flow and transport properties. Chlorite precipitation was obtained in all simulation cases. A maximum of one percent volume fraction of chlorite could be formed, whose process may reduce swelling and sorption capacity of bentonite clay, affecting the performance of the repository. llitisation process was not obtained from the present simulations.
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 Sciences
Bentonite is used as a buffer material in most high-level radioactive waste (HLW) repository designs. Smectite clay is the main mineral component of bentonite and plays a key role in controlling the buffer’s physical and chemical behaviors. Moreover, the long-term functions of buffer clay could be lost through smectite dehydration under the prevailing temperature stemming from the heat of waste decay. Therefore, the influence of waste decay temperatures on bentonite performance needs to be studied. However, seldom addressed is the influence of the thermo-hydro-chemical (T-H-C) processes on buffer material degradation in the engineered barrier system (EBS) of HLW disposal repositories as related to smectite clay dehydration. Therefore, we adopted the chemical kinetic model of smectite dehydration to calculate the amount of water expelled from smectite clay minerals caused by the higher temperatures of waste decay heat. We determined that the temperature peak of about 91.3 °C occurred...
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.
2009
The bentonite barrier is an essential part of a safe spent fuel repository in granitic bedrock. In this work COMSOL Multiphysics® is used in modelling the thermal (T), hydrological (H), mechanical (M) and chemical (C) phenomena and processes taking place in a bentonite buffer. Special interest lies in systems in which the density of bentonite or bentonite pore water varies. Typically, variation occurs during the erosion and wetting stage of bentonite. The reason for developing COMSOL models for this purpose is the lack of commercially available software, which typically covers either THC or THM models; M and C are not modelled together.
Geochemical model of the granite–bentonite–groundwater interaction at Äspö HRL (LOT experiment)
Applied Clay Science, 2003
The bentonite buffer material has become an integral component of many HLNW repository designs since its introduction by SKB in the KBS-3 concept. The bentonite buffer provides mechanical stability, hydrological isolation and chemical buffering and radionuclide retardation. The three functions are interconnected and, consequently, the geochemical stability of the bentonite buffer is essential for the performance of the HLNW repository.
2008
A numerical model of coupled saturated/ unsaturated water flow, heat transfer and multi-component reactive solute transport is presented to evaluate the longterm geochemical evolution in bentonite, concrete and clay formation for a potential geological radioactive waste repository. Changes in formation porosity caused by mineral dissolution/precipitation reactions are taken into account. Simulations were carried out with a general-purpose multicomponent reactive transport code, CORE 2D V4. Numerical results show that pH in the bentonite porewater can vary from neutral to up to 13 over a time scale of 1 Ma although dissolution of silica minerals and precipitation of secondary calcium silicate hydrate (CSH) minerals in bentonite buffer the effect of the hyperalkaline plume. Mineral precipitation reduces the volume of pore space in bentonite close to the bentonite-concrete interface due to the precipitation of CSH minerals. Model results indicate that bentonite porosity decreases less than 25%. The hyperalkaline plume from the concrete only extends to a distance of 0.7 m in the clay formation over the time range of 1 Ma.
Journal of contaminant hydrology, 2017
Radioactive waste disposal in deep geological repositories envisages engineered barriers such as carbon-steel canisters, compacted bentonite and concrete liners. The stability and performance of the bentonite barrier could be affected by the corrosion products at the canister-bentonite interface and the hyper-alkaline conditions caused by the degradation of concrete at the bentonite-concrete interface. Additionally, the host clay formation could also be affected by the hyper-alkaline plume at the concrete-clay interface. Here we present a non-isothermal multicomponent reactive transport model of the long-term (1Ma) interactions of the compacted bentonite with the corrosion products of a carbon-steel canister and the concrete liner of the engineered barrier of a high-level radioactive waste repository in clay. Model results show that magnetite is the main corrosion product. Its precipitation reduces significantly the porosity of the bentonite near the canister. The degradation of the...
Clays and Clay Minerals, 2010
Bentonite is often proposed as an engineered-buffer material in high-level radionuclide wastemanagement systems. For effective design of the barrier that will provide protection over the long time periods required, the physical/thermal/chemical processes taking place in the barrier material must be understood thoroughly. These processes, which interact, include the flow of water and gas, the flow of heat, and the transport and reaction of chemical constituents. The purpose of this study was to better understand the processes that occurred in a small-scale experiment within a confined bentonite space. A conceptual and mathematical model (FADES-CHEM) was built in order to simulate the results of an experiment conducted in 2000, and thereby to gain a better understanding of the controlling processes. In that experiment, a block of compacted bentonite was placed in an airtight cell and subjected, for 6 months, to simultaneous heating and hydration from opposite sides. The bentonite block was then sliced into five sections each of which was then analyzed in order to obtain a series of physicochemical parameters illustrating the changes that had occurred. Before modeling, the chemical composition of the bentonite pore waters was restored in order to account for different processes such as gas outgassing and cell cooling. Modeling indicated that gas-pressure build up was a relevant process when computing the saturation of bentonite, and the computations in the present study suggested that evaporation/condensation processes played a crucial role in the final distribution of the water content. Gas pressure and evaporation/ condensation also affected the geochemical system, and the numerical model developed gives results that were consistent with the experimental values and trends observed. The model reproduced the results obtained and enable use at the repository scale and over longer time frames, provided that adequate data are available.