Underground Nuclear Waste Repository Research Articles

Swelling Behavior of Bentonite-Based Backfilling Materials in Nuclear Waste Repository Conditions

During the postconstruction phase, the stress equilibrium conditions within the proposed underground nuclear waste repositories may alter because of various factors, such as far-field mining activity, fluctuation of the groundwater table, and seepage erosion. A reduction in the transmitted stress from the host rock may destabilize the isochoric condition, leading to swelling displacement of backfilling and buffer materials. In the current study, the swelling behavior of bentonite-based backfilling materials was studied. Laboratory tests were carried out on compacted bentonite (dry density=1.26 Mg/m3) and bentonite-sand (70% bentonite, 30% sand, dry density=1.50 Mg/m3) specimens in a strain-controlled oedometer. First, compacted specimens were hydrated with distilled water under constant volume condition. Next, predetermined displacements were introduced in a step-wise manner that caused expansion of the specimens. The swelling pressures of the specimens were measured under a constant-volume condition at each induced strain. The test results showed that induced strains caused a decrease in the swelling pressures of the saturated backfilling materials. A decrease in the swelling pressure for a maximum induced strain of approximately 1.6% was found to be approximately 19% for the bentonite specimen and approximately 30% for the bentonite-sand specimen. It was shown how the swelling index of backfilling materials can be used to assess the recoverable swelling pressure following any induced strain.

Hydrologic issues associated with nuclear waste repositories

Significant progress in hydrology, especially in subsurface flow and solute transport, has been made over the last 35 years because of sustained interest in underground nuclear waste repositories. ...

Non-invasive detection of fractures, fracture zones, and rock damage in a hard rock excavation — Experience from the Äspö Hard Rock Laboratory in Sweden

A key requirement for licensing of the construction of underground repositories for nuclear waste is the demonstrated capability to verify design assumptions involving the presence and extent of the excavation damage zone around tunnels, shafts, emplacement holes and caverns. As part of ongoing work to select and refine key technologies and techniques towards this end, geophysical surveys were performed at two locations within the Äspö Hard Rock Laboratory in Sweden. Earth resistivity (RES), induced polarization (IP), and Ground Penetrating Radar (GPR) data were collected using a variety of survey parameters; Light Detection and Ranging (LiDAR) data were collected as a reference for surface structures, surface topography, and site geology. Based on an analysis of the data, models for the Highly Damaged Zone (HDZ) and Excavation Damage Zone (EDZ) at both sites were developed. The HDZ was found to be approximately 5 to 10cm in thickness, and the EDZ was found to extend between 15 and 35cm below the excavation surface. Two-dimensional (2D) RES profiling generated the most reliable assessment of the HDZ, whereas chargeability data and GPR data were more useful in the estimation of the EDZ dimensions.

France testing viability of underground nuclear waste repository

France testing viability of underground nuclear waste repository

Chlorine transport processes through a 2000 m aquifer/aquitard system

In the Paris Basin, in France, the Callovo-Oxfordian (COx) is currently studied over a 250 km2 surface area by the French national radioactive waste management agency in order to assess the feasibility of long-term underground nuclear waste repository. The COx is a 140 m thick clay-rich layer, which is part of the 2000 m aquitard/aquifer system constituting the sedimentary cover. In such sedimentary context, the transport processes of potential contaminants can be represented by both vertical diffusion and horizontal advection through the most permeable layers.Chloride is used as a natural conservative tracer, and is monitored in term of concentrations and isotopic composition (δ37Cl) for both pore and groundwater. During this study, the samples were collected from three boreholes located in the center of the studied zone, one of them (EST433) going down to 2000 m depth. The main solute transport process is shown to be vertical diffusion from the massive Keuper halite level to the rest of the sedimentary pile. This global diffusive system can be occasionally disturbed by horizontal circulation of groundwater occurring in the Oxfordian and Dogger limestone formations. Therefore, these circulations cut the global diffusive system in a succession of independent diffusive systems. In this study the data set was implemented in a simplified 2D solute transport model and scenarii reproducing known history in term of paleo circulations inside the system, were applied and allowed to obtained a good fit of the data. Model results showed that paleo circulations, occurring between −145 Ma and −110 Ma, still have an impact on current distribution of chloride in the system, especially for δ37Cl. The model highlights the need of the presence of a circulation spatially limited at the base of the Liassic formation to fit the data. The best fit obtained indicated current residence time of 500 ka in the Dogger and Oxfordian, with respective onset of the circulations at −20 Ma and −5 Ma.

Investigation of the THM behaviour of the buffer and rock-buffer interaction during the canister retrieval test performed in the ASPÖ Hard Rock Laboratory

In the framework of the THERESA European project, numerical modelling of coupled thermo-hydro-mechanical (THM) and thermo-hydro-mechanical-chemical (THMC) behaviour of buffer (bentonite) and buffer-rock interfaces for deep underground nuclear waste repositories has been undertaken, with focus on the performance assessments.A major step of the project was the analysis of a large scale test, called the Canister Retrieval test, which has been performed in Aspö Hard Rock Laboratory. It consists in a full scale test of the emplacement of a canister with the surrounding buffer material. A deposition hole was first bored, and then the canister with heaters was installed together with bentonite blocks. The gap between the rock and the bentonite blocks was filled with bentonite pellets. The whole set was artificially wetted from its external boundary in order to accelerate the expected natural rehydration by the surrounding rock. The evolution of the THM processes was recorded over 5 years.Before analysing the whole CRT experiment, a preliminary simpler problem has been defined, which consisted in modelling a disc of buffer at canister mid-height. Thanks to the available experimental recorded measurements, it has been possible to numerically investigate the respective influence of the various THM parameters involved in the modelling of the physical processes. The theoretical model is based on one hand on the Richard's approximation for the flow calculation, and on the other hand on a Biot's type model for the hydro-mechanical behaviour. It has revealed the large influence of the liquid relative permeability, which is unfortunately in general not directly available from experiments and must be determined through inverse analysis techniques.Then, in a second stage, the whole CRT experiment has been analysed. For simplicity reasons, an axisymetrical model has been adopted, although the presence of a neighbouring experiment did influence the CRT results. The comparisons of the most relevant features of the test, between predicted and measured values, are presented and discussed in the paper.

Hydration Behavior of MX80 Bentonite in a Confined-Volume System: Implications for Backfill Design

AbstractBentonites are considered suitable backfill material for planned underground nuclear-waste repositories because of an inherent capacity to self-seal and retain contaminants when hydrated. Barrier effectiveness, however, depends on the physical properties of bentonite after placement in a repository site, where hydration state and bulk density can vary. The objective of the present study was to investigate commercial bentonite MX80 hydration rates and mechanisms during water infiltration into dry, moist, and wet samples using the ‘wet-cell’ X-ray diffraction technique. During experimentation, water enters a small flow-through cell and induces swelling within a confined reaction volume, analogous to clay barriers in excavated underground sites. Results demonstrated the importance of using dry, well compacted (>1.4 g/cm3) bentonite, which became saturated slowly (<2.0 ×10−9 m/s) with minimal water in noninterlayer sites (external-surface sites, or within pores). The significant degree of interlayer expansion dominated by the formation of two and eventually three water layers developed as hydration clusters with greater probabilities for the same thickness to lie in adjacent interlayer sites. The relatively thicker particles and the less accessible surface area of hydrated, initially dry bentonite probably resulted in less pore-controlled diffusion, but also less potential radionuclide adsorption by surface complexation. Moist MX80 had the greatest water uptake, the smallest (1.23 g/cm3) dry bulk density, and the greatest proportion of water in pores and on external surfaces. Water that initially accumulated in pore spaces subsequently acted as a reservoir for interlayer hydration and probable gel formation in trapped voids, which is expected to occur in more loosely filled gaps within an excavated repository.

Bentonite Alteration Due to Thermal-Hydro-Chemical Processes During the Early Thermal Period in a Nuclear Waste Repository

After closure of an underground nuclear waste repository, the decay of radionuclides will raise the 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 thermohydrologic 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. The illitization process was not obtained from the present simulations.

Hydromechanical behaviour of a heterogeneous compacted soil: experimental observations and modelling

The paper describes a theoretical and experimental study of the coupled hydromechanical behaviour of a compacted mixture of bentonite powder and bentonite pellets intended as sealing material in underground repositories for nuclear waste. One of the main advantages of the use of powder/pellets mixtures is the reduction of the compaction effort required to achieve the value of average dry density necessary to attain the required swelling potential. However, the heterogeneous fabric of the material requires special approaches in order to describe adequately its behaviour during hydration. A double porosity formulation is presented to account for the presence of two distinct structural levels in the material. Hydraulic equilibrium between the two porosities is not assumed; instead a water exchange term between them is postulated. The formulation is applied to the modelling of a number of one-dimensional swelling pressure tests performed in the CEA (Commisariat à l'Énergie Atomique, France) and CIEMAT (Spain) laboratories. A very satisfactory quantitative description of the experimental observations is obtained that includes a number of complex behaviour features such as size effects and non-monotonic development of swelling pressures. Some microfabric observations using X-ray tomography and mercury intrusion porosimetry lend support to the conceptual approach adopted. The formulation is then applied to the analysis of a long-term large-scale sealing test performed at the Hades underground facility in Belgium, using the same set of hydraulic and mechanical parameters employed in the modelling of the laboratory tests. Although the field observations exhibit a much higher degree of scatter, the basic behaviour of the field sealing test is satisfactorily simulated. A formulation that incorporates basic features of the microfabric of the mixture is thus able to span successfully over a large range of space and time scales.

Dissolved helium distribution in the Oxfordian and Dogger deep aquifers of the Meuse/Haute-Marne area

The 140-m-thick, clay-rich Callovo-Oxfordian (COX) layer of the eastern Paris Basin, France, is being considered by the French Nuclear Waste Agency (Andra) as a long-term underground nuclear waste repository. Andra has selected a 250 km 2 area (transposition zone) to be further characterised, especially in view of the confinement properties exhibited by the COX. This study reports the helium concentrations and isotopic ratios of water samples from the aquifers above and below the COX, which are the Oxfordian and Dogger aquifers, respectively. The samples were collected from five drilling sites (2007–2008). Both the Oxfordian and Dogger groundwaters are of meteoric origin and have accumulated radiogenic He with a 3He/ 4He ratio of approximately 0.02 Ra. The He concentrations in the groundwaters are two orders of magnitude higher than in the air-saturated water in the Oxfordian and approximately 10 times higher in the Dogger than in the Oxfordian. One borehole was drilled down into the Triassic sediments, allowing a sample to be collected from the Bundsandstein aquifer. Here, the He concentration is of the same order of magnitude as those measured in the Dogger, but the isotopic ratio is slightly higher at 0.04 Ra. However, this ratio is approximately 10 times lower than those measured by Marty et al. (2003) in waters collected closer to the Trias recharge and tagged with mantle-derived He. This lower ratio is most likely due to a significant and rapid slowdown of the circulation in this aquifer, allowing substantial radiogenic He accumulation. A key conclusion of this study concerns the lateral non-homogeneity of the studied area: He concentrations are higher in the northern part of the transposition zone. This observation can be explained by a longer residence time of the waters and/or a higher input of He from the basement. A simplified 2D model of He transport shows that the second hypothesis alone does not allow fitting of the data, thus implying more stagnant water in this area. The indicative residence times derived from this coarse model are 0.3–0.5 Myr (Oxfordian) and 0.5–0.6 Myr (Dogger) for the borehole located in the centre of the investigated zone; for the northernmost borehole, they reach 0.8–1.4 Myr for the Oxfordian and 1.5 Myr for the Dogger.

Corrosion susceptibility of potential rock bolts in aerated multi-ionic simulated concentrated water

Rock bolts used for the reinforcement of underground mines, tunnels and nuclear waste repositories are made up of low and medium carbon steels, and high strength low alloy steels. Typical rock bolt systems used for rock reinforcement are mechanically anchored bolts, grout anchored, frictional rock stabilizers and strand anchors. For nuclear waste repository sites such as Yucca Mountain (YM) as well as for mines and tunnels, in addition to mechanical properties, corrosion properties are also important due to potential seepage of water through the fractures or pores in the rock. During temporary rock support period of 50–100 years, the temperature of the tunnel at YM should be maintained at ambient conditions. For any reason if the rock bolts are exposed to YM waters and high temperatures in the tunnel then there is a chance of corrosion of steel rock bolts. In this study an attempt was made to study the corrosion properties of various potential rock bolts for YM tunnel support via the aid of electrochemical corrosion testing. At ambient temperature (25 °C) all the rock bolts that were studied showed good corrosion resistance in these waters. At higher temperatures, 60 °C and 90 °C, corrosion resistance of rock bolts decreased, but due to special stress relief heat treatment of one of the frictional rock stabilizers (Swellex Mn 24) the corrosion rates were lower than all other tested rock bolts. Note: Swellex, Split set and Williams are the proprietary names of Atlas Copco, International Roll Forms, Inc. and Williams Form Engineering Corp, respectively.

Averaging of a Singular Random Source Term in a Diffusion Convection Equation

We consider a simplified model for the radionuclides migration in an underground nuclear waste repository, based on a linear partial differential equation of diffusion convection type. This partial differential equation has a source term constituted by a large number of local sources spatially periodically distributed and lying on the porous domain median plane. The behavior of each source is spatially homogeneous but their time dependence is uncertain; their release curve (source emission versus space and time) parameters are random both in space and in time. Starting from the mesoscopic model described above, our aim is then to obtain by upscaling a model with a deterministic averaged source term, describing the global evolution of such a system, and to prove the convergence, estimate (under proper mixing assumptions) the rate of convergence, and characterize the asymptotic behavior of the corrector.

Scaling up of an underground nuclear waste repository including a possibly damaged zone

We derive a macroscopic model for an underground nuclear waste repository consisting of long storage cells linked by a possibly damaged drifts. As the first result we find a simple first-order approximation. Secondly, we compute a corrector using a matched expansion around the drift. We prove an appropriate convergence result.

Existence of solutions for a model of multiphase flow in porous media applied to gas migration in underground nuclear waste repository

We prove existence of solutions for a new model of two compressible and partially miscible phase flow in porous media, applied to gas migration in underground nuclear waste repository. This model, modelling fully and partially water saturated situations, consists of a coupled system of quasilinear parabolic partial differential equations. We seek a new set of variables in order to obtain a system which belongs to the class of equations considered by Alt and Luckhaus such that it would be possible to use their existence theorem.

Modeling coupled THM processes of geological porous media with multiphase flow: Theory and validation against laboratory and field scale experiments

A FEM model for analysis of fully coupled multiphase flow, thermal transport and stress/deformation in geological porous media was developed based on the momentum, mass and energy conservation laws of the continuum mechanics and the averaging approach of the mixture theory over a three phase (solid–liquid–gas) system. Six processes (i.e. stress–strain, water flow, gas flow, vapor flow, heat transport and porosity evolution processes) and their coupling effects are considered, which not only makes the problem well-defined, but renders the governing PDEs closed, complete, compact and compatible. Displacements, pore water pressure, pore gas pressure, pore vapor pressure, temperature and porosity are selected as basic unknowns. The physical phenomena such as phase transition, gas solubility in liquid, thermo-osmosis, moisture transfer and moisture swelling are modeled. As a result, the relative humidity and other related variables in porous media can be evaluated on a sounder physical basis. A three dimensional computer code, THYME3D, was developed, with eight degrees of freedom at each node. The laboratory CEA Mock-up test and the field scale FEBEX benchmark test on bentonite performance assessment for underground nuclear waste repositories were used to validate the numerical model and the software. The coupled THM behaviors of the bentonite barriers were satisfactorily simulated, and the effects and impacts of the governing equations, constitutive relations and property parameters on the coupled THM processes were understood in terms of more straightforward interpretation of physical processes at microscopic scale of the porous media. The work developed enables further in-depth research on fully coupled THM or THMC processes in porous media.

Hydrogeochemistry of groundwaters in and below the base of thick permafrost at Lupin, Nunavut, Canada

Summary Shield fluids are commonly understood to evolve through water–rock interaction. However, fluids may also concentrate during ice formation. Very little is currently known about groundwater conditions beneath thick permafrost in crystalline environments. This paper evaluates three possible Shield fluid evolution pathways at a crystalline Shield location currently under 500+ meters of permafrost, including surfical cryogenic concentration of seawater, in situ cryogenic concentration and water–rock interaction. A primary goal of this study was to further scientific understanding of permafrost and its role in influencing deep flow system evolution, fluid movement and chemical evolution of waters in crystalline rocks. Precipitation, surface, permafrost and subpermafrost water samples were collected, as well as dissolved and free gas samples, fracture fillings and matrix fluid samples to characterize the site. Investigations of groundwater conditions beneath thick permafrost provides valuable information which can be applied to safety assessment of deep, underground nuclear waste repositories, effects of long-term mining in permafrost areas and understanding analogues to potential life-bearing zones on Mars. The study was conducted in the Lupin gold mine in Nunavut, Canada, located within the zone of continuous permafrost. Through-taliks beneath large lakes in the area provided potential hydraulic connections through the permafrost. Na–Cl and Na–Cl–SO4 type permafrost waters were contaminated by mining activities, affecting the chloride and nitrate concentrations. High nitrate concentrations (423–2630 mg L−1) were attributed to remnants of blasting. High sulfate concentrations in the permafrost (578–5000 mg L−1) were attributed to naturally occurring and mining enhanced sulfide oxidation. Mine dewatering created an artificial hydraulic gradient, resulting in methane hydrate dissociation at depth. Less contaminated basal waters had medium sulfate concentrations and were Ca–Na dominated, similar to deeper subpermafrost waters. Subpermafrost waters had a wide range of salinities (2.6–40 g L−1). It was unclear from this investigation what impact talik waters would have on deep groundwaters in undisturbed environments. In situ cryogenic concentration due to ice and methane hydrate formation may have concentrated the remaining fluids, however there was no evidence that infiltration of cryogenically concentrated seawater occurred since the last glacial maximum. Matrix waters were dilute and unable to affect groundwater salinity. Fracture infillings were scarce, but calcite fluid inclusion microthermometry indicated a large range in salinities, potentially an additional source of salinity to the system.

A model of multiphase flow and transport in porous media applied to gas migration in underground nuclear waste repository

We prove existence of solutions for a new model of two compressible and partially miscible phase flow in porous media, applied to gas migration in an underground nuclear waste repository. This model, modeling fully and partially water saturated situations, consist of a coupled system of quasilinear parabolic partial differential equations. We seek a new set of variables in order to obtain a system which belongs to the class of equations considered by Alt and Luckhaus such that it would be possible to use their existence theorem. A simulation of a numerical test case is performed in order to numerically demonstrate the ability of this model to take in account the appearance of one phase. To cite this article: F. Smaï, C. R. Acad. Sci. Paris, Ser. I 347 (2009).

Two asymptotic models for arrays of underground waste containers

We study the homogenization of two models of an underground nuclear waste repository. The nuclear waste cells are periodically stored in the middle of a geological layer and are the only source terms in a parabolic evolution problem. The diffusion constants have a very large contrast between the fuel repository and the soil. It is thus a combined problem of homogenization and singular perturbation. For two different asymptotic contrasts we give the homogenized limit problem which is rigorously justified by using two-scale convergence. Eventually we perform 2D numerical computations to show the effectiveness of using the limit model instead of the original one.

Multiple-point statistical prediction on fracture networks at Yucca Mountain

In many underground nuclear waste repository systems, such as Yucca Mountain project, water flow rate and amount of water seepage into the waste emplacement drifts are mainly determined by hydrological properties of fracture network in the surrounding rock mass. Natural fracture network system is not easy to describe, especially with respect to its connectivity which is critically important for simulating the water flow field. In this paper, we introduced a new method for fracture network description and prediction, termed multi-point-statistics (MPS). The process of Multi-point Statistical method is to record multiple-point statistics concerning the connectivity patterns of fracture network from a known fracture map, and to reproduce multiple-scale training fracture patterns in a stochastic manner, implicitly and directly. It is applied to fracture data to study flow field behavior at Yucca Mountain waste repository system. First, MPS method is used to create fracture network with original fracture training image from Yucca Mountain dataset. After we adopt a harmonic and arithmetic average method to upscale the permeability to a coarse grid, THM simulation is carried out to study near-field water flow in surrounding rock of waste emplacement drifts. Our study shows that connectivity or pattern of fracture network can be grasped and reconstructed by Multi-Point-Statistical method. In theory, it will lead to better prediction of fracture system characteristics and flow behavior. Meanwhile, we can obtain variance from flow field, which gives us a way to quantify uncertainty of models even in complicated coupled THM simulation. It indicates that Multi-Point Statistics is a potential method to characterize and reconstruct natural fracture network in a fractured rock mass with advantages of quantifying connectivity of fracture system and its simulation uncertainty simultaneously.

Dissolution kinetics of clinoptilolite and heulandite in alkaline conditions

Temperature and pH-dependent dissolution kinetics of natural clinoptilolite and a ‘pure’ heulandite phase have been determined and compared. Si dissolution rates increased with increasing temperature and pH. Heulandite has a slightly higher dissolution rate than clinoptilolite in the pH range 8–10. Measured reaction order was 0.23 and 0.14 for clinoptilolite and heulandite, respectively. This is slightly lower than for other aluminosilicate phases at comparable temperatures. The measured dissolution rates indicate a 1 mm spherical crystal would dissolve in 38 848 years and 17 301 years for clinoptilolite and heulandite, respectively. If solution composition is maintained, and remains unsaturated with respect to Si, Al, etc., the presence of zeolites in the vicinity of an underground nuclear waste repository would be beneficial as a sorption barrier.