High-level Nuclear Waste Repository Research Articles

Non-isothermal water retention curve model for adsorption

Non-isothermal water retention curve (WRC) in high suction range is a fundamental and interpretative tool to describe the behavior of bentonite and is crucial to ensure the safety and effective operation of high-level nuclear waste repository. In this study, an adsorption equation at given water content is proposed according to Kelvin equation, Clausius-Clapeyron equation and the adsorption enthalpy equation. Then, the proposed adsorption equation is incorporated into van Genuchten-Mualem model to establish a non-isothermal WRC model for adsorption. Four series of non-isothermal WRC data in a temperature range (293∼353 K) and high suction range (>1 MPa) from four different soils are employed to illustrate the application and performances of the proposed model. The proposed WRC model provides good agreement between the calibrated/predicted and measured non-isothermal WRCs.

Trust, mistrust and distrust as blind spots of Social Licence to Operate: illustration via three forerunner countries in nuclear waste management

The notion of social licence to operate (SLO) has become a widely applied concept for companies in mining and resource extraction industries to manage their social and community relations, in the face of local criticism and opposition. SLO literature and practice have highlighted earning the trust of the local community as a key requirement for an SLO. This article addresses three weaknesses in how the current SLO literature addresses trust. The arguments are illustrated via examples from nuclear waste management in Finland, France and Sweden – three forerunners in implementing high-level nuclear waste repository projects. Nuclear waste management constitutes a relevant case for analysis, as an industry that faces significant risk-related challenges of local acceptance, ethics, economics, and democratic debate. Focussing on the oft-used SLO framework of Boutilier and Thomson, with its emphasis on interactional and institutionalised trust between the company and the local community, we address three gaps in the SLO literature: 1) insufficient conceptualisation of trust, in particular the dynamics between different dimensions of trust, mistrust and distrust; 2) lack of attention to the potential virtues of mistrust and distrust; and 3) the downsides of taking the institutionalisation of trust as the ultimate criterion of a strong SLO, especially in contexts entailing significant asymmetries of power. The article concludes by suggesting ways of alleviating the identified weaknesses, via greater recognition of the multidimensionality of trust, mistrust and distrust, the virtues of mistrustful civic vigilance, and greater attention to trust dimensions that lie beyond the community-company relations.

Theoretical and Numerical Study on Electrical Resistivity Measurement of Cylindrical Rock Core Samples Using Perimeter Electrodes

The estimation of hydraulic and mechanical properties of bedrock is important for the evaluation of energy-related structures, including high-level nuclear waste repositories, hydraulic fracturing wells, and gas-hydrate production wells. The hydraulic conductivity and stress–strain curves of rocks are conventionally measured through laboratory tests on cylindrical samples. Both ASTM standards for hydraulic conductivity and compressive strength involve the use of the planar bases of a cylindrical sample. Hence, an alternative test method is required for the simultaneous measurement of hydraulic conductivity and stress–strain curves. This study proposes a novel electrical resistivity estimation method using two perimeter electrodes for the estimation of hydraulic properties. The theoretical background for the perimeter electrode setup is derived and the COMSOL MultiPhysics® finite element numerical simulation tool is employed to verify the derived theoretical equation. The accuracy of the numerical simulation tool is first validated by simulating the ASTM standard testing method for electrical resistivity. The electrical resistance values derived from the theoretical equation and numerical simulation are compared for different electrical resistivity and electrode radius. The assumed equidistant, circular equipotential surface results in a theoretical lower bound for the measured electrical resistance in the cylindrical specimen. The introduction of a phenomenological distortion factor to correct for the theoretical equipotential surface results in a good fit with the numerical simulation results. The effects of electrode length and equivalent strap electrodes were investigated to assess the applicability of the suggested method for laboratory testing. Consequently, this study presents an effective alternative theoretical assessment method for the lower bound electrical resistivity of cylindrical rock core samples under confining conditions when the installation of base electrodes is infeasible.

Cracking and sealing behavior of the compacted bentonite upon technological voids filling

The technological voids, which are inevitably formed during the construction of the deep geological repository for high-level radioactive nuclear waste, have negative influence on the hydro-mechanical properties of the engineered barrier. In this study, hydration tests along with image acquisition were carried out on compacted GMZ bentonite samples with simulated technological voids to investigate the self-sealing behavior and research emphasis was placed on the cracks appearing during the hydration. Results showed that with the presence of technological voids, the self-sealing process of the hydrating sample can be visually divided into cracking and closing stages, both of which were affected by the sample preparation process, the void width, and the sample height. The hydration cracks occurred at the early phase was considered as tensile cracks induced by the failure of tensile strength against the tensile stress induced by the difference of swelling strain. At the end of hydration, though the cracks were closed with the sealing of technologicalvoids, the distributions of water content and dry density along the sample diameter were found in a decreasing and an increasing trend with the increasing distance from the technological voids, respectively. By applying the image processing method, the cracking and sealing behaviors were further studied with the reciprocal verification of digital images and grey scale profiles. A good agreement between the grey scale profiles and digital images was found and the quantifiable grey values could accurately reflect the color difference on the samples which was difficult to be distinguished by the naked eye, showing the applicability and superiority of image processing technology.

Monte-Carlo based investigation of individual dosimetry in deep geological repository for high-level nuclear waste with consideration of realistic body postures

Deep geological disposal with provisions for retrieval is one of the most promising strategies for the definite management of high-level nuclear waste. However, retrieval of disposed waste might require certain extensive work of personnel near the waste package that might also enhance the level of radiation exposure. Hence, a precise estimation of the personal dose with consideration of realistic body postures is highly desired for optimization of individual working scenarios. In this study, the near field of a generic geological disposal facility was modeled with a horizontal emplacement drift in saline host rock. Inside the drift, a shielded disposal cask loaded with spent nuclear fuel was placed on the ground. A whole-body stylized phantom with moveable limbs was applied to represent a reference worker. Several relevant realistic body postures that might be encountered in a disposal facility were investigated. The corresponding radiation exposure was calculated and compared with a general-purpose Monte Carlo code.

Active source seismic imaging on near-surface granite body: case study of siting a geological disposal repository for high-level radioactive nuclear waste

In order to research whether it is suitable to set a geological disposal repository for high-level radioactive nuclear waste into one target granite body, two active source seismic profiles were arranged near a small town named Tamusu, Western China. The study area is with complex surface conditions, thus the seismic exploration encountered a variettraveltimey of technical difficulties such as crossing obstacles, de-noising harmful scattered waves, and building complex near-surface velocity models. In order to address those problems, techniques including cross-obstacle seismic geometry design, angle-domain harmful scattered noise removal, and an acoustic wave equation-based inversion method jointly utilizing both the and waveform of first arrival waves were adopted. The final seismic images clearly exhibit the target rock’s unconformable contact boundary and its top interface beneath the sedimentary and weathered layers. On this basis, it could be confirmed that the target rock is not thin or has been transported by geological process from somewhere else, but a native and massive rock. There are a few small size fractures whose space distribution could be revealed by seismic images within the rock. The fractures should be kept away. Based on current research, it could be considered that active source seismic exploration is demanded during the sitting process of the geological disposal repository for nuclear waste. The seismic acquisition and processing techniques proposed in the present paper would offer a good reference value for similar researches in the future.

Influence of ionic strength on hydrogen generation during interaction of copper with deoxygenated neutral solution

Interaction of copper with deoxygenated solutions is key to quantify corrosion during long-term exposure in a final high-level nuclear waste repository. To evaluate the effect of ionic strength on such interaction, electrochemical measurements in 0.001−0.1 mol dm−3 NaClO4 were performed at 22, 50 and 70 °C. An extended model including a homogeneous reaction is proposed to account for the extra features in impedance spectra, and the effect of perchlorate concentration (i. e. ionic strength) on its kinetic parameters is discussed. Using the model predictions, the rates of hydrogen production in pure water are estimated and compared to literature data.

Impact of rock fracture geometry on geotechnical barrier integrity – A numerical study

The effect of fracture geometry on bentonite erosion for a generic repository site in crystalline host rock environment was investigated by means of 2-d numerical simulations. Fracture geometry was varied systematically using random aperture normal distributions with a mean aperture of 1 mm and standard deviations between 0 and 0.7 mm, respectively. Moreover, two aperture correlation lengths (0.2 m and 2 m) were applied. Based on the synthetic fracture aperture fields generated the cubic law in conjunction with the Darcy equation is used to simulate fracture flow fields for mean flow velocities in the fracture between 1 × 10−5 m/s and 1 × 10−7 m/s. These flow fields are used in a two-way coupling approach to bentonite erosion simulations.The results of the study clearly show the influence of variable fracture aperture on bentonite erosion behaviour and erosion rates (kg/a). Increasing fracture aperture standard deviation leads to increasing heterogeneous flow velocity distributions governing the erosion behaviour and erosion rates. Calculated steady state erosion rates are in the range of ~0.25 kg/a down to ~0.014 kg/a. The highest erosion rate is calculated for the highest mean flow velocity in conjunction with the highest standard deviation. The effect of aperture heterogeneity diminishes for the lowest flow velocities.In summary, the results show the effect of fracture heterogeneity on bentonite erosion, especially for high to medium mean flow velocities combined with high to medium fracture heterogeneity under the model boundary conditions and model capabilities and limitations considered. An increase of up to ~83% in erosion rate compared to the constant aperture case highlights the need to consider fracture aperture heterogeneity and its effect on the bentonite erosion in the assessment of the safety and evolution of a high-level nuclear waste repository.

Uranium and neptunium retention mechanisms in Gallionella ferruginea/ferrihydrite systems for remediation purposes

The ubiquitous β-Proteobacterium Gallionella ferruginea is known as stalk-forming, microaerophilic iron(II) oxidizer, which rapidly produces iron oxyhydroxide precipitates. Uranium and neptunium sorption on the resulting intermixes of G. ferruginea cells, stalks, extracellular exudates, and precipitated iron oxyhydroxides (BIOS) was compared to sorption to abiotically formed iron oxides and oxyhydroxides. The results show a high sorption capacity of BIOS towards radionuclides at circumneutral pH values with an apparent bulk distribution coefficient (Kd) of 1.23 × 104 L kg−1 for uranium and 3.07 × 105 L kg−1 for neptunium. The spectroscopic approach by X-ray absorption spectroscopy (XAS) and ATR FT-IR spectroscopy, which was applied on BIOS samples, showed the formation of inner-sphere complexes. The structural data obtained at the uranium LIII-edge and the neptunium LIII-edge indicate the formation of bidentate edge-sharing surface complexes, which are known as the main sorption species on abiotic ferrihydrite. Since the rate of iron precipitation in G. ferruginea-dominated systems is 60 times faster than in abiotic systems, more ferrihydrite will be available for immobilization processes of heavy metals and radionuclides in contaminated environments and even in the far-field of high-level nuclear waste repositories.

Iron speciation in Opalinus clay minerals

Clay formations are under considerations in various countries to host a deep geological repository for high-level nuclear waste owing to beneficial physicochemical properties. These properties can, however, be influenced by the chemical form of structural Fe. The goal of this study was to obtain information on speciation of Fe associated with the clay fraction separated from the Opalinus clay rock, one of the clay formations at an advanced stage of repository planning. The clay rock was purified and the clay fraction was separated. Analyses by X-ray powder diffraction evidenced the presence of 1:1 (kaolinite) and 2:1 layer minerals (e.g., illite and possibly illite/smectite mixed layers), no other phase in significant amounts could be detected. Infrared spectroscopy corroborated the presence of both 1:1 and 2:1 layer minerals, and the absence of Fe3+2-OH bending in the spectrum ruled out extensive Fe clustering and hinted at a rather homogeneous distribution within the structure. Information on Fe speciation was obtained by X-ray absorption spectroscopy (XAS) at the K-edge. To reduce uncertainties on structural parameters, overlapping contributions from in-plane and out-of-plane shells were filtered by recording polarized XAS data. The position of the pre-edge features indicated the prevalence of Fe3+, while the intensity and splitting were typical of Fe located at octahedral site. The pre-edge of the reference kaolinite differed from that of the Opalinus clay sample, excluding the prevalence of Fe within that 1:1 layer mineral. Data thus suggested the prevalence of Fe3+ within the octahedral sheet of 2:1 layers. Polarized XANES and EXAFS exhibited significant angular dependencies hinting at the presence of various neighboring shells with specific orientations. The first O shell located at 2.03 Å contains about six O atoms oriented in-plane. At higher distances, three in-plane Al/Mg and Fe and about four out-of-plane Si atoms were detected at ~3.05 Å and 3.23 Å, respectively, suggesting that Fe is located in a dioctahedral octahedral sheet of 2:1 layers. The presence of significant amounts of Fe in the tetrahedral sheet was excluded. Combining results from XAS with that from chemical analysis enabled to estimate first the structural formula of the 2:1 layer fraction and subsequently the proportions of 1:1 and 2:1 layer minerals within the purified clay fraction of the Opalinus rock. The estimated proportions of 1:1 and 2:1 layer minerals were in reasonable agreement with that of the whole Opalinus rock.

Serpentinization and H2 production during an iron-clay interaction experiment at 90C under low CO2 pressure

Interactions between steel canisters and clay-rich material in deep geological repositories for high-level nuclear waste were studied by reacting metallic iron and a claystone from the Callovo-Oxfordian formation of the Paris Basin (COx). The experiment ran at 90 °C for 14 weeks in the presence of water and under 20 mbar of initial CO2 partial pressure (p(CO2)). At the end of the experiment, the pressure in the autoclave reached 50 bar due to H2 production. Water was almost entirely consumed and poorly crystallized 7 Å-greenalite-type clays were formed at the expense of illite and mixed-layered illite-smectite minerals (I/S). The dissolution of quartz and pyrite and the crystallization of pyrrhotite were also observed. Considering the results of Mössbauer and Fourier Transformed Infrared (FTIR) spectroscopies, iron carbonate and and/or ferrous hydroxycarbonate were suspected to precipitate but in very small amounts. A follow-up of the mineralogical evolution by XRD (X-ray diffraction), SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) shows that serpentinization of clays (formation of greenalite) is accompanied by a significant consumption of water and the production of H2 gas due to the oxidation and dissolution of metallic iron. Experimental results are in good agreement with geochemical modelling with the PHREEQC code. The kinetics of the iron corrosion was evaluated from H2 production and shows that diffusion processes are the rate limiting step.

Canister spacing in high-level radioactive nuclear waste repository

One of the core problems of waste canisters layout in high-level radioactive waste repository is the evolution of the temperature field. On basis of the layered thermal analysis model for single waste canister, the expression of temperature increment at any location in surrounding rock in the repository was obtained by the superposition principle. The initially estimated value of the canister spacing (CS) was determined according to the temperature design criterion. Finally, the influence of relevant parameters on the canister surface temperature (CST) was analyzed. The results were drawn as follows: (a) Taking thermal conductivities of 2.4 and 2.8 W/(m × K) for the rock as examples, the appropriate CS is 12.2 and 13.5 m under the tunnel spacing of 40 m, respectively. (b) The greater the CS, the greater the thermal conductivity of bentonite and rock, the smaller the CST would be. (c) The thicker the buffer layer, the less the heat flux inside the canister would spread out.

Towards higher temperatures in nuclear waste repositories

Recently, there is a tendency to explore the possibility of increasing the maximum design temperature in deep geological repositories for high-level nuclear waste and spent fuel. In the paper, a number of issues related to the use of higher temperatures are reviewed. Both bentonite barriers and argillaceous host rocks are addressed. An application involving the modelling of a large-scale field test conducted at a maximum temperature of 140ºC is presented. It is shown that currently available theoretical formulations and computer codes are capable to deal with temperatures above 100ºC and to reproduce satisfactorily the thermally-induced overpressures in the rock.

Fault slip envelope: a new parametric investigation tool for fault slip based on geomechanics and 3-D fault geometry

Abstract. By combining a 3-D boundary element model, frictional slip theory, and fast computation method, we propose a new tool to improve fault slip analysis that allows the user to analyze a very large number of scenarios of stress and fault mechanical property variations through space and time. Using both synthetic and real fault system geometries, we analyze a very large number of numerical simulations (125 000) using a fast iterative method to define for the first time macroscopic rupture envelopes for fault systems, referred to as “fault slip envelopes”. Fault slip envelopes are defined using variable friction, cohesion, and stress state, and their shape is directly related to the fault system 3-D geometry and the friction coefficient on fault surfaces. The obtained fault slip envelopes show that very complex fault geometry implies low and isotropic strength of the fault system compared to geometry having limited fault orientations relative to the remote stresses, providing strong strength anisotropy. This technique is applied to the realistic geological conditions of the Olkiluoto high-level nuclear waste repository (Finland). The model results suggest that the Olkiluoto fault system has a better ability to slip under the present-day Andersonian thrust stress regime than for the strike-slip and normal stress regimes expected in the future due to the probable presence of an ice sheet. This new tool allows the user to quantify the anisotropy of strength of 3-D real fault networks as a function of a wide range of possible geological conditions and mechanical properties. This can be useful to define the most conservative fault slip hazard case or to account for potential uncertainties in the input data for slip. This technique therefore applies to earthquake hazard studies, geological storage, geothermal resources along faults, and fault leaks or seals in geological reservoirs.

Effects of NaCl solution on the swelling and shrinkage behavior of compacted bentonite under one-dimensional conditions

Because of its low hydraulic conductivity, high swelling capacity, and good adsorption properties, Gaomiaozi (GMZ) bentonite has been proposed as a suitable buffer/backfill material for construction of artificial barriers in deep geological repositories for high-level nuclear waste (HLW) disposal in China. By varying the concentration of NaCl solution and total suction, the swelling and shrinkage characteristics of compacted GMZ bentonite were studied under one-dimensional conditions. Results show that the swelling strain of compacted GMZ bentonite decreases as the concentration of NaCl solution increases under one-dimensional conditions. During drying with suction control by vapor phase technique, the density increases with suction. Simultaneously, the sample saturated with salt solution has a larger void ratio than that of the sample saturated with distilled water under the same suction. The void ratio and water content of the samples decrease with increasing suction. For a given suction, the sample saturated with salt solution has a higher degree of saturation than the sample saturated with distilled water. With the obtained water content (w), void ratio (e), and the degree of saturation (Sr), soil-water retention curves (SWRCs) were calculated for samples initially hydrated with different concentrations of NaCl solution. The SWRCs reveal that the sample with salt solution has higher water retention capacity than the sample with distilled water. A modified SWRC equation was proposed to account for the effect of NaCl solutions on the drying process of compacted GMZ bentonite under one-dimensional conditions. The results confirm that the modified SWRC equation can effectively describe the SWRCs of compacted GMZ bentonite considering the effects of NaCl solutions.

Representation of Volume Change for Bentonite in Saline Solution based on Modified Effective Stress

Bentonite is usually selected as the buffering barrier material for high-level nuclear waste repository due to its characteristics of high expansibility and low permeability, yet the swelling properties are influenced by the saline solution existing in the fissures of surrounding rocks. Quantifying the effects of saline solution concentration on the swelling property of compacted bentonite is significant for assessing the security of engineering barrier used to dispose high-level radioactive waste. In this study, the modified effective stress incorporating with osmotic suction generated from saline solution is proposed to depict the volume changes for bentonite in saline solution with different concentrations by a unique equation. The osmotic suctions are calculated with the simplified Debye-Hückel equation and the results are validated by comparing with the experimental data in other literatures. The published experimental data prove the uniform relationship between the volume change and the modified effective stress in saline solution with different concentrations. Therefore, the quantitative predictions of volume change of bentonite in saline solution with different concentrations are presented using the modified effective stress concept.

Peak shear strength of compacted GMZ bentonites in saline solution

Knowledge of the peak shear strength of compacted bentonites is of fundamental importance to high-level nuclear waste repositories, as it forms the basis of the analysis of repository stability. The shear strength properties of GMZ bentonites are throughout investigated by undrained direct shear tests with different concentrations of NaCl solutions, and are explained in the light of the modified effective stress concept, which takes into consideration of the osmotic suction. The modified effective stress concept is expressed as p’ = p + π(p/π)Ds-2, incorporated with the normal stress (p) and osmotic suction (π) based on the bentonite surface fractal model. The peak shear strength behavior of GMZ bentonites in NaCl solutions appears to be explained satisfactorily by the modified effective stress. According to the Mohr-Coulomb criterion, the peak shear strength of bentonites saturated by distilled water and NaCl solutions is expressed by the same unique line in the τ-p’ plane. GMZ bentonite has the same values of the peak friction angle and the peak cohesion in distilled water and NaCl solutions of different concentrations.

Modelling groundwater contamination above a potential nuclear waste repository in the Columbia River Basalt, USA

Potential groundwater contamination above the one-time candidate repository for high-level nuclear waste in the Columbia River Basalt (CRB), USA, is evaluated in a flow-transport model with 260,895 elements for a period of 1 million years. The hydrogeologic database originates from the Basalt Waste Isolation Project, which ended in 1987, giving way to the Yucca Mountain project. The CRB data have been scanned recently with the object of finding a storage site for natural gas and a sequestration site for CO2. However, the impact on the nuclear issue in the USA or other countries with similar large flood-basalt provinces has never been evaluated since the suspension of the project. The peak radioactive dose predicted for a potential repository in the CRB is at least eight orders of magnitude lower than the US generic regulatory limit (0.15 mS/a) even for the most pessimistic scenario.

A Study on the Manufacturing Characteristics and Field Applicability of Engineering-scale Bentonite Buffer Block in a High-level Nuclear Waste Repository

A Study on the Manufacturing Characteristics and Field Applicability of Engineering-scale Bentonite Buffer Block in a High-level Nuclear Waste Repository

Radioactivity recorded by clay minerals in the Shea Creek area, Athabasca Basin (Canada): Implications for uranium transfers and exploration

The understanding of uranium mobility in the geosphere is a prerequisite for the modelling of high-level nuclear waste repositories and economic uranium deposit genesis. To complement more classical geochemical and mineralogical approaches, this understanding can be improved by measuring the record of past cumulative radioactivity as stable radiation-induced defects in clay mineral structure. This study focuses on world-class unconformity-related uranium deposits of the Athabasca Basin (Canada) for which the source, timing, and paths of the uranium-bearing fluids are still matters of debate. A set of 46 samples collected from the Athabasca Group sandstones in the Shea Creek area of the western Athabasca Basin, up to 634 m above either the unconformity (barren drill hole) or uranium mineralization, was selected in order to locate the paleo-occurrences of radioelements. A relevant three-dimensional view is shown by plotting (i) the concentrations of radiation-induced defects (RID's) in clay minerals, (ii) the present dose rate, and (iii) the distance to the mineralization or unconformity. The results clearly reveal different cases, such as geochemical background, equilibrated dose rate, late accumulations of radioelements, and/or records of their past temporary occurrence. Noticeable paleo-occurrences, now leached away, are revealed within 100 m of the structures hosting present-day mineralized bodies, which is in line with a recent model of long range lateral paleofluid flow in a basinal permeable formation, and may be useful for exploration, albeit within a proximal range. Such results rely on the detection of RID's in clay minerals, as chemical analysis or gamma counting alone detect only the present concentration of radioelements and are unable to distinguish between accumulations, equilibration of transfers, or temporary occurrences of uranium. This study represents a first step toward spatial 3D quantitative reconstruction of U transfers, which will require time constraints and artificial dosimetry to improve models of genesis of high-grade unconformity-related deposits and to identify paleo-pathways of U migration.