Repository For Disposal Research Articles

Continuous Wavelet Transformation to Quantify small-scale Cycles of Petrophysical Properties; a New Approach Applied in a Potential Disposal Repository of Nuclear Waste, SW Hungary

Continuous Wavelet Transformation (CWT) was applied to study the small-scale repetitive oscillations of porosity distribution patterns in a 5 m silty-claystone core sample of the Boda Claystone Formation. We handled the fluctuations in voxel porosity averages over unequal depth distributions as signals over uneven time intervals. The strength of wavelet analysis lies in the ability to study the fluctuation of a signal in detail, i.e., the wavelet transforms permit automatic localization of the cyclic attributes' sequences both in time (the depth domain) and according to their frequency (the frequency domain). Thereupon, three main frequency branches (cycles) were discerned: small scale (5, 6.67, and 11 cm), intermediate scale (20, 30 cm), and large scale (66.67 cm). Depending on the CWT coefficients magnitude plot, we were able to detect the developments of porosity oscillation according to the depth variable. Thus, small-scale cycles were seen throughout the core sample., the intermediate-scale cycles were strong in the upper parts of the core sample and dwindled toward greater depths, and the large cycle was predominant in the lower part of the core sample. The cross-correlation of the wavelet coefficients of porosity and rock-forming components allows a detailed study of the inter-dependence of such parameters as their relationship changes over time. The distinct peaks at zero lag indicates that the measured wavelet coefficient series were contemporaneously correlated; their strong positive correlations suggest that both examined series respond similarly and simultaneously to other exogenous factors. The results emphasize that cyclical porosity fluctuations at all scales would concern three main factors; sediment deposition, diagenetic processes, and structural deformation (i.e., convolute laminations).

Compressibility behavior of colemanite added bentonite under short and long-term high temperature

Introduction
 The increasing energy demand and the limited fossil fuel resources make searching for new sustainable clean energy sources. In this regard, it is vital to use energy geo-structures as a renewable and clean energy source. The geo-structures are in direct contact with the soil and cause temperature changes throughout the soil mass.
 Bentonite is considered suitable as an engineering barrier in deep geological disposal repositories for spent nuclear fuel, mainly because of its favorable swelling properties and extremely low permeability [1]. It was reported by many researchers that the engineering properties of clayey soils change at high temperatures [2,3]. Therefore, there is a need for soil materials that can maintain their long-term engineering properties under high temperatures. Borates are naturally occurring minerals. They can be found mainly in sediments and sedimentary rocks. The most commercially important boron minerals are tincalconite, colemanite, and ulexite [4].
 In the present study, it was tried to improve the compressibility behavior of bentonite by adding colemanite under high temperature. The oedometer tests were performed under a constant temperature (80 ºC) on the bentonite-colemanite mixtures. In this context, samples exposed to short and long-term high temperature (80 ºC) were used and the results were compared with the room temperature results.
 Material Characterization and Methods
 The Ca-bentonite sample used which is activated with sodium bicarbonate. Colemanite was added to bentonite at a rate of 10% of the bentonite by dry weight. In this context, B10C sample represents a 10% colemanite added bentonite mixture. The liquid limits of bentonite and colemanite are 270% and 37%, respectively. The samples (smaller than 75 μm) were obtained by mixing the mixture powder with tap water at a water content of 1.5 times the pre-determined liquid limit value of the mixtures. The slurries were consolidated under a vertical pressure of 12.5 kPa for 14 days. Samples 70 mm in diameter and 19 mm in height were obtained by trimming. The samples were placed in oedometers for tests at room and high temperature (80 ºC). The experimental system was modified for high-temperature tests. The modified system consists of a conventional apparatus, a heat ring, a thermostat, and a water tank. Thus, by heating the cell water, the temperature of the sample was indirectly increased to 80 °C. For long-term experiments, samples were placed in thermal pools in clamped molds (constant volume). These molds were kept in the thermal pools under a constant temperature of 80 °C for 6 months. Then, the consolidation tests of these samples were performed at 80 °C.
 Results and Discussions
 The result of the consolidation tests of bentonite mixtures at 80 °C after being kept in the thermal pool for 6 months is given in Figure 1. For comparison, the test results at room temperature and 80 °C are also shown. The compression amount of bentonite increased with the increase in temperature, while the swelling amount decreased. The total compression deformation of the bentonite sample, which showed 53% compression at room temperature, reached 65% when the temperature was increased to 80 °C. However, swelling deformation decreased from 7% to 3% when the temperature was increased. When the volumetric deformation behavior of bentonite (NC) is examined, thermal contraction behavior at high temperature (90 °C) was reported [2]. In other words, the vertical compression amount of NC bentonite increases at elevated temperatures. In addition, since the structure of the clay deteriorated at high temperature and the deformations were permanent, it was an expected result that the rebound (swelling) behavior would decrease at high temperature.
 There was a decrease in the amount of compression in the sample cured for 6 months, compared to 80 °C. However, although the compression amount of the 6-month cured sample decreased, the amount of compression still slightly increased compared to the room temperature. Compression-swelling deformation amounts of the samples cured for 6 months and tested at 80 °C are presented in Table 1 in detail. When the swelling behavior of additive-free bentonite was examined, increasing temperature decreased the rebound behavior of bentonite. In addition, the swelling amount of the sample cured for 6 months was determined to be almost the same as the sample at 80 °C. The reason for this was that the deformations that occurred at high temperatures were permanent. 
 When the temperature of the colemanite-added bentonite mixtures was increased, the amount of deformation increased. However, the amount of deformation of additive-free bentonite decreased with the colemanite addition at 80 °C, and returned to its room temperature value (approximately 53%). In other words, the addition of colemanite had a positive effect by reducing the deformation amount of bentonite at both short and long term high temperatures. The short and long term effects of 80 °C temperature on colemanite added samples were almost the same.

Experimental investigations of groundwater inflow-induced piping phenonomenon in compacted bentonite buffers for HLW repository

This study introduces a series of laboratory-scale experimental tests of piping erosion in buffer in order to investigate the phenomenon of piping erosion around the buffer material when rapid groundwater inflow occurs in a disposal repository. In a cylindrical cell, specimens of compacted bentonite were exposed to continuous water injection under varying conditions of flow rate, chemical composition, and flow direction. The erosional behaviors of bentonite buffer in piping are analyzed, including the formation of piping channels, critical inflow water pressure, and self-healing. In addition, X-ray CT scanning was utilized to analyze the post-erosion characteristics of the bentonite buffer. The outcomes will inform the development of requirements for the reference design and construction of a secure HLW repository.

Stable microbial community in compacted bentonite after 5 years of exposure to natural granitic groundwater.

The Materials Corrosion Test (MaCoTe) at the Underground Research Laboratory in Grimsel, Switzerland, assesses the microbiology and corrosion behavior of engineered barrier components of a deep geological repository (DGR) for long-term disposal of high-level nuclear waste. Diversity and temporal changes of bentonite-associated microbial community profiles were assessed under DGR-like conditions for compacted Wyoming MX-80 bentonite (1.25 g/cm3 and 1.50 g/cm3 targeted dry densities) exposed to natural groundwater. Using culture-dependent and molecular techniques, samples taken from the outside layer of 5-year borehole modules revealed up to 66% and 23% of 16S rRNA gene sequences affiliated with Desulfosporosinus and Desulfovibrio, respectively. Putatively involved in sulfate reduction, these taxa were almost undetectable within the bentonite core. Instead, microbial profiles of the inner bentonite core were similar to uncompacted bentonite used to pack modules years earlier, and were consistent with a previously published 1-year time point, revealing no detectable microbial growth. Abundances of culturable aerobic and anaerobic heterotrophic bacteria in the uncompacted bentonite were relatively low, with less than 1,000 and 100 colony-forming units (CFUs) per gram dry weight, respectively. Nearly 5 years after emplacement, culturable heterotrophic bacterial CFUs and sulfate-reducing bacteria did not change significantly inside the bentonite core. Phospholipid fatty acid data indicated similar lipid abundance, and corresponding cell abundance estimates, for inner 5-year MaCoTe bentonite samples compared to those previously obtained for 1-year incubations. Collectively, our results provide complementary evidence for microbial stability inside highly compacted bentonite exposed to conditions that mimic engineered barrier components of a deep geological repository. IMPORTANCE The long-term safety of a deep geological repository for used nuclear fuel is dependent on the performance of the engineered and natural barriers. Microbial activity can produce chemical species that can influence the corrosion of the disposal containers for used nuclear fuel. Although previous studies have evaluated the microbiology of compacted bentonite clay within subsurface environments, these have been limited to relatively short incubations (i.e., 1 year). The current study provides a unique 5-year perspective that reinforces previous findings of growth inhibition for bentonite clay exposed to in situ subsurface conditions.

Public acceptance of nuclear waste disposal sites: a decision-making process utilising the ‘veil of ignorance’ concept

This study demonstrates that a decision-making process utilising ‘the veil of ignorance’ concept, defined in process terms as beginning from a blank slate encompassing the entire country as potential sites and shortlisting candidate sites based on scientific (geological) safety, promotes public acceptance of siting a repository for the geological disposal of high-level radioactive waste and fosters procedural fairness. A hypothetical scenario experiment was conducted in Japan, manipulating the site selection process by setting two conditions—one being the application/proposal condition that the Japanese government currently employs, such as an application by municipalities or a proposal by the government, and the other being the veil of ignorance condition, in which multiple candidate areas are selected from a blank slate for the entire land area based purely on geological factors. Three stages of acceptance were presumed—at the level of general management policy, the site selection process itself with a specified decision policy, and the siting of a repository in their area of residence. Two hypotheses were tested: (a) the veil of ignorance condition will be evaluated as a more acceptable and fairer procedure and will engender increased national consensus than the application/proposal condition at the site selection and repository siting stages, and (b) procedural fairness and national consensus will impact acceptance at each stage; these variables at each stage help shape the same variables in the next stage. The results supported these hypotheses. This study discusses the importance of the site selection process, beginning where any de facto site can be a candidate and shortlisting the candidate sites based on scientific criteria.

Experimental and numerical investigation on meso-fracture behavior of Beishan granite subjected to grain size heterogeneity

Research on crack evolution mechanism and failure pattern of Beishan granite is vital for evaluation of the engineering stability of disposal repository. The mechanical properties, especially the strong heterogeneity, significantly affect the cracking behavior of granite. To investigate the fracture behavior of heterogeneous granite, mesoscopic three-point bending tests were conducted on Beishan granite using an in situ scanning electron microscope (SEM) with a loading device, and the entire cracking process was observed in real time. Additionally, the effect of grain size heterogeneity on strength and deformation behavior of granite was quantitatively investigated by introducing a statistically defined heterogeneity index into the grain-based model in two-dimensional Particle Flow Code (PFC-GBM), and the crack development was characterized through fractal theory. The results demonstrated that fracture mechanism of granite is mainly tensile and shear failure, and the fracture surface of tensile failure is rough and irregular, which is mainly caused by the tearing between various mineral crystals, whereas the fracture surface of shear failure is relatively smooth, which is mainly caused by the dislocation and slip between minerals. The crack edge can be regarded as the damage zone, and the development of the main crack is promoted by the continuous generation and combination of micro-damage at the crack tip. The crack propagation path from a mesoscale perspective is a process of continuous crack orientation, which depends on the competition mechanism between mineral properties and maximum tensile stress. Generally, the granite strength tends to increase with the increase of heterogeneity.

Perceptions of surface facilities of a potential disposal site and the role of place attachment

Abstract. Building a deep geological repository for the final disposal of nuclear waste implies not only changes in the underground but also landscape changes on the surface. Transformations of the surface – the landscape – are accompanied by changes in the perceptions of places and the activities of those living close-by, e.g. regarding leisure activities (cf. Devine-Wright and Batel, 2017; Gailing et al., 2020). Therefore, changed perceptions of a landscape may also have effects on the place attachments of individuals and collective groups who live in those transformed regions (cf. Knaps et al., 2022; Mihaylov and Perkins, 2014). Place attachment is defined as the emotional bond of individuals or collectives to certain locations or landscapes, which include both physical objects and social aspects (cf. Lewicka, 2011). Potentially affected local people want to be able to have an idea of what a repository might look like, even on the surface, how this would fit into the surrounding landscape, and what effects the construction and operation of a repository might have on a region. In this context, participation plays an important role, as shown, for example, by the extensive discussion on the placement of surface facilities in the sectoral plan procedure for deep geological repositories in Switzerland (cf. Neles, 2022). In the project TRANSENS, a transdisciplinary project on disposal research, we conducted an empirical study with different actors on the role of place attachment and perceptions of models of surface facilities of a repository. Our research question is as follows: how do residents with different place attachments perceive landscape changes? For this purpose, we designed an experimental workshop setting with different actors (students, people from different regions in Germany, and transdisciplinary partners in TRANSENS). In this experimental setting, we conducted two surveys which were centred, first, on place attachment and, second, on perceptions of the visualized models of surface facilities integrated in different landscapes (which we showed the participants in a 3D virtual space). We also included a group discussion on the different perceptions and evaluated how the discussion changed the perception. The results of the workshop show that place attachment among the participants of the workshop is more or less pronounced and influences the perception of the visualized surface models differently (e.g. those who define “home” by its landscape elements attach great importance to changes in the landscape). Aesthetic aspects in relation to the embedding in the landscape are emphasized in the discussion. In addition, the group discussion had a rather positive influence on the perception of surface facilities. The findings of the experimental workshop are integrated in a transdisciplinary research design on spatial effects and governance in which we have already conducted several interviews (e.g. on place attachment) with individuals from three different regions of Germany. With our research, we aim to elaborate on how these findings are to be included in a place-sensitive and participatory long-term governance concept (cf. Mbah and Kuppler, 2021).

Insight on the Swelling Pressure-Suction Relationship of Compacted Bentonite during Hydration.

Investigation of the swelling pressure of buffer/backfill materials is a critical aspect in the design of high-level radioactive waste (HLW) disposal repositories. In this study, to clarify the swelling pressure-suction relation for compacted bentonite upon the hydration path, constant-volume swelling pressure tests with suction control were conducted. The swelling pressure-suction curves indicated that the swelling pressure of the specimens increased significantly with increasing dry density, while the shape of the curves during hydration depended on the dry density. Moreover, the swelling pressure-suction curves exhibited a distinction between unsaturated and saturated segments divided by the critical saturated state (CSS) curve, which proves the unique existence of a CSS curve in the stress space independent of the stress path. With the introduction of the CSS curve into the s-p space, the conventional stress space of unsaturated soil could expand to that of unsaturated expansive soil. The results obtained in this study could provide the mechanical parameters for the construction of disposal repositories. In addition, the stress space with CSS curve proposed in this study provides a new approach to building constitutive models of bentonite materials.

Computational Materials Design for Ceramic Nuclear Waste Forms Using Machine Learning, First-Principles Calculations, and Kinetics Rate Theory.

Ceramic waste forms are designed to immobilize radionuclides for permanent disposal in geological repositories. One of the principal criteria for the effective incorporation of waste elements is their compatibility with the host material. In terms of performance under environmental conditions, the resistance of the waste forms to degradation over long periods of time is a critical concern when they are exposed to natural environments. Due to their unique crystallographic features and behavior in nature environment as exemplified by their natural analogues, ceramic waste forms are capable of incorporating problematic nuclear waste elements while showing promising chemical durability in aqueous environments. Recent studies of apatite- and hollandite-structured waste forms demonstrated an approach that can predict the compositions of ceramic waste forms and their long-term dissolution rate by a combination of computational techniques including machine learning, first-principles thermodynamics calculations, and modeling using kinetic rate equations based on critical laboratory experiments. By integrating the predictions of elemental incorporation and degradation kinetics in a holistic framework, the approach could be promising for the design of advanced ceramic waste forms with optimized incorporation capacity and environmental degradation performance. Such an approach could provide a path for accelerated ceramic waste form development and performance prediction for problematic nuclear waste elements.

A Multimodel Framework for Quantifying Flow and Advective Transport Controlled by Earthquake-Induced Canister Failures in a Reference Case for Radioactive Waste Geological Disposal

Characterizing flow and transport for earthquake-induced shear canister failure is critical for the performance and safety assessment of radioactive waste geological disposal. The study presents a modeling framework that integrates multiple models to account for fractures produced by shear displacements, evaluate canister failures, and simulate flow and advective transport in a conceptual repository site based on a selected reference case in an offshore island in western Taiwan. The typical KBS-3 disposal concept associated with 500 realizations of the shear-induced fracture properties is employed to quantify the uncertainty of flow and advective transport in the geological disposal site. The radionuclides in canisters are assumed to migrate through the shear-induced fractures surrounding the deposition holes. The results from 500 realizations show that two types of fractures produce a high potential to destroy canisters induced by the shear displacements. The earliest canister failure time influenced by possible shear movements is 0.23 million years for the reference case. The modeling framework identifies five canisters and the associated shear-induced fractures for flow and advective transport simulations. Based on the results of the density-dependent flow fields, the particle tracking algorithm enables the calculations of flow and transport parameters, including equivalent initial flux, equivalent flow rate, path length, travel time, and flow-related transport resistance for the identified five canisters. These parameters are critical for the performance and safety assessments of buffer erosion and canister corrosion near the disposal repository and the far field of the radioactive waste disposal site.

Disposal of high-level radioactive waste in crystalline rock: On coupled processes and site development

Safe disposal of high-level radioactive nuclear waste (HLW) is crucial for human health and the environment, as well as for sustainable development. Deep geological disposal in sparsely fractured crystalline rock is considered one of the most favorable methods for final disposal of HLW. Extensive research has been conducted worldwide and many countries have initiated their own national development programs for deep geological disposal. Significant advancements of national programs for deep geological disposal of HLW in crystalline rock have been achieved in Sweden and Finland, which are currently under site development stage, focusing on detailed site characterization, repository construction, and post-closure safety analysis. Continued research and development remain important in the site development stage to ensure long-term safety of the HLW disposal repository. This work presents an overview and discussion of the progress as well as remaining open scientific issues and possibilities related to site development for safe disposal of HLW in crystalline rock. We emphasize that developing a comprehensive and convergent understanding of the coupled thermal, hydraulic, mechanical, chemical and biological (THMCB) processes in fractured crystalline rock remains the most important yet challenging topic for future studies towards safe disposal of HLW in crystalline rock. Advancements in laboratory facilities/techniques and computational models, as well as available comprehensive field data from site developments, provide new opportunities to enhance our understanding of the coupled processes and thereby repository design for safe geological disposal of HLW in crystalline rock.

Importance of Stress and Temperature-Dependent Permeability of Rocks and its Application in Underground Nuclear Waste Disposal

Water flow is an essential factor in the sealability of any underground cavern, including those for nuclear waste disposal, and is significantly affected by the permeability of the rock. The permeability of rocks is affected by various factors, including stress and temperature. The rock stress changes by excavating a cavern, and rock temperature changes by decay heat from nuclear waste, and the temperature change induces thermal stress. Therefore, water flow around such caverns must be evaluated considering the effects of stress and temperature. Numerical analyses of water migration around underground nuclear waste disposal caverns have been carried out. However, studies considering the stress and temperature-dependent permeability may not be published yet. To demonstrate the necessity to consider the stress and temperature-dependency in permeability, equations that represent the post-failure permeability as a function of average effective stress and temperature were proposed. The water inflow was numerically calculated for a simple underground nuclear waste disposal cavern with or without stress and temperature dependency which showed the significance of the dependency. Also, the importance of rock types was demonstrated by considering the three rocks of granite, sandstone, and tuff for a full-scale underground radioactive disposal site for the stress and temperature-dependent permeability. A high sealability could be expected for granite and sandstone but not for the tuff. Introducing the stress and temperature-dependent permeability could contribute to the thoughtful design of an underground repository for radioactive waste disposal considering rock types.

A model for describing advective and diffusive gas transport through initially saturated bentonite with consideration of temperature

Understanding the gas migration mechanism in an initially saturated bentonite is of great importance for evaluation of the long-term performance of a deep geological repository for disposal of high-level radioactive wastes. Previous researches usually neglect the gas diffusion process, but adopted the advective gas flow theory (visco-capillary flow or dilatant gas flow) to describe the gas transport in the initially water-saturated bentonite clays. In this study, gas migration process was regarded as results of a combination of diffusive transport of dissolved gas through pore water and advective flow along the gas pathways formed in the bentonite matrix. Meanwhile, dilation of the gas flow pathways with variation of the external stress and gas pressure during the advective flow was also considered. Based on this concept, a new model was proposed to describe the advective and diffusive gas transports with consideration of temperature. Validation was conducted using the gas injection tests performed under different boundary conditions in literature. Comparisons show that the diffusion and advection-controlled process provided by the proposed model can effectively simulate gas transport in the compacted GMZ bentonite. For gas migration under lower injection pressures, the contributions of advective gas flow to the gas flux are comparable to that of diffusion. With the increase of the injection pressure, the advective gas fluxes would gradually dominate over the diffusive transport. Meanwhile, predicted degree of saturation of the compacted bentonite is maintained at above 99% during the whole gas migration process, indicating that advective movement of gas was highly discrete in initially saturated bentonite.

Study on Damage Behavior and Its Energy Distribution of Deep Granite at High-Temperature Conditions

The phenomenon of surrounding rock damage and rupture caused by high temperatures is widespread, and has become a potential threat to the safety of nuclear waste disposal repositories. In order to reveal the energy distribution pattern of fractured granite during the failure process under different confining pressures, triaxial compression tests were carried out on rocks with different initial thermal damage. Firstly, the rock was treated at a high temperature to analyze the change rule of the porosity of the rock after high-temperature treatment, define the equivalent damage coefficient, and analyze the influence of confining pressure and equivalent damage coefficient on the peak stress and peak strain of the rock. The results show that, after high-temperature treatment, the porosity increases with the increase in temperature. The peak stress and corresponding strain of rock samples with similar equivalent damage factors increase with the increase in confining pressure. By comparing the rock samples with the same confining pressure and different initial thermal damage, the larger the confining pressure, the smaller the difference of peak stress of different initial thermal damage specimens. Then, the energy density of rock in a triaxial compression test is quantitatively analyzed by energy theory. The results show that, as long as the confining pressure is the same, the proportion of the dissipated energy of the specimen has a similar evolution trend with the strain. When the confining pressure is the same, the proportion of dissipated energy decreases rapidly with the change of strain due to the increase in equivalent damage factor, but the rate of decline will gradually slow down; however, when the confining pressure increases, the difference caused by the equivalent damage factor will gradually decrease, because the fracture is bound by the confining pressure. Finally, we analyze the maximum dissipated energy during rock deformation and failure. According to the inflection point of maximum dissipated energy, the optimum time for critical support of the key rock mass is determined.

Validation of the fully-analytical solution with temperature superposition for the nuclear waste repository

Temperature distributions and evolutions in high-level radioactive waste (HLW) disposal repository are usually obtained by numerical simulation or analytical solution. For the use of analytical solution to calculate the temperature distributions of one disposal panel in the repository, the temperatures from every nuclear waste canister were superposed for calculating the whole effects of all the canisters, and the disposal tunnels, which are filled with backfill materials, were ignored to simplify the deducing process. Yet the accuracy of the temperature superposition has not been validated and the influence of tunnels on temperature has not been evaluated. In this paper, a disposal panel with 1189 waste canisters was modelled by the finite element method for analyzing the temperature distributions and evolutions. The size of numerical domain was determined by studying the temperature distributions. When the distance between the outer edges and outermost canisters surface was set as 700 m or above, the influence of boundaries on calculated results can be ignored. A disposal panel with tunnels was modelled to study the influence of tunnels on temperature evolution, and the results show that the tunnels have practically no effect on the temperature evolution. Finally, the comparison of temperatures calculated by numerical simulation and analytical solution was done to validate the analytical solution with the superposition method.

Impact of microbial processes on the safety of deep geological repositories for radioactive waste.

To date, the increasing production of radioactive waste due to the extensive use of nuclear power is becoming a global environmental concern for society. For this reason, many countries have been considering the use of deep geological repositories (DGRs) for the safe disposal of this waste in the near future. Several DGR designs have been chemically, physically, and geologically well characterized. However, less is known about the influence of microbial processes for the safety of these disposal systems. The existence of microorganisms in many materials selected for their use as barriers for DGRs, including clay, cementitious materials, or crystalline rocks (e.g., granites), has previously been reported. The role that microbial processes could play in the metal corrosion of canisters containing radioactive waste, the transformation of clay minerals, gas production, and the mobility of the radionuclides characteristic of such residues is well known. Among the radionuclides present in radioactive waste, selenium (Se), uranium (U), and curium (Cm) are of great interest. Se and Cm are common components of the spent nuclear fuel residues, mainly as 79Se isotope (half-life 3.27 × 105 years), 247Cm (half-life: 1.6 × 107 years) and 248Cm (half-life: 3.5 × 106 years) isotopes, respectively. This review presents an up-to-date overview about how microbes occurring in the surroundings of a DGR may influence their safety, with a particular focus on the radionuclide-microbial interactions. Consequently, this paper will provide an exhaustive understanding about the influence of microorganisms in the safety of planned radioactive waste repositories, which in turn might improve their implementation and efficiency.

Scaling effect on cesium diffusion in compacted MX-80 bentonite for buffer materials in HLW repository

Abstract In this study, radionuclide behavior in high-level radioactive waste (HLW) disposal repositories is complicated because of the spatial heterogeneity of porous media, coupled flow-transport mechanisms, and multiple chemical reaction processes. Discrepancies in the diffusion behavior of a non-sorbing tracer (HTO) and a reactive tracer (137Cs) in porous media have long been recognized but are not yet fully understood, which hinders effective assessment of the capabilities of buffer materials. This paper was dedicated to exploring and explaining the discrepancies in the transport behavior of non-sorbing and reactive tracers through laboratory experiments and an investigation of contributing mechanisms. Our results showed that for a bentonite sample of the same thickness, 137Cs has smaller apparent and less effective diffusion coefficients than those for HTO. These discrepancies can be attributed to the negative surface electric effects, atomic properties, and chemical reactions. In the case of bentonite samples with different thicknesses (0.5, 0.75, 2.0, 2.5 cm), the apparent and effective diffusion coefficients show an increasing trend with bentonite thickness. According to the experimental data and fitting results, the apparent and effective diffusion coefficients are highly related to bentonite thickness. Thus, scaling effects on transport parameters were proposed to explain the results, which were attributed to the nonuniform distribution of the pore space in the bentonite sample. The scale effect behavior of radionuclide was quantified through a regression analysis. The results can be used to improve buffer designs for radionuclides diffusion.

Investigation of nuclide migration in complex fractures with filling properties

Natural fractures in rock masses commonly feature filling properties, which greatly impact the process of fluid flow and mass transport in the geological disposal of high-level radioactive waste. This study develops a unified pipe-network method to numerically investigate the mechanism of nuclide migration in rough-walled fractures with filling properties based on mobile–immobile models (MIMs). Benchmark tests are conducted against analytical results with respect to the concentration distribution along a single filled fracture to demonstrate the numerical reliability. A case study is performed at the Xinchang (Beishan) selected site as the geological disposal repository for high-level radioactive waste. The migration process of U-238 in a fracture-network system close to a water-conducting zone is investigated with varying fracture-filling properties, fracture-filling proportions in mobile and immobile domains, and hydrodynamic properties. A safety classifier is developed using a support vector machine (SVM) based on 500 sets of numerical results for the concentration evolution of nuclides in complex fractures. This is to effectively evaluate the avoidance distance between the repository and the water-conducting zone with respect to factors including the fracture delay coefficient, the porosity of the fracture mobile domain, the hydraulic gradient, and the equivalent hydraulic aperture.

Forty-five years of joint research programmes on geological disposal of radioactive waste and the pioneering role of the HADES Underground Research Laboratory

Abstract Since 1975, the European Commission has supported research in the field of radioactive waste management and geological disposal through the Euratom Research and Training Programme. During the first two community programmes (1975–85), the research activities focused on basic knowledge, feasibility and safety assessments of such geological disposal repositories. It was during this first decade of collaborative research activities that the site characterization, preliminary design studies and the application for authorization to construct the first underground research laboratory, the High Activity Disposal Experimental Site facility, took place.

Confronting the Uncertainties Associated with Long-Time Scales: Analysis of the Modes of Preservation of Memory of Radioactive Waste Burial Sites

The burial of radioactive waste in deep geological repositories has gradually imposed itself since the 1980s in various countries. Considered more stable, safe, and responsible than storage above ground, this solution is seen as a way of keeping the waste out of human reach, and of freeing oneself of the obligation to monitor repositories. However, it soon became clear that the idea of relying on the relative stability of deep geological repositories for the safe disposal of nuclear waste to remove it from the ‘all too human’ risks associated with history’s turbulences, has produced new uncertainties, and raised new questions, given the multi-millennial time scales involved. This article presents a study of the strategies adopted by the actors in charge of radioactive waste management in the face of the temporal constraints imposed by the length of their radioactive life. More specifically, it is intended to question the representations of temporality that allow the designers of these projects to assume their responsibilities by transmitting information and warnings towards to an extremely distant future, to recipients who are not easily representable and for whom they already have a responsibility even though they do not yet exist.