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The long-lived radiotoxicity of the high-level radioactive waste generated by nuclear power plants requires safe isolation from the biosphere for many hundreds of thousands of years. An international consensus has emerged that such isolation can best be provided by disposal in mined geologic repositories, a strategy that today is pursued by most countries dealing with radioactive waste. However, the need to predict the performance of such repositories over very long time periods generates large uncertainties that have to be accounted for in safety assessments. The findings from such safety assessments need to be conveyed to all stakeholders in a clear way, such that public confidence in geologic disposal solutions can be achieved. It is suggested here that close international collaboration on the technical aspects of geologic waste disposal has helped, and will continue to help, building trust and increasing confidence. This paper discusses a particular international collaboration initiative referred to as DECOVALEX, which brings together multiple teams and disciplines to collectively tackle complex experimental and modeling challenges related to geologic disposal. By describing how DECOVALEX works and by providing joint research examples, a case is made that such international collaboration contributes to knowledge transfer and confidence building in radioactive waste disposal science.

The MOS model (acronym coming from the French MOdèle Simplifié) was born from the desire to have a simple tool that can quantify the contribution of the diffusive reactive environment to the alteration of a vitrified nuclear waste package in deep geological disposal conditions. In the model, this environmental contribution consists partly of the ability of iron, metallic casing corrosion products, and argillite to consume silicon, and partly of the brake on diffusive transport provided by silicon through the successive layers of environmental material. It is a modeling tool serving as an intermediary between operational modeling for the calculation of the source term from the glass, mathematically more simple and giving higher upper margins, and models that use geochemistry and transport, giving greater accuracy for the interactions between glass and its environment. The goal of the MOS model is to calculate the possible impact of silicon reactive diffusion on the alteration rate within the different layers of material surrounding nuclear glass. This article lists the simplifying hypotheses on which the MOS is based, presents the digital resolution method for an environment consisting of several successive layers with different reactivity and transport properties, and explains the model's implementation. [ABSTRACT FROM AUTHOR]

The U.S. Department of Energy is funding research into studying the consequences of postclosure criticality on the performance of a generic repository by (1) identifying the features, events, and processes (FEPs) that need to be considered in such an analysis, (2) developing the tools needed to model the relevant FEPs in a postclosure performance assessment, and (3) conducting analyses both with and without the occurrence of a postclosure criticality and comparing the results. This paper describes progress in this area of research and presents the results to date of analyzing the consequences of a postulated steady-state criticality in a hypothetical saturated shale repository. Preliminary results indicate that postclosure criticality would not affect repository performance. [ABSTRACT FROM AUTHOR]

Bentonite, used as a barrier material in landfill systems and the geological disposal of radioactive waste, undergoes various processes such as the cation exchange with landfill leachate and repository groundwater, desiccation shrinkage due to heat released from solid waste or nuclide decay. In the context of these engineering applications, it is crucial to consider the effect of interlayer cation composition on the desiccation-shrinkage behavior of bentonite for assessing the long-term performance of barrier systems. This study investigates this impact by conducting desiccation tests on four different cation-type bentonite samples (i.e., K-, Na-, Ca-, and Mg-bentonite) and elucidating the underlying mechanisms through mercury intrusion porosimetry test, X-ray diffraction analysis, and suction measurements. The results indicate that K-bentonite exhibits the highest water evaporation rate and the lowest shrinkage potential, with no shrinkage cracks detected, and that Na-bentonite evaporates water at a relatively fast rate but presents the most pronounced volumetric shrinkage. In contrast, Ca- and Mg-bentonite have the highest water-retention capacity, and their shrinkage potentials are less than Na-bentonite. An improved shrinkage model is proposed, which is shown to describe the measured shrinkage curves for four types of homoionic bentonite samples. Additionally, the mechanism behind the effect of different interlayer cations was interpreted in terms of the interlayer–cation–water interaction, pore structure evolution, and soil suction variation. [ABSTRACT FROM AUTHOR]

Bentonite is an integral part of the engineered barrier system (EBS) in deep geological repositories (DGR) for nuclear waste, but its indigenous microorganisms may jeopardize long-term EBS integrity. To predict microbial activity in DGRs, it is essential to understand microbial reactions to the early hot phase of DGR evolution. Two bentonites (BCV and MX-80) with varied bentonite/water ratios and saturation levels (compacted to 1600 kg.m− 3 dry density/powder/suspension), were subjected to heat (90–150 °C) and irradiation (0.4 Gy.h− 1) in the long-term experiments (up to 18 months). Molecular-genetic, microscopic, and cultivation-based techniques assessed microbial survivability. Exposure to 90 °C and 150 °C notably diminished microbial viability, irrespective of bentonite form, with negligible impacts from irradiation or sample type compared to temperature. Bentonite powder samples exhibited microbial recovery after 90 °C heating for up to 6 months but not 12 months in most cases; exposure to 150 °C had an even stronger effect. Further long-term experiments at additional temperatures combined with the mathematical prediction of temperature evolution in DGR are recommended to validate the possible evolution and spatial distribution of microbially depleted zones in bentonite buffer around the waste canisters and refine predictions of microbial effects over time in the DGR. [ABSTRACT FROM AUTHOR]

Site selection and characterisation are key factors influencing the safe, stable disposal of spent nuclear fuel. Site selection can be based on geophysical surveys, and geophysical well logging is performed at the detailed survey stage. Typically, gamma – gamma density logging is conducted to continuously obtain in situ density information regarding the well depth. However, gamma – gamma density logging is often influenced by the geological characteristics of the site. The ratio of atomic number to atomic weight is required to convert the electron density obtained through density logging counts into bulk density. Because it is difficult to calculate these ratios for the entire depth, an equation based on water-filled limestone is typically used for conversion. Therefore, as the difference in the ratio of the atomic number and atomic weight increases relative to that of limestone, the difference between the converted bulk density and the actual bulk density increases. In this study, equations were derived for various mineral compositions and applied to the density logging results. Matrix density was analyzed using a pycnometer, and the ratio between the atomic number and weight was calculated according to X-ray fluorescence analysis results. This approach improved the accuracy of the density logging data and reduced density errors. The proposed method is expected to be particularly useful for estimating in situ density at high-level radioactive waste disposal sites without complex geological formations. [ABSTRACT FROM AUTHOR]

Fractured crystalline rocks have been chosen or are under consideration by several countries as host rock formations for deep geological repositories for spent nuclear fuel. In such geological formations, flow and solute transport are mostly controlled by a network of connected natural fractures, each of them being characterised by internal heterogeneity, also denoted as roughness. Fractures are, in turn, subject to variable load caused by various factors, such as the presence of thick ice sheets formed during glaciation periods. Understanding how coupled hydro-mechanical (HM) processes affect flow and transport at the scale of a single natural fracture is crucial for a robust parameterisation of large-scale discrete fracture network models, which are not only used for nuclear waste disposal applications but are also of interest to problems related to geothermics, oil and gas production or groundwater remediation. In this work, we analyse and model an HM experiment carried out in a single natural fracture and use the results of both, the experimental and the modelling work, to get insights into fundamental questions such as the applicability of local cubic law or the effect of normal load on channeling. The initial fracture aperture was obtained from laser scanning of the two fracture surfaces and an equivalent initial aperture was then defined by moving the two fracture surfaces together and comparing the results obtained using a Navier–Stokes based computational fluid dynamics (CFD) model with the experimental flowrate obtained for unloaded conditions. The mechanical effect of the different loading stages was simulated using a high-resolution contact model. The different computed fracture apertures were then used to run groundwater flow simulations using a modified Reynolds equation. The results show that, without correction, local cubic law largely overestimates flowrates. Instead, we show that by explicitly acknowledging the difference between the mechanical aperture and the hydraulic aperture and setting the latter equal to 1/5 of the former, cubic law provides a very reasonable approximation of the experimental flowrates over the entire loading cycle. A positive correlation between fluid flow channeling and normal load is also found. [ABSTRACT FROM AUTHOR]

Flash sintering can prepare ceramics in extremely short time, which can significantly improve fabrication efficiency and is promising for processing nuclear waste. Herein, simulated TRPO waste (16 components containing low-melting components) was directly added into as-prepared Gd 2 Zr 2 O 7 , and a series of waste forms were successfully prepared by flash sintering at 950 °C for only 3 min. It was demonstrated that samples prepared by flash sintering showed almost no loss of simulated low-melting-point radionuclide, while samples prepared by pressureless sintering have certain losses of simulated low-melting-point radionuclide. The 42 d leaching rates of Mn, Ce, Nd, and Gd of the flash sintered samples were 8.21 × 10−5, 5.31 × 10−7, 4.04 × 10−6, and 1.04 × 10−6 g m−2 d−1, respectively, showing excellent chemical durability. This work proves that flash sintering technology can effectively suppress nuclear leaks during immobilization, along with excellent chemical durability. [ABSTRACT FROM AUTHOR]

This research paper focuses on the thermo-hydro-mechanical (THM) modeling of brine migration in a heated borehole test conducted as part of the ongoing Brine Availability Test in Salt (BATS) at the Waste Isolation Pilot Plant (WIPP) in New Mexico. It is a component of the international collaboration project DECOVALEX-2023 (DEvelopment of COupled models and their VALidation against EXperiments), which aims to understand the THM processes governing brine flow in heated rock salt repositories through collaborative analysis by multiple research teams. Using the TOUGH–FLAC simulator, THM simulations were performed and compared with data from the BATS phase 1a. This experiment involved two identical horizontal-borehole arrays, one heated and one serving as a control, both equipped with sensor arrays. Analysis of measurements revealed water flow rate surges during heater power transitions, with the highest jump observed during cooling. Acoustic emission activity exhibited distinct patterns in response to heater power changes, suggesting that damage to rock salt is particularly pronounced during the cooling phase. The THM simulations successfully captured these phenomena, highlighting the significance of thermal effects, brine migration, and mechanical behavior in predicting brine availability in heated and damaged rock salt. Our modeling also revealed the critical interplay between heating and cooling-induced damage and its influence on flow properties, particularly affecting brine inflow estimation. Notably, we found that cooling-induced brine inflow spikes result from increased permeability due to tensile dilatancy. These findings have important implications for the development of robust containment strategies and enhance our understanding of the complex processes involved in repository performance. Highlights: Numerical modeling study accurately replicates the thermal-hydromechanical behavior of rock salt surrounding a heated borehole. Jumps in brine flow rate during transitions in heater power are observed, with the highest jump during cooling. Acoustic emission patterns suggest pronounced damage to rock salt during the cooling phase. Thermal pressurization and tensile-induced damage are identified as the primary contributors to brine inflow spikes. Findings improve predictions of brine availability, aiding the development of robust containment strategies for radioactive waste disposal. [ABSTRACT FROM AUTHOR]

A sealed container for the geological disposal of spent nuclear fuel and vitrified high-level waste is the only component of a deep geological repository that provides complete containment of radionuclides. As such, attention is focused on its lifetime. The lifetime of the container is influenced by material degradation processes during disposal and is typically of the order of several millennia and, for some container materials, up to one million years. Designing, manufacturing, and predicting the performance of containers over such long periods requires an indepth understanding of their material properties, fabrication processes, and degradation mechanisms. Scientific and technological progress can improve both the performance of containers and the robustness of lifetime predictions. Optimization of these aspects is of primary importance for many national radioactive waste disposal programs. In this article, the state of the art of complex coupled degradation processes, as well as the optimization potential of novel container materials, is presented. Furthermore, the existing tools allowing the prediction of long-term barrier integrity are discussed. [ABSTRACT FROM AUTHOR]

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Radiation and radioactive substances result in the production of radioactive wastes which require safe management and disposal to avoid risks to human health and the environment. To ensure permanent safe disposal, the performance of a deep geological repository for radioactive waste is assessed against internationally agreed risk‐based standards. Assessing postclosure safety of the future system's evolution includes screening of features, events, and processes (FEPs) relevant to the situation, their subsequent development into scenarios, and finally the development and execution of safety assessment (SA) models. Global FEP catalogs describe important natural and man‐made repository system features and identify events and processes that may affect these features into the future. By combining FEPs, many of which are uncertain, different possible future system evolution scenarios are derived. Repository licensing should consider both the reference or "base" evolution as well as alternative futures that may lead to radiation release, pollution, or exposures. Scenarios are used to derive and consider both base and alternative evolutions, often through production of scenario‐specific SA models and the recombination of their results into an assessment of the risk of harm. While the FEP‐based scenario development process outlined here has evolved somewhat since its development in the 1980s, the fundamental ideas remain unchanged. A spectrum of common approaches is given here (e.g., bottom–up vs. top–down scenario development, probabilistic vs. bounding handling of uncertainty), related to how individual numerical models for possible futures are converted into a determination as to whether the system is safe (i.e., how aleatoric uncertainty and scenarios are integrated through bounding or Monte Carlo approaches). [ABSTRACT FROM AUTHOR]

Rock salt is considered a suitable medium for the permanent disposal of heat‐generating radioactive waste due to its isolation properties. However, excavation damage and heating induce complex and heterogeneous thermal‐hydrological‐mechanical (THM) processes across different zones. Quantifying this heterogeneity is crucial for accurate long‐term performance assessment models, but traditional methods lack the necessary resolution. This study employs 4D electrical resistivity tomography (ERT) monitoring during controlled heating experiments in a salt formation to unravel the spatiotemporal dynamics of THM processes. Advanced time‐lapse inversion and clustering analysis quantify subsurface properties and map the heterogeneity of THM dynamics. The ERT results can estimate subsurface properties and delineate the damaged and intact zones, enabling appropriate parameterization and representation of processes for long‐term modeling. This approach may be used in further improving the predictive models and ensuring the safe long‐term disposal of radioactive waste in rock salt. Plain Language Summary: Rock salt is great for the disposal of heat‐generating radioactive waste because it prevents releasing harmful radiation into the environment. However, mining and heating can cause complex changes in temperature, water flow, and pressure, affecting rock salt in various ways. It is important to understand these changes accurately for planning how the disposal will affect the rock salt, but the traditional ways of studying them are not detailed enough. In this study, we used a special technique called 4D ERT while heating up the rock salt to see how these changes happen over time and space. We analyzed the data in advanced ways to measure the underground properties and understand the complex changes. The information can improve how we model and plan the disposal of heat‐generating waste. This new insight makes it easier to ensure that storing heat‐generating radioactive waste in rock salt is safe for a long time. Key Points: We employ 4D electrical resistivity tomography (ERT) monitoring during a controlled heating experiment in salt formationsThe resistivity data exhibits contrasting trends throughout and post‐heating, indicating the heterogeneous thermal‐hydrological‐mechanical processes in rock salt are likely linked to the different levels of excavation damagesEmploying time‐series clustering technology enables quantification and spatial distributions of excavation damage/disturbed zones within the salt formations which may help further improve the parameterization of long‐term predictive models [ABSTRACT FROM AUTHOR]

In the present study, the chemical composition, mineralogy, and mechanisms of alteration of a cementitious grout based on a CEM III/C with addition of smectite, hydrotalcite, and silica fume, are studied using a combination of chemical and physical methods. This material was designed in the context of geological repository of radioactive wastes, with a twofold aim: first, to fill the technical voids left by drilling operations at the interface between the geological formation and the disposal galleries. Second, to neutralize a potential acidic transient due to pyrite oxidation, and to create an environment that favors low corrosion rates of carbon steels. The grout is mainly composed of calcium silicate hydrates having a Ca/Si ratio of ~0.8, incorporating Al in the bridging site of the Si chains (C-A-S-H), and accounting for 29–36 wt.% of the sample. It also contains silica fume (38–48 wt.%), smectite with interlayer Na (11–17 wt.%), hydrotalcite with interlayer CO32− (3–4 wt.%), and lower amounts of portlandite, calcite, and possibly gibbsite and gypsum. Upon alteration by water in a flow-through reactor, the main modifications affecting the sample are calcite and gypsum dissolution, hence releasing aqueous Ca2+ that is adsorbed in smectite interlayer by replacing Na+, and stoichiometric C-A-S-H dissolution. The evolution of solution chemistry and of the solid phase composition are reproduced successfully using a thermokinetic model. [ABSTRACT FROM AUTHOR]

Due to the development and utilization of nuclear energy, the safe disposal of nuclear waste needs to be urgently addressed. In recent years, the utilization of microorganisms' adsorption capacity to dispose of radioactive waste has received increasing attention. When compared with conventional disposal methods, microbial adsorption exhibits the characteristics of high efficiency, low cost, and no secondary pollution. In the long term, microbial biomass shows significant promise as specific chemical-binding agents. Optimization of biosorption conditions, identification of rare earth element binding sites, and studies on the sorption capacities of immobilized cells provide compelling reasons to consider biosorption for industrial applications in heavy metal removal from solutions. However, the interaction mechanism between microorganisms and radioactive nuclides is very complex. This mini-review briefly provides an overview of the preparation methods, factors affecting the adsorption capacity, and the mechanisms of microbial adsorbents. [ABSTRACT FROM AUTHOR]

This work focused on evaluating the evolution of the corrosion product layer and associated corrosion rates on archeological artifacts from 16 sites. Sites with clay soils and maximum waterlogging (pond bottoms and wetlands) were monitored. The corrosion products were evaluated by X‐ray imaging, X‐ray diffraction, scanning electron microscope, Brunauer–Emmett–Teller, and nanoindentation. Two stages of corrosion attack with slightly different mechanisms were observed. Corrosion rates observed at sites with continuous flooding show average corrosion rates of 0.4–1 μm.a−1. These values demonstrate a minimum service life of the container outer case of 15 × 103 years, which is several times longer than the required lifetime of the outer case. [ABSTRACT FROM AUTHOR]

[Display omitted] Solvent expulsion away from an intervening region between two approaching particles plays important roles in particle aggregation yet remains poorly understood. In this work, we use metadynamics molecular simulations to study the free energy landscape of removing water molecules from gibbsite and pyrophyllite slit pores representing the confined spaces between two approaching particles. For gibbsite, removing water from the intervening region is both entropically and enthalpically unfavorable. The closer the particles approach each other, the harder it is to expel water molecules. For pyrophyllite, water expulsion is spontaneous, which is different from the gibbsite system. A smaller pore makes the water removal more favorable. When water is being drained from the intervening region, single chains of water molecules are observed in gibbsite pore, while in pyrophyllite pore water cluster is usually observed. Water-gibbsite hydrogen bonds help stabilize water chains, while water forms clusters in pyrophyllite pore to maximize the number of hydrogen bonds among themselves. This work provides the first assessment into the energetics and structure of water being drained from the intervening region between two approaching particles during oriented attachment and aggregation. [ABSTRACT FROM AUTHOR]

Experimental investigation of the elastic behaviours of lacustrine shales remains sparse, although they play an essential role in source rock evaluation, unconventional reservoir exploration and development, and the seal integrity evaluation for geological storage of CO2 and nuclear waste disposal. We make the ultrasonic velocity measurement of 63 organic‐rich shale samples (Chang 7, Qingshankou and Lucaogou formation) from three typical lacustrine basins in China. It is found that the P‐ and S‐wave velocity of Chang 7 and Qingshankou shale corresponding to the fresh‐brackish lacustrine depositional environment is mainly impacted by the clay and organic matter content, whereas their elastic anisotropic magnitude is mostly influenced by clay content. The P‐ and S‐wave velocities of Lucaogou shale corresponding to the saline lacustrine depositional environment are mainly affected by the total organic carbon and porosity and exhibit weak anisotropy linked to organic matter enrichment. Exponential law well captures the relationship between anisotropic magnitude and velocity perpendicular to bedding for both saline and fresh‐brackish water lacustrine shales, although there exists notable discrepancy, particularly at low velocities. The disparity in elasticity between laminations has a profound impact on the magnitude of elastic anisotropy and shapes the trend of velocity variations. [ABSTRACT FROM AUTHOR]

Characterizing the changes in the meso-structure of granite against thermo-hydro-mechanical (THM) effect is of great significance to understand the variation mechanism of granite hydro-mechanical properties in disposal system. In this study, an improved meso-structure-based DEM model (I-MSBM) was first developed to adequately consider the weakening effects of the mineral grain bonds caused by THM loading, in which the aperture-dependent strength and elastic modulus weakening factors of pores were determined by experiments. Comparisons between the numerical and experimental results indicated that the developed model was suitable to distinguish the THM responses of varying minerals, mineral boundaries and pores in the granite. Based on the I-MSBM, the changes in the permeability and macro mechanical properties, i.e., the uniaxial compressive strength and elastic modulus of granite were investigated under the conditions of coupled temperatures (25–225 °C), pore pressures (0–12 MPa) and confining pressure (15 MPa), and the main mechanisms affecting the varied macro hydro-mechanical properties were further discussed. The results indicate that the permeability and macro mechanical properties of granite are more sensitive to the high pore pressure (P > 4 MPa) at moderate temperatures (T < 125 °C), which is mainly due to the variation in the pore aperture induced by high pore pressure. Furthermore, the macro hydro-mechanical properties are more susceptible to temperature higher than 125 °C owing to the accelerated development of the thermal-induced micro cracks. The findings of this study are helpful for understanding the complex THM coupling damage to the granite in the nuclear waste disposal systems. Highlights: An improved meso-structure-based DEM model was developed to characterize the THM coupled effects on weakening the bonds among the mineral grains, in which the aperture-dependent weakening factors of pores were determined. The THM responses of varying minerals, mineral boundaries and pores in the granite were distinguished via the improved meso-structure-based DEM model The changes in the macro hydro-mechanical properties of granite subjected to THM coupling in disposal system were clarified, and the mechanisms that affect the varied macro hydro-mechanical properties were further revealed. [ABSTRACT FROM AUTHOR]

The stability of cementitious materials under the harsh environment they will experience when used for radioactive waste disposal is incredibly important. Therefore, understanding the irradiation resistance of geopolymer cement, a potential alternative binder for the treatment of nuclear waste, is of the utmost importance when trying to develop a safety case for these materials. The study presented here addresses the structural and chemical changes of metakaolin‐based geopolymers, designed with different water contents, and exposed to a total cumulative dose of 1 MGy of gamma radiation. The range of formulations that were tested showed a significant loss of free water related to the irradiation process, which has led to an increase in the porosity. Analysis of the chemical structure has shown minimal changes in the main binding type‐gel phase, demonstrating high microstructural stability. Results showed that in samples cured for longer than 20 h, the bound/gel structure water remained in the sample when the water content was kept low enough. As the porosity and water content increase, more gel water is removed due to radiation exposure. However, the degree to which the water is removed from the gel structure is very small, and minimal changes can be seen across the geopolymers tested. Overall, metakaolin‐based geopolymers appear resistant to irradiation up to 1 MGy, which offers a potentially viable alternative for the immobilization of problematic intermediate‐level waste. [ABSTRACT FROM AUTHOR]

This study concerns the project of high and intermediate level radioactive waste disposal (Cigéo project for Centre Industriel de stockage GEOlogique). Millions of tons of argillite will be excavated during the construction of the site. Therefore, the aim of this work is to reuse this argillite in the formulation of an injectable geopolymer grout and study its evolution in different storage conditions (T = 20 and 80 °C). To achieve this challenge, a metakaolin and calcined Callovo-Oxfordian (COx) argillite were used with potassium silicate solutions. The grouts were successfully obtained from both metakaolin and COx argillite in both temperatures 20 and 80 °C. The range of viscosity of several mixtures respected the required specifications (viscosity ≤5 Pa s and setting time superior to 6 h). The metakaolin-based geopolymers didn't display any major changes no matter the storage temperature, except for the pore size, a coalescence phenomenon was observed with the increase of temperature. In contrast, for argillite or argillite-metakaolin based samples, a decrease of pore size as well as in pH values were observed with the increase of temperature. This behaviour was attributed to the dissolution of argillite's impurities such as illite, providing more oligomers to allow the formation of different geopolymer networks. [ABSTRACT FROM AUTHOR]

Safe confinement of fission iodine isotopes for long-term radioactive waste disposal remains a formidable challenge, as conventional sorbents provide inherently weak iodine-host interactions. We report here a novel halogen bond (X-bond) directed strategy to sequester volatile iodine in hydrogen-bonded (H-bonded) frameworks with unprecedented stability. Charge-assisted H-bonded frameworks bearing open halide sites are developed, showing distinctive iodine encapsulation behaviors without compromising the crystallinity. Direct crystallographic evidence indicates the formation of X-bonds, i.e., ∣–∣⋯Cl− and ∣–∣⋯Br−, within the confined pore channels. Unusual polyhalogen anions, i.e., [I2Cl2]2− and [I2Br2]2−, sustained in H-bonded frameworks are identified for the first time. The X-bond reinforced host-guest interaction affords robust iodine trapping without leaking out even at elevated temperatures up to 180 °C. By integrating the halogen-bond chemistry with H-bonded frameworks, this study offers fresh concepts for developing effective host reservoirs to secure fission iodine isotopes from spent fuel reprocessing off-gases. [ABSTRACT FROM AUTHOR]

Featured Application: The materials studied may be used in the radioactive waste vitrification process. Additionally, the proposed simulation method may be useful in optimizing glass composition and predicting the glass elements migration. Among the many possible applications of iron phosphate glasses, one of them is that they are promising materials in waste vitrification, particularly for radioactive waste. In vitrified form, waste elements should be permanently immobilized in a glass network as they are susceptible to harsh environmental conditions. The self-diffusion of the vitrified material species may limit the potential usefulness of the glasses. This paper presents the possibility of using molecular dynamics simulations to study this process and the substitution of SrO into an iron phosphate glass network. It was evidenced that the self-diffusion mechanism differed significantly depending on whether the glass was in a solid or liquid state. The proposed method also offered a relatively easy prediction of glass characteristic temperatures, such as transformation and flow. We also observed, and here describe, an aggregation process of the glass elements that may drive their crystallization. The obtained results are discussed in light of the experimental and theoretical structural feature literature data. [ABSTRACT FROM AUTHOR]

The article presents the results of an experimental modelling of the influence of Ca2+ ion concentration and pH of a model solution on the elemental composition and sorption capacity of natural (NB) and sodium-modified (SMB-20) bentonites from the Cherkasy deposit for 137Cs and 90Sr. The processes of the Cs and Sr sorption from multicomponent solutions with different pH, Eh and salt composition depend on the insoluble or sparingly soluble compounds precipitated on the bentonite surface. Based on the computer simulation data, it was found that small amounts of sediments, mainly oxides, hydroxides, Fe oxyhydroxides (hematite, goethite, limonite), and Ca carbonates (calcite, aragonite, dolomite) were present in the model solution with alkaline pH after adding CaCl2 solutions. Oxides and hydrated oxides of Fe, and calcium carbonates play an important role in the Cs and Sr sorption processes on bentonites. They can concurrently sorb radionuclides and block the sorption centres in montmorillonites. The concentration of the main structural elements (Si, Al) in the bentonites practically does not change with the increased pH and Ca2+ ion concentration in the model solution compared to the initial samples of NB and SMB-20, which indicates their structural stability. At the same time, an increase in the calcium concentration and a decrease in the sodium concentration were found in the ionexchange complex of montmorillonite — the main bentonite mineral if compared to the initial samples. This can eventually lead to the transformation of SMB-20 bentonite from Na, Ca form to Ca, Na form. The 137Cs and 90Sr sorption on NB is shown to proceed more effectively, compared to SMB-20 in conditions of the increased pH and Ca2+ ion concentration. The results of this research may justify the bentonite clay application as a component of the isolating engineered barriers of radioactive waste disposal facilities. [ABSTRACT FROM AUTHOR]

Selecting a site to build a deep geologic repository for high-level nuclear waste is a process fraught with social tension and few countries have managed to do so successfully. Canada is poised to be next, with a site selection decision due in 2024. Two potential sites are under consideration, one in northern Ontario and one in southern Ontario, each with different geologic settings and different social and economic landscapes. We review the consent-based process that has led to this point, the complexities involved in expressing consent in Canada, and some of the hurdles that remain. [ABSTRACT FROM AUTHOR]

In recent decades, with the growth of population and the development of metropolitans around the world, the issue of landfill and waste management has become more and more critical. Due to development of radioactive material applications, disposing these radioactive waste material became an important geo-environmental concern. Bentonite clay as an eco-friendly and natural shielding material is used as the main material for landfill barriers. The radiation shielding performance of bentonite clay modified by different percentages of basalt fiber and Polyvinyl alcohol (PVA) resin additives is investigated using experimental measurement, Monte Carlo N-Particle (MCNP) simulation, and XCOM database methods, in Gamma-ray energy levels of 60Co (1173.2 and 1332.5 keV). The employed sample mixtures include 0.25, 0.5, 1, and 2 percent of basalt fiber and PVA resin additive. The obtained results for Gamma-ray shielding measurement from these three methods have a good agreement, depicting an increasing trend in the linear attenuation coefficient for higher percentages of additives. The maximum increment is observed at 0.5 percent of additive, from 7.85 to 9.69 m-1, and 9.3 to 11.23 m-1, in energy levels of 1132.5 and 1173.2 keV, respectively. This improvement may decrease the bentonite clay Gamma-ray barrier thickness (half-value layers (HVL), tenth value layers (TVL)), up to 20 and 23 percent for 1173.2 and 1332.5 keV, correspondingly. Moreover, the hydraulic and gas permeability of the mixtures are controlled to find the standard and optimum mixture. In which the 0.5 to 1.0 percent of additives will keep the permeability requirement in an allowable and preferable range.

Safety-focused process system analysis is an important approach to ensure the safety of radioactive waste disposal facilities throughout their lifecycle, including siting, construction, operation, and closure. In recent years, the uncertainty in the analysis process receives increasing attention, and uncertainty analysis and management become integral parts of safety-focused process system analysis and safety evaluation. International documents such as the Safety-Focused Process System Analysis and Safety Evaluation for Radioactive Waste Disposal by IAEA and domestic regulations like the Safety Regulations for Near-Surface Disposal of Low and Intermediate-Level Radioactive Solid Waste emphasize the importance of uncertainty analysis in these processes. Properly addressing uncertainty in safety-focused process system analysis and safety evaluation is crucial for ensuring the credibility of the evaluation results. Therefore, conducting uncertainty analysis on case studies for long-term safety evaluations is particularly important. External factors, such as environmental changes and human behavior, which cannot be controlled in advance, significantly impact the safety of various nuclear facilities, including radioactive waste disposal facilities, and thus require uncertainty assessment. Many countries and international organizations in the nuclear industry conducted related research, focusing on probability, risk guidance, and uncertainty analysis. They developed some general evaluation methods, such as Monte Carlo simulation and Latin hypercube sampling, for uncertainty analysis according to specific circumstances. In the uncertainty analysis of radioactive waste disposal repositories, the computational process of parameter uncertainty analysis is more complex compared to the common application scenarios of Monte Carlo simulation. It involves a large number of random input parameters and complex models, which necessitates smaller sample sizes to reduce computation time and improve sampling efficiency. In this case, the sampling process with small sample sizes and multiple random parameters will inevitably face a challenge: Whether the uniformity and correlation between the dimensions of the sampled samples meet the computational requirements. To address the issue of uniformity in the sampling process, adopting Latin hypercube sampling (LHS) coupled with Monte Carlo simulation is a common choice. In this study, LHS was improved by applying the Cholesky decomposition method in the parameter uncertainty analysis of safety-focused process system analysis for radioactive waste disposal repositories. The improved LHS method through this approach can use smaller samples to meet the correlation requirements of the computation, thereby enhancing sampling efficiency. In the scenario of this study, the improved sampling method only requires one-tenth of the sample size needed in the original method.

This work is focused on Magneto hydrodynamic unsteady flow of Newtonian fluid flow over a perpendicular porous plate along with chemical reaction, heat, and mass transfer. The induced magnetic field and viscous and magnetic dissipation properties are considered throughout the porous plate. Heat source is the added effect in the model to observe the nature of flow in this work. This study finds its applications in understanding the storage of flues, disposal of radioactive waste materials, flow in water purifies, etc. The nonlinear behavior of the governing equation motivates us to use a finite difference approach for solving equations. The major resolution of this research is to work on how the physical elements affect velocity, temperature, concentration, and magnetic field. Fascinating facts are noticed as the unsteady fluid velocity rises with the heat source parameter because as the heat of a material increases, the movement of the fluid particle will be fast. Due to the presence of a magnetic field high thermal radiation is observed at high temperature and concentration. As the magnetic parameter and magnetic field are inversely proportional according to an induced magnetic field, it is noticed that their magnetic field declines for higher values of magnetic Prandtl number & magnetic parameter. This work has dynamic prominence in the field of medicine and engineering, which develops interest among young researchers. [ABSTRACT FROM AUTHOR]

The Arctic region is facing growing demands for energy to support various economic activities, while also grappling with the profound impacts of climate change. Black carbon particulate matter emissions reduction is a key objective to mitigate the susceptibility of the Arctic's ecosystems to the impact of climate change. Nuclear power has been suggested as a potential source of clean energy to decarbonize maritime transport in the Arctic. However, although the operation of nuclear-powered vessels and floating nuclear power platforms in the region ensures energy security and reduces black carbon emissions, it may pose significant risks of nuclear material release and radiological accidents and raise concerns about improper radioactive waste disposal. In regulating these nuclear-powered vessels and floating nuclear power platforms in the Arctic, the existing international legal regime faced a series of challenges. This research employs a method of policy analysis to analyze these legal challenges and explores how the international community could work together to cope with the challenges that arise in the Arctic during the operation of nuclear-powered vessels and platforms for maritime decarbonization purposes. [ABSTRACT FROM AUTHOR]

Fluid-solid interactions in nanoporous materials underlie processes fundamental to natural and engineered processes, including the thermochemical transformation of argillaceous materials during high-level nuclear waste disposal. Operando fluid-solid resolution at the nanoscale, however, is still not possible with existing optical and electron microscopy approaches that are constrained by the diffraction limit of light and by vacuum-fluid incompatibility, respectively. In this work, we develop an operando scanning electron microscopy (SEM) platform that enables the first direct in situ imaging of dynamic fluid-solid interactions in nanoporous materials with spatio-temporal-chemical resolutions of ~2.5 nm per pixel and 10 fps, along with elemental distributions. Using this platform, we reveal necessary conditions for thermochemical pore and fracture generation in shales and measure their surface wetting characteristics that constrain the feasibility of high-level nuclear waste containment. Notably, we show that low heating-rate conditions typical of radioactive decay produce hydrocarbon liquids that wet fracture and pore surfaces in a selfsealing manner to impede aqueous radionuclide advection. [ABSTRACT FROM AUTHOR]

Cruise 92 of the R/V Akademik Mstislav Keldysh was organized by Shirshov Institute of Oceanology within the long-standing program "Marine Ecosystems of the Siberian Arctic" October 5–November 7, 2023. A total of 76 scientists from institutes of the Russian Academy of Sciences, National Research Center Kurchatov Institute, Moscow State University, Moscow Institute of Physics and Technology, VNIRO, and MES participated in the cruise. Coordinated hydrophysical, hydrochemical, biooceanological, and geochemical research were carried out in the eastern Kara Sea in the period of seasonal ice formation, as well as in the Ob River estuary and fjords of the Novaya Zemlya archipelago. The conditions of largest radioactive waste disposals in the Kara Sea were evaluated. [ABSTRACT FROM AUTHOR]

The present work deals with diffusion of gases in fully saturated porous media. We test and validate the gas transport mechanism of dissolution and diffusion, implemented in the TH2M process class in the open-source finite-element software OpenGeoSys. We discuss the importance of gas diffusion for the integrity of the multi-barrier system. Furthermore, we present a multi-component mass balance equation implementation in Python, which serves as a reference for the two-component TH2M implementation and allows for a discussion of multi-component gas diffusion in liquids. We verify and validate the numerical implementations as follows: First, we come up with a set of numerical benchmarks in which solutions obtained by the two-component TH2M and multi-component implementations are compared. Thus, we show under which conditions predictions made by the TH2M model can be used for multi-component gas systems. Finally, the work is validated using a through diffusion experiment performed at Belgium's Nuclear Research Centre SCK CEN and a sensitivity analysis is conducted based on the featured experiment. The results of this work illustrate that predictions by both the two- and four-component models match the laboratory findings very well. Therefore, we conclude that also the two-component implementation can reflect the multi-component processes well under the given constraints such as full saturation. Highlights: Numerical modelling of a laboratory scale gas diffusion experiment using Boom Clay samples. Validation of numerical implementation by comparison of experimental and numerical results. Extension of a two-component two-phase flow model towards a multi-component model including comparison of the two implementations. Sensitivity analysis identifies and quantifies the influence of key diffusion parameters, such as the Henry coefficient and diffusivity. [ABSTRACT FROM AUTHOR]

The French National Radioactive Waste Management Agency is leading the design of a deep geological radioactive waste disposal to be located in the Callovo-Oxfordian formation. At the disposal main level, large-diameter galleries will ensure access to the storage cells and connection between zones. After a long operational period, the disposal will be closed by sealing structures located at some key positions to ensure post-closure safety over the long term. The seals are intended to prevent the flow of water and the migration of radionuclides from the disposal to the biosphere throughout the entire storage life. Thus, the long-term safety of the disposal relies to a large extent on the performance of these structures. The paper presents the work carried out to assess and simulate the phenomena underlying the response and performance of a large-diameter sealing concept under real disposal conditions. The complexity of the problem is addressed by considering in the simulations large-scale 3D geometries, advanced constitutive models, complex coupled phenomena, key geometric details at decimetre scale, and all the phases from the excavation to the post-closure period (~ a few thousand years). Highlights: The three-dimensional hydro-mechanical modelling of a large structure concept, foreseen to seal access galleries in a clayey rock deep disposal, has been carried out. Model considers large geometry (order of tens of meters), anisotropy, constitutive laws of different complexity, interfaces, and relevant geometrical details (order of cm). Results evidence a progressive hydration and swelling of the sealing core during more or less 2000 years after disposal closure. The concrete plugs placed at both ends of the sealing structure move longitudinally, but core density and swelling pressure do not change significantly in seal central part. Swelling core recompresses partially the excavation-induced damaged zone in the area of lining deposition. [ABSTRACT FROM AUTHOR]

Fracture filling minerals and the smectite-group clay minerals in particular play a significant role in the geotechnical stability of the fractured rock mass. Smectites have multiple uses as geomaterials in waste management solutions, of which the most technically demanding is the geological disposal of radioactive waste. Here, the stability and occurrence of smectites as fracture filling minerals were studied with two main objectives: 1) to test the capability of short-wave infrared spectroscopy as a tool to streamline the site assessment where clay mineralogy needs to be characterised in detail, and 2) to provide a first assessment of naturally occurring smectites in various sample types collected from two Finnish sites (Olkiluoto and Hyrkkölä) and their potential as natural analogues of smectite stability for bentonite used in the engineered barrier systems of geological disposal repositories. The short-wave infrared (SWIR) method has been validated to be a useful tool in fracture mineral characterisation, specifically due to its fast acquisition and interpretation time compared to the conventional methods. The study confirms the in situ occurrence of smectites at both sites studied showing that, especially in Hyrkkölä, the smectite-group member is close to montmorillonite, the main constituent of bentonite. At Olkiluoto, the smectites studied show more interstratified features. The differences in the mode of occurrence are related to the history of their formation, especially hydrothermal alteration, and parent rock composition. Highlights: Fracture filling mineral analysis using short-wave infrared spectroscopy for site characterisation is non-destructive providing an efficient method for investigating large sample sets Fast data collection and interpretation from field for rock engineering can indicate presence of swelling minerals that may cause rock collapse Fracture filling smectites, specifically montmorillonites, provide natural analogues for bentonites used in geological disposal facilities for radioactive waste Short-wave infrared spectroscopy methods can partially exceed X-ray diffraction-based smectite identification especially for bentonites (e.g., beidellite vs. montmorillonite) which is important in quality assurance of bentonites used in geological disposal. [ABSTRACT FROM AUTHOR]

In the Mont Terri Rock Laboratory (Switzerland), an interdisciplinary examination program is carried out to increase knowledge about coupled hydro-mechanical effects in Opalinus Clay, which are of significant interest regarding the stability and integrity of a potential storage facility for high-level radioactive waste. This article focuses on the characterization of the claystone in the near field of excavations and related hydraulic effects due to excavation and ventilation. Electrical resistivity tomography (ERT) is applied to characterize the OPA: Several open fractures correlate with regions of high resistivity values, indicating potential preferential flow paths that are relevant for transport processes. Due to the combined interpretation of ERT long-term monitoring and seasonally repeated nuclear magnetic resonance (NMR) measurements, a relationship between electrical resistivity and water content can be established, resulting also in a time-dependent map of the water content around excavations with different climatic conditions. The statistical interpretation of these measurements indicates the existence of small-scale singularities in contrast to dominating, more homogeneous zones. The presented approach leads to a better process understanding of these heterogeneous near field effects and provides a valuable basis for a pragmatic approach to safety assessment. Highlights: Methodic approach to provide a time-dependent map of the water content around an excavation in the sandy facies of Opalinus Clay (OPA). Identification and statistical evaluation of potential flow paths (micro-cracking areas) in the near field of excavations with regard to numerical safety analysis. Establishment of the relationship between electrical resistivity and water content in the sandy facies of OPA based on combined evaluation of ERT and NMR measurements. Impact of heterogeneity and technical boundary conditions on the evolution of the water content and the permeability. Combined interpretation of multi-disciplinary methods aiming on a detailed process understanding and characterization of coupled hydraulic-mechanical effects in Mont Terri OPA. [ABSTRACT FROM AUTHOR]

To analyse the effect of temperature on the mechanical behaviour of the Callovo-Oxfordian claystone in the context of deep geological disposal of radioactive waste in France, compression tests were performed in a triaxial cell under pseudo-drained condition with strains measurements, at different temperatures (20, 40, 60, 80, 100 and 150 °C), confining pressures (0, 4 and 12 MPa) and samples orientations (parallel and perpendicular to the bedding plane). The analysis of elastic coefficients indicates that in all cases an anisotropic damage develops during the deviatoric loading due to the opening of axial microcracks. In addition, the peak deformation and strength increase with the confining pressure. The initial heating stage generates a transitory pore water overpressure due to thermal expansion, which creates microcracks probably parallel to the bedding plane. Despite the scattering of some results, an overall decrease of the peak strength with increasing temperature (until 100 °C) is observed because of to the thermo-hydro-mechanical damage induced by the initial heating. For the parallel samples under uniaxial conditions, this decrease is the most important and volumetric dilatancy develops during the loading for the highest temperatures. For all other conditions, the decrease is more moderate and there is no dilatancy because confining pressure reduces the creation of initial thermo-induced microcracks, which are also closed when the axial stress increases during the compression tests when the orientation is perpendicular. There is no noticeable impact of temperature up to 100 °C on the evolutions of the elastic coefficients. The peak strength increases at the highest temperature (150 °C) in all cases due to the water vaporisation and then strong samples desaturation, which induces the development of a very significant capillary suction. Highlights: Compression tests were performed on the Callovo-Oxfordian claystone at different temperatures (20–150 °C), confining pressures (0, 4 and 12 MPa) and orientations (parallel and perpendicular). An anisotropic microcracking damage develops during the compression test and the increase of confining pressure increases the ductility and strength of the material. Initial heating stage generates a transitory pore water overpressure due to thermal expansion, which creates microcracks mainly under uniaxial condition. Despite the scattering of some results, an overall decrease of the peak strength with increasing temperature (until 100 °C) is observed due to the initial thermo-hydro-mechanical damage. The peak strength increases at 150 °C, due to the water vaporisation and then strong samples desaturation, which induces very significant capillary suction. [ABSTRACT FROM AUTHOR]

This paper presents several series of multi-stage permeability tests in laboratory with both steady-state and pulse decay methods on core samples of the low permeable Callovo-Oxfordian claystone from Meuse/Haute-Marne (France). The focus is to explain why water permeability values obtained in laboratory with the steady-state method are lower than those obtained with the pulse decay method and those obtained in situ in the Underground Research Laboratory designed for a feasibility of a deep geological repository of radioactive waste. The focus is also to assess the influence of the percolating water chemistry and the duration of resaturation and steady flow stages on the permeability values, and the impact of these permeability measurements on the mechanical properties (elastic coefficients, peak strength). The water permeability values measured with the pulse decay method are homogeneous and around 10−20 m2. Those obtained with the steady flow method are significantly lower (up to one order of magnitude) when using the synthetic water of ANDRA. This difference can be explained by an additional hydration of the clay minerals during steady-state flow which induces a swelling and then a decrease of the interparticle porosity. These physicochemical mechanisms slow down the water flow during the steady flow tests. When using a chemically balanced water and reducing the test duration, the difference is only about half an order of magnitude. Sample resaturation and steady-state flow induce a significant damage of the material which is mainly due to the opening of the bedding planes of swelling clay minerals. Highlights: Laboratory permeability tests with steady state and pulse decay methods were performed on core samples of Callovo-Oxfordian claystone from Meuse/Haute-Marne (France) Permeability values measured with the steady flow method are significantly lower than those obtained from the pulse decay method when using the synthetic water of ANDRA This difference could be explained by an additional hydration of the clay minerals during steady state flow, which induces swelling and slows down the water flow A more accurate assessment of the permeability with the steady state method was obtained using a chemically balanced water and experiment with shorter run time Initial resaturation and steady state flow induce a significant damage of the material which is mainly due to the opening of the bedding planes of swelling clay minerals [ABSTRACT FROM AUTHOR]

Clay formations are one of the options currently being considered for the storage of radioactive wastes worldwide. In France, the National Radioactive Waste Management Agency (Andra) operates the Meuse/Haute-Marne Underground Research Laboratory in the Callovo-Oxfordian (COx) clay formation to assess the feasibility and safety of an industrial radioactive waste repository. A good level of understanding of the thermo-hydromechanical behaviour of the host rock is paramount for the safety assessment. A new anisotropic elastoplastic damage and viscoplastic model is proposed to describe the hydromechanical behaviour of the COx claystone based on a large dataset of experimental evidence. The model is based on the Hoek and Brown criterion and considers recent findings of the COx hydromechanical behaviour. The key mechanisms considered are: plastic strain hardening prior to reaching the peak strength, a post-peak behaviour characterised by strain softening in the frame of continuum damage mechanics and a residual stage represented by a perfectly plastic behaviour. Time-dependent deformations are also included based on a creep model, which in this work is coupled with damage. The proposed model was implemented with a regularisation scheme based on the non-local implicit gradient in Comsol Multiphysics® with the purpose of performing THM modelling (1D, 2D and 3D) in the framework of the Cigéo project. The numerical implementation is first validated based on several simulations of creep tests at different deviatoric stress and triaxial compression tests at different confining pressures and angles between the loading direction and the bedding, α. Then, the GCS drift hydromechanical behaviour is simulated considering transverse isotropic conditions. It is shown that the model is capable of reproducing the measured peak of pore pressure in sensors near the GCS wall as well as the drift convergence. Importantly, the predicted extent of damaged zones around the drift is consistent with the in situ observations. The impact of damage on the time-dependent behaviour and the permeability was investigated numerically. It was found that the magnitude and anisotropy of drift convergence and the pore pressure drop are sensitive to this coupling and the best agreement was obtained when this effect was taken into account. Finally, the performance of the regularisation scheme is demonstrated with a set of simulations of the drift with different mesh refinements. It is concluded that the proposed model captures the key features of the hydromechanical behaviour of the COx claystone. Highlights: An advanced constitutive model that takes into account the recent findings on the key mechanisms of deformation and failure of COx claystone is proposed. Strain hardening/softening, anisotropic elasticity and plasticity, damage described in the framework of CDM and time-dependent behaviour of COx claystone are considered. The effect of damage and shear fracturation on the transport and viscous properties are also addressed in the model. The model with a regularisation scheme is implemented in Comsol Multiphysics®, which allows several physics to be considered, then to perform 1D, 2D and 3D THMC modellings. The operational nature of the proposed model is successfully evidenced on the GCS drift of the MHM URL, for which the in situ observations are the most challenging to reproduce. [ABSTRACT FROM AUTHOR]

A deep geological facility (Cigéo project) is planned, if authorized, in the Callovo-Oxfordian claystone (COx) for the disposal of all long-lived radioactive waste produced in France. The heat emitted from the most exothermic wastes, emplaced in horizontal parallel micro-tunnels, will lead to a temperature increment within the COx. The resulting thermal pressurization may lead to tensile effective stresses and induce thermal hydrofracturing. One of the design criteria for this area is to avoid any damage induced by thermal loading, especially in the far field. The French National Radioactive Waste Management Agency (Andra) has conducted an in situ heater experiment at the Meuse/Haute-Marne Underground Research Laboratory to better understand the thermal hydrofracturing process of the COx. The experiment's design was conceived in a way that the thermal pressurization will be sufficiently high to generate effective tensile stresses larger than the effective tensile strength of the COx. To induce thermal hydrofracturing, the COx was submitted to temperatures and pore pressures higher than those expected in the Cigéo project. A breakdown pore pressure, a sign of a fracture event, was registered by the measurements during the second heating cycle. These observations were also confirmed by the post-test core drilling. Furthermore, the numerical modelling of the experiment shows that the thermo-poro-elastic approach reproduces the temperature and pore pressure measurements well in a wide range of locations up to just before the occurrence of the fracture event. This satisfactory reproduction provides confidence for the design of the Cigéo project. Highlights: An in situ experiment was performed in the Meuse/Haute-Marne Underground Research Laboratory to study the thermal hydrofracturing of the Callovo-Oxfordian claystone. The heating strategy consisted of applying a thermal load to reach the tensile strength and to fracture the rock. The measurements, the numerical modelling, and the post-test core drilling indicate a fracture was created after two heating phases. The thermal pressurization induced a hydraulic fracture during the second phase after 28 days of heating. The thermo-poro-elastic approach reproduces well the measurements in a large range of the measurement locations. [ABSTRACT FROM AUTHOR]

In the geological disposal of high-level radioactive waste in argillaceous rocks, studying the barrier integrity after gas transport and the pathway closure thanks to self-sealing capacity is a crucial aspect for the safety assessment. This paper presents experimental research in Boom Clay (a potential host rock in Belgium) to evaluate the effectiveness of self-sealing and possible fissure reactivation during a second gas invasion event. Initial water permeability under oedometer conditions was first measured on samples at two bedding orientations, being higher the sample with bedding planes parallel to flow, highlighting marked anisotropy. Then, gas injection tests at a constant volume rate were performed. Results indicated that Boom Clay underwent expansion and degradation during gas injection due to the development of fissures that were quantified using microstructural techniques. The computed effective gas permeability was not significantly dependent on bedding orientation and was slightly larger than the initial intrinsic water permeability. The re-saturation of the samples led to a recovery of the initial water permeability for both orientations, replicating the original anisotropy. The microstructural analyses confirmed the gas pathways' closure, indicating good self-sealing and the regaining of the hydraulic barrier function. However, a small volume of large unconnected pores was detected on undrained unloading before the microstructural study. An additional gas injection after the self-sealing resulted in a higher effective gas permeability and a larger increase in pore volume, suggesting the reopening of fissures generated during the first injection. Finally, the experimental data were compiled within a multi-scale phenomenological model to relate the microstructural information to macroscopic flow transport properties capturing the intrinsic permeability increase on gas invasion and its recovery during self-sealing. Highlights: Gas transport on Boom Clay occurs along fissures, increasing its permeability The re-saturation process induces the recovery of the initial water permeability, demonstrating an excellent self-sealing capacity. A subsequent gas injection after the re-saturation suggests the reopening of previous gas paths. A multi-scale model is proposed to account for the permeability changes derived from a microstructural damage variable. [ABSTRACT FROM AUTHOR]

Drainage (drying) and imbibition (wetting) cycles are expected in many applications involving subsurface flows, such as underground CO 2 storage, hydrocarbon extraction and production, and deep geological disposal of radioactive waste (particularly in the potential production of hydrogen due to corrosion and radiolysis). In the presence of imbibition/drainage cycles, the two-phase constitutive relations between water saturation, capillary pressure and relative permeability to liquid and gas, cannot be represented by simple one-to-one functions: they depend on the history of wetting/drying processes. Recent numerical and experimental studies highlight the importance of hysteresis and its effect on two-phase flows in porous media. In this study, an extensive critical review of conceptual hysteretic models is first presented. The performance of these models is evaluated and tested from a theoretical point of view based on their assumptions, and by confronting them with measured data for various porous materials extracted from the literature (cement, sands, Bentonite MX80). The advantages and disadvantages of the different hysteretic constitutive models are discussed. Finally, we implement the five hysteresis models in the iTOUGH2 code, and we develop a numerical simulation of a two-phase flow experiment with immiscible fluids at the laboratory scale (meters), to compare the flow results obtained with or without hysteresis, and also, with or without capillary entrapment. The results highlight the need to take into account hysteresis in the modeling of multiphase porous media flows, especially when they are subjected to drying–wetting cycles. It is suggested that the model of Beriozkin and Mualem (Adv Water Resour 115:253–263, 2018) should be preferred both from a theoretical standpoint and for its computational advantages. Highlights: Quantitative assessment of several significant and representative conceptual models for hysteretic two-phase flow in porous media. Identification of best hysteresis model in terms of physical parametrization, accuracy, and ease of implementation. Gas entrapment in hysteresis models is found to be essential for accurate simulations of gas–liquid flow in porous media. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of the swelling behaviour of a compacted bentonite–sand mixture subjected to hydration under constant volume conditions. Contrary to previous studies, the tested sample was isotropically compacted before being hydrated under constant volume conditions until full saturation was reached. The total axial pressure, total radial pressures at four different heights of the sample, and injected water volume were recorded over time. The experimental data reveal a complex and non-uniform evolution of the axial and radial stresses over time, as well as anisotropy of the total stresses, which persist at the saturated equilibrated state. To gain further insights, a numerical analysis was performed using an advanced hydromechanical framework for partially saturated porous media, accounting for the evolving microstructure of the material. The complex evolution of the total axial and radial pressures with time is attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The good agreement between the numerical results and the experimental data enables validation of the developed framework. Implications for engineered barriers in deep geological disposal of radioactive waste are discussed. Highlights: The swelling behaviour of an isotropically compacted bentonite-based material under constant-volume conditions is investigated. Hydration of the sample generates stress heterogeneity and anisotropy, which persist at the saturated equilibrated state. Numerical modelling shows that the complex evolution of the total axial and radial pressures can be attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The relationship between the local dry density and the radial stress seems to follow the global dry density-swelling pressure trend determined on small-scale samples. [ABSTRACT FROM AUTHOR]

The DIGIT experiment was launched at the Tournemire Underground Research Laboratory URL with the aim of determining the effects of temperature on the transfer of analogues of most mobile radionuclides (i.e., 36Cl, 129I and 79Se) in the Toarcian clay rock, the properties of which are close to host rocks being considered for future deep geological disposal of high-level (HL) radioactive wastes. The experimental principle involves the monitoring of an exchange between a test water traced with stable halides and deuterium at constant concentration and the porewater of the Toarcian clay rock submitted to various temperatures. This experiment seeks to partially address questions regarding the potential spread of contaminants during the thermal phase of High Level Waste (HLW) waste packages. Specifically, the in situ experiment aims to evaluate the role of scale effects and thermodiffusion, a process that combines Fick's law and the Soret effect, in the transfer of radionuclides. This paper presents the first steps of the study, including the scoping calculations, the experimental set-up and the first results obtained during the unheated phase. The study started with the acquisition of the initial parameters, including the rock thermal properties, the concentrations of the four tracers (chloride, bromide, iodide and deuterium) naturally present in the clay porewater and their diffusive transport parameters by using four diffusion exchange techniques (phase 0). A model coupling heat and mass transfers was then developed using Comsol Multiphysics®, integrating data acquired so far with existing literature data. A test water with a tracer concentration around 1000 times higher than those in the pore water was proposed with a temperature imposed at the test section wall of 70 °C. A large test zone of 50 cm height and 1 m in diameter and installed in a 3 m deep vertical well located in a sound zone at the URL was then proposed. The installation of the experiments required the realization of one shaft and of nine peripheral boreholes for the monitoring of temperature, water pressure and deformation. The experiment started with phase 1, involving a traced, unheated water start-up for a period of 5 months. Then, a core sampling was conducted in the emptied well, and the same diffusion exchange techniques were applied. The results of anionic tracers were compared to simulations based on initial parameters (phase 0), revealing that tracer penetration at the end of phase 1 exceeded simulated values by approximately 2 cm. This result is interpreted as an increase in the accessible porosity to tracers, possibly due to the excavation damaged zone. Future simulations should incorporate these adjusted diffusive transport parameters. Following phase 1, the heating system was activated, applying a temperature of 70 °C to the test zone. New data will enable the comparison of tracer penetration and assess the actual impact of temperature on tracer transfer. [ABSTRACT FROM AUTHOR]

Palladium-107 is one of the selected radionuclides in the safety assessment of geological disposal of radioactive waste. Although isosaccharinic acid (ISA) forms strong complexes with many elements and enhances element solubility, the thermodynamic evaluation of the complex of Pd with ISA has not been conducted. In this study, the solubility of Pd(OH)2 at pH 8.5–12.5 in the presence of ISA was investigated under inert gas (N2) atmosphere. Furthermore, the coordination state of aqueous Pd and ISA was investigated by X-ray absorption and Fourier transform infrared spectroscopy, respectively. According to experimental results, the number of OH ligands in the mixed complex depends on pH. The thermodynamic model and conditional equilibrium constants of the Pd-ISA complex were estimated by slope analyses of solubility experiments at different pH levels and ionic strengths based on the specific ion interaction theory. Hence, the impact of complexation with ISA on Pd(II) solubility under disposal conditions could be quantified using the proposed thermodynamic models in this study. [ABSTRACT FROM AUTHOR]

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We present an analysis and interpretation of potential cliff stability at a low-level waste disposal facility at Los Alamos National Laboratory, New Mexico, using cliff morphologic and fracture characteristics coupled with carbon-14 surface exposure dating. Our study is important as it directly bears on the licensing criteria for low-level radioactive waste sites. We find that future characteristic cliff failures will likely not breach disposal pits and shafts over the 1000-year minimum regulatory period. Further, we find, using a multivariate regression model, that slope angle and cliff face aspect are sub-equal in importance to predict regions of high risk of failure when combined with surface exposure ages and assuming that old exposure ages are most indicative of stability (instability) and therefore can aid decision making in final design implementation. [ABSTRACT FROM AUTHOR]

The disposal of high-level radioactive waste (HLW) and spent nuclear fuel (SF) presents a unique challenge for the prediction of the long-term performance of corrodible structures since HLW/SF containers are expected, in some cases, to have lifetimes of one million years or longer. Various empirical and deterministic models have been developed over the past 45 years for making predictions of long-term corrosion behaviour, including models for uniform and localised corrosion, environmentally assisted cracking, microbiologically influenced corrosion, and radiation-induced corrosion. More recently, fracture-mechanics-based approaches have been developed to account for joint mechanical–corrosion degradation modes. Regardless of whether empirical or deterministic models are used, it is essential to be able to demonstrate a thorough mechanistic understanding of the corrosion processes involved. In addition to process models focused on specific corrosion mechanisms, there is also a need for performance-assessment models as part of the overall demonstration of the safety of a deep geological repository. Performance-assessment models are discussed in Part 2 of this review. [ABSTRACT FROM AUTHOR]

The disposal of high-level radioactive waste (HLW) and spent nuclear fuel (SF) presents a unique challenge for the prediction of the long-term performance of corrodible structures since the HLW/SF canisters are expected, in some cases, to have lifetimes of one million years or longer. Various empirical and deterministic models have been developed over the past 45 years for making predictions of the long-term corrosion behaviour, including models for uniform and localized corrosion, environmentally assisted cracking and microbiologically influenced corrosion. As well as process models focused on specific corrosion mechanisms (described in Part 1 of this review), there is also a need for performance assessment models as part of the overall analysis of the safety of a deep geological repository (DGR). Performance assessment models are often based on simplified or abstracted process models. The manner in which various international waste management programs have predicted the long-term performance of HLW/SF containers with copper, steel, Ni and Ti alloy corrosion barriers is discussed. Performance assessments are repeated periodically during the development and implementation of a DGR, and the corrosion models are constantly updated in light of new mechanistic understanding and/or more information about the deep geological environment. Two examples of how the container performance assessment models evolve over time are also described. Performance assessment models cannot easily be validated, so it is important to build confidence in the long-term predictions using other methods, including natural analogues and large-scale in situ tests and the use of complementary models. [ABSTRACT FROM AUTHOR]