Repository Environment Research Articles

Insight into the elastoplastic behavior of Beishan granite influenced by temperature and hydraulic pressure

Granite is an ideal host rock for high-level radioactive waste (HLW) repositories. In the deep environment of HLW repositories, granite is subjected to variable temperature and hydraulic pressure. In order to investigate the influence of temperature and hydraulic pressure on the elastoplastic behavior of the Beishan granite, the present paper implemented a set of triaxial compression experiments at various temperatures and hydraulic pressures corresponding to the environment of HLW repositories. The deformation processes, damage evolution, elastic coefficients, and strength of the Beishan granite were analyzed, resulting in a limited change versus temperature and hydraulic pressure. The residual strengths of granite are similar, but a higher temperature surpasses about 70 °C degrades the friction strength of granite. Evidence from the acoustic emission (AE) hits shows the damage rate is accelerated by improved temperature and hydraulic pressure. According to the experiment-based mechanism, a modified elastoplastic model was proposed which considered thermal pressurization and changeable compressibility. The results of computation based on the proposed model match well the experiment data.

Analysis of natural organic matter chemistry in bentonite clay under compaction using different dry densities and duration

The Nuclear Waste Management Organization of Canada has adopted an adaptive phased management plan for the long-term storage of used nuclear fuel in a Deep Geological Repository (DGR). The DGR barrier system employs copper-coated used fuel containers (UFCs) surrounded by buffer material which is composed of highly compacted bentonite clay. The natural organic matter (NOM) composition of the compacted bentonite is of practical importance for the safety assessment as it regulates the biogeochemical processes at the interface between the UFCs and the buffer layer. However, insufficient investigations on NOM constituents limits the understanding of biogeochemical dynamics in bentonites compacted under different dry densities. This study analyzed the carbon content and NOM composition using targeted compound analysis and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with bentonites compacted at 1.1, 1.4 or 1.6 g/cm3 dry density for 1, 3, 6, 12 or 18 months. Organic carbon contents were similar across bentonites with different compaction densities at various durations as compared to the reference clay (powdered bentonite sample before compaction). The overall NOM composition measured by solid-state 13C NMR suggested a predominance of alkyl and aromatic carbon in bentonite samples. No marked variations were observed for the NOM components (i.e., alkyl, O-alkyl and aromatic carbon) between the compacted bentonites and the reference clay. Targeted compound analysis revealed that the extractable lipid concentrations in the compacted clays did not significantly vary from the reference bentonite with only some compounds exhibiting nano-gram level differences. Bentonites with relatively lower dry densities (1.1 and 1.4 g/cm3) demonstrated slightly higher extractable compound concentrations, but these differences were not statistically significant. In contrast, the compound concentrations of bentonites compacted to 1.6 g/cm3 were similar or slightly lower compared with the reference bentonite, likely associated with limited microbial growth under high dry compaction density. Taken together, these results suggested that NOM composition and quantity did not significantly alter in bentonites under various pressures nor with longer experimental durations. These findings highlight that compacted bentonite exhibited geochemical stability under the simulated repository environments, which provides critical insight for design and performance of engineered barrier system in a DGR concept.

The effect of cerium, neodymium, and ytterbium doping on UO2 dissolution

The dissolution rate of spent nuclear fuel under environmental conditions has been studied heavily to understand the impacts of a failed waste package on potential radionuclide release from a geologic repository. Multiple countries have evaluated these scenarios in oxidizing and reducing environments respective to their relevant repository conditions. The repository environment (e.g., water chemistry, temperature, oxygen content) and the fuel itself (i.e., chemical composition of the fuel) both heavily impact the dissolution rate of the spent fuel. This work examined the impact that rare earth element dopants (Ce, Nd, Yb) have on the fuel dissolution under repository relevant conditions with decreasing oxidizing conditions using a single pass flowthrough system. UO2 samples were doped with Ce, Nd, or Yb with concentrations between 1 and 5 at%. The addition of dopants to the samples reduced the dissolution rates on most samples relative to pure UO2 samples. Preliminary experiments that occurred in a less oxidizing environment showed a reduction in dissolution rate compared to fully oxidizing conditions. The results within highlight the importance of dopant behavior in used fuel dissolution modeling.

Fungal diversity in environments of repository of the national archive of the Republic of Cuba from the 80s to 2022

This work aims to perform a compilation of the fungal diversity in the environment of the National Archive of the Republic of Cuba (NARC) repositories from the 80s of the last century and until 2022, which includes the fungal genera and species isolated from: air, settled dust and document surface, as ecological niches that contribute to the environmental mycological quality of these spaces in the institution. A total of 55 references were analyzed (31 of air, 4 of settled dust and 20 of different types of documents surface). A total of 168 fungal genera and 54 species were isolated from air, 36 genera and 12 species from dust as well as 98 genera and 26 species were detected on the document surfaces with a marked predominance of the genera Aspergillus, Cladosporium and Penicillium in the three niches, although 24 rare genera were detected in air after the year 2000 (e.g. Acremonium, Aureobasidium, Beltraniella, Bipolaris, Blastomyces, Botryoderma, Botryotrichum, Chaetomium, Chrysonilia, Exophiala, Gilmaniella, Harposporium, Hyalodendriella, Itersonilia, Nodulisporium, Ovulariopsis, Papularia, Scolecobasidium, Sepedonium, Sprorobolomyces, Torula, Trichophyton, Wardomyces, Zygosporium). Only six genera were similarly isolated from air, dust and the document surfaces (9.7%), while 19 species were found in these three niches (9.5%) (e.g. A. flavus, A. niger, A. versicolor, Cl. caryigenum, Cl. cladosporioides, Cl. herbarum, P. chrysogenum, P. citrinum, P. commune, P. janczewskii). Although fungal isolations from dust and the document surfaces are still scarce, it is evident that the greatest diversity of species was isolated from the air and that the similarities of species between these three niches are low, therefore air is the matrix that contributes with the most fungal species to the NARC environment. This compilation of the fungal diversity in the environment of the NARC repositories will serve as a reference for future studies in Cuban archives and other countries.

Investigation of leaching behaviors of carbonation-cured CSA cements using an electrically accelerated leaching test

The present study provides the investigation of leaching behaviors of carbonation-cured calcium sulfoaluminate (CSA) cements with varying initial sulfate contents using an electrically accelerated leaching test. The CSA cement samples were normally or carbonation-cured for 28 days before subjecting them to an electrically accelerated leaching condition for 14 days. The samples were characterized experimentally before and after accelerated leaching. In addition, the leaching behaviors of the CSA cement samples over a wide range of cumulative leached water were thermodynamically modeled to determine their likely performance in repository environments. The results indicate that the reaction of ye'elimite was significantly promoted by accelerated leaching, resulting in the formation of ettringite after leaching, regardless of the curing regime used. Calcium sulfates and C-(A)-S-H formed in the carbonation-cured samples were the first phases to decompose upon accelerated leaching, which is in good agreement with the thermodynamic modeling calculations. For the normally cured samples, the use of higher calcium sulfate in the mixture is preferred, considering the amount of leached ions, yet the opposite holds true for the carbonation-cured samples. In particular, the changes in the calculated volumes of the reaction products upon simulated leaching were smaller for the carbonation-cured samples than for the normally cured samples.

Suppressed grain growth and enhanced irradiation resistance of nano-grained waste form under electronic energy loss

The effects of both nuclear energy loss and electronic energy loss need to be taken into consideration in the ceramic-based waste forms under repository environment. However, the irradiation responses of ceramic-based waste forms to each type of energy loss are somewhat different. In this study, the microstructure evolutions of ultrafine nano and micro Gd2Zr2O7-based waste forms were systematically studied under predominant electronic energy loss simulated by multi-energy He+ irradiation, and compared to those under predominant nuclear energy loss. The results reveal that the fewer He bubble chains, ribbon-like He bubbles and smaller microcracks were observed in the irradiated nano-grained sample. Additionally, nano-grained sample displayed a lower degree of amorphization and higher atomic order compared to micro-grained samples when subjected to predominant electronic energy loss. Moreover, the irradiation dominated by nuclear energy loss can easily induce the grain growth of nano-grained Gd2Zr2O7-based waste form, but in the present study this phenomenon was not observed under multi-energy He+ irradiation. Consequently, under predominant electronic energy loss, the thermodynamic instability and driving force for grain growth due to excess surface energy in the ultrafine nano sample can be suppressed. As a result, the sample demonstrated enhanced irradiation resistance due to the more efficient absorption and elimination of defects at grain boundaries induced by electronic excitation. We elucidated that enhanced irradiation resistance of the waste forms by tailoring the grain size requires the consideration of the effects of electronic energy loss and nuclear energy loss, which can provide guidance for the design and optimization of highly irradiation-resistant nuclear waste forms.

Assessment of canister degradation for the encapsulation of spent nuclear fuel: Key research issues encountered in recent regulatory reviews and government decision making in Sweden

Abstract. Regulatory review of a licence application for construction and operation of a geological repository for radioactive waste should not only be based on information provided by the applicant but also on viewpoints brought up by stakeholders and decision makers. In the Swedish Radiation Safety Authority's (SSM) licensing review of a spent fuel repository at the Forsmark site, the main concerns brought up in court hearings, from national consultations and by the Swedish government were focused around the long-term protective capacity of the copper canisters with cast iron inserts. The most debated canister degradation mechanisms were anoxic corrosion of copper (in oxygen gas free water), localised sulphide corrosion (pitting corrosion and stress corrosion cracking), the influence of gamma radiation, and strain hardening of the canister cast iron insert. The main question addressed by SSM is not whether such effects can be ruled out altogether, but rather their potential extent and influence on canister integrity during a sufficiently long period in the repository environment. Constraints are mainly based on rock properties and associated hydrological and geochemical factors known from site investigation and knowledge about the protective capability of the buffer barrier. The key factors are in general well characterised and cover for instance sulphide availability, groundwater flow, mass transfer rates and expected mechanical loading on canisters in the repository environment. The canister corrosion/degradation mechanisms reviewed in this paper are judged by SSM to have a small or limited importance for demonstration of regulatory compliance, taking into account the characteristics of the site, the function of the buffer, and the margins provided by the significant thickness and mechanical strength of the proposed canister.

Bentonite alteration and retention of cesium and iodide ions by Ca-bentonite in alkaline and saline solutions

The nuclide retention capacity of bentonite can be affected by changes in the repository environments, such as alkaline and saline conditions. This study investigated the effects of bentonite alteration on the nuclide retention capacity of Ca-bentonite. A series of batch experiments was conducted at room temperature (25°C) for 7 d using Ca-bentonite and alkaline and saline solutions spiked with 10 mM each of cesium (Cs) and iodide (I). Cation exchange and a decrease in the basal spacing (d001) of montmorillonite were observed, as was the formation of zeolite minerals in bentonite samples reacted with an alkaline solution (0.1 M KOH and 0.1 M NaOH). Illitization, partial dissolution, and aluminum ions substitution for silicon ions in the tetrahedral sheets of montmorillonite were also observed in the bentonite samples reacted with an alkaline solution (0.1 M KOH). However, the Cs retention capacity of bentonite was not significantly affected by the mineral and physicochemical changes of montmorillonite in alkaline and saline solutions. Competing ions, including Mg, Ca, K, and Na ions in solutions with high ionic strength, all played a crucial role in Cs retention. Further, the cation exchange capacity of bentonite samples increased due to the formation of zeolite minerals under alkaline conditions.

Remediation by Degrees

In 2022, staff at J. Murrey Atkins Library initiated a metadata remediation project project that inserted FAST controlled vocabulary terms for subjects into legacy ETD records in its Niner Commons repository through an automated process and normalized data in other elements, all with a view to improving discoverability and the user experience. This case study describes the stages of the remediation process, the difficulties that were encountered in the course of the work, the results and findings of the project, and the challenges of ETD metadata remediation work in the contemporary institutional repository environment.

Bentonite colloids mediated Eu(III) migration in homogeneous and heterogeneous media of Beishan granite and fracture-filling materials

Accurate prediction of radionuclide migration directed by colloids in a repository environment is critical for the long-term safety assessment of radioactive waste disposal. Yet, there remains a paucity of research focusing on the migration and release processes of radionuclides combined with colloids associated with the repository, e.g., granite, and especially fracture-filling materials (FFMs). Further, the impact of heterogeneity on radionuclide migration in these media remains unclear. Thus, laboratory-scale column experiments were conducted to explore the migration behaviors of Eu(III) and bentonite colloids (BCs) in both homogeneously and heterogeneously configured columns. It was observed that the migration of BCs can be influenced by the configuration of media and ionic strength of solution. FFMs exhibited a higher retardation capacity for BCs than granite, and the retention ability of the heterogenous media (Granite-FFMs) was intermediate between that of granite and FFMs. The retardation of BCs increased with increasing ionic strength. Despite the “irreversibility” of Eu(III) adsorption on these media with insensitivity on ionic strength, the presence of BCs can grab the immobilized Eu(III) and carry more amount of adsorbed Eu(III) into the mobile phase at higher ionic strength. Even with the rinse of BCs flow, FFMs still revealed a better retardation ability for Eu(III) than granite. These findings are essential for predicting the geological fate and the release risk of radionuclides in the Beishan repository environment.

First approach to microplastics occurrence in sandy beaches in the province of Islay (Arequipa), southern Peru

Plastic pollution in marine and coastal environments has been widely recognized as a growing environmental concern. Plastic particles generated mainly by the degradation of larger plastic debris have been called microplastics (< 5 mm), which are being widely studied in all regions of the world. In South America there has been a constant increase of the research on this topic mainly on the Pacific coast. Specifically, in Peru, the number of publications has also increased, but the northern and southern zones are not well studied. Due to the importance of determining the presence of these contaminants, especially in coastal regions, which are considered as repository environments for these plastics, it is necessary to establish baselines of their current situation. This research aimed to determine and characterize the presence of microplastic particles (< 5 mm) on five sandy beaches in the province of Islay in southern Peru and to consider the possible effect of the Tambo River mouth on the transport and deposition of microplastics on two adjacent beaches. Three sampling stations were determined for each selected beach, consisting of three to two quadrants (1 m2) sampled at 5 cm depth. The results confirm the presence of microplastics. A total of 304 particles were found, with an average density between 1 to 4 part/m2. The areas with the highest concentration of microplastic particles and frequency of occurrence were the beaches adjacent to the river estuary where the most frequent types of particles were fragments and fibers, followed by foams and films, but no pellets.

The anoxic corrosion of mild steel in a cementitious repository environment

AbstractThe long‐term disposal of low‐ and intermediate‐level radioactive waste is anticipated to involve the encapsulation of the material within a cementitious matrix before placement in a deep geological repository. After sealing, the waste repository will become anoxic and the corrosion of emplaced metals will result in the production of hydrogen gas. Knowledge of the rate at which this hydrogen is generated is valuable in that it permits the assessment of pressure build‐up within the repository. The corrosion behavior of mild steel has been monitored through the generation of hydrogen gas, under simulated repository conditions that mimic various stages of cement evolution. It was found that, when cement is used to encapsulate steel, the passage of several years is required for the steel and cement to reach a steady hydrogen production rate. Steel that is directly immersed in simulated cement pore water requires only months to stabilize. All measured steel uniform corrosion rates were significantly below 1 nm year−1 at 50°C, regardless of the test environment, after more than 4 years of exposure.

Thermal Assessment of Repository Footprint Requirement for Advanced Fuel Cycles

Decay heat is an important constraint for repository size and design because it can drive processes that affect performance and compromise critical materials. This paper investigates how compliance with repository thermal limits is affected by three decay heat management strategies: waste package loading, waste package spacing, and surface storage time. In particular, this paper focuses on how repository area, a result of package spacing, is impacted by waste loading and surface storage time. A two-part analytical heat transfer model is presented and executed iteratively to determine the minimum allowable repository area. The analysis considers three generic close-contact repository designs along with the wastes generated from the 40 fuel cycle analysis examples used to generate metric data for the Fuel Cycle Evaluation and Screening study. Detailed results are presented for two fuel cycles: the once-through use of low-enriched uranium fuel in light water reactors and the continuous recycling of U and Pu in sodium fast reactors. Two limits for surface storage time are identified: the time required for disposal to be possible at all and the time at which further surface storage time yields no gains. The impact of waste loading is also diminished with increasing surface storage time. In general, the generic salt repository is most flexible to accept high-heat-generating wastes with less surface storage time than other repository environments. Limited-recycle fuel cycles are shown to pose a disposal challenge because of elevated, sustained decay heat generation in the waste.

The Use of Microfluidic Platforms with Raman Spectroscopy for Investigating the Co-Precipitation of Metals and Radionuclides in Carbonates

In the context of long-term safety assessments of deep geological repositories for radio-active wastes, a rigorous understanding of the retention of radionuclides such as 226Ra due to co-precipitation with carbonate and sulphate minerals is important for a realistic prediction of radionuclide migration behaviour in the repository near and far field. The co-precipitation of 226Ra in sulphate minerals, in particular barite, has been studied experimentally and numerically in detail throughout the last decade to establish the thermodynamic properties and mixing behaviour of its solid solutions over a wide range of temperatures. However, so far, few studies have been dedicated to the incorporation of 226Ra into carbonates, and little is known about the mixing behaviour of 226Ra and calcium carbonate phases such as calcite, aragonite, or vaterite. The aim of the work presented here was to develop and explore innovative microfluidic experiments in combination with in situ Raman spectroscopy that can be used to investigate co-precipitation processes of radionuclides in carbonate minerals, using stable Ba as a chemical analogue for 226Ra in the first step, due to their similar ionic radii. Different microfluidic set-ups were developed to address co-precipitation in bulk solution as well as in confinement or under diffusive flow regimes. It could be shown by XRD and SEM-EDX analyses that high amounts of Ba can be incorporated into the lattice of calcite when formed via an amorphous precursor phase, suggesting that the formation of calcium carbonates can contribute to the retention of 226Ra in the repository environment, which will be verified in future studies using the presented microfluidic platforms.

The corrosion behavior of nonferrous metals in deep geological repository environments

AbstractThe advanced technologies of modern civilization produce radioactive wastes that require careful disposal if they cannot be recycled. These materials can originate from a variety of activities, such as scientific research, medicine, or nuclear power generation and, as such, can result in numerous waste forms. In this paper, the corrosion behavior of several less‐common metals is studied, specifically: aluminum, copper, lead, magnesium, zinc, and zirconium, all under simulated cementitious environments. The data reported rely on the production of hydrogen as a corrosion end‐product to calculate the uniform corrosion rate as a function of time. At 50°C, in either young cement water (pH 13.5) or saturated portlandite (pH 12.5) and after approximately 2 years of exposure, magnesium was found to corrode at ∼10 µm/year; aluminum at 1 µm/year (portlandite only); zinc at ∼100 nm/year; lead at <1 nm/year and both copper and zirconium at less than 0.1 nm/year.

Immobilization of radioactive waste material in concrete matrix–A study of leachability aspects

The biomass, Ganoderma Lucidum, is a naturally growing macro-fungus that grows in rubber plantations, and has been found to remove uranium from low level radioactive wastewaters. Subsequent to the sorption of uranium, the loaded biomass needs to be safely and scientifically disposed. This is done by first immobilizing the waste biomass material in a solid matrix such as concrete or resin, and then disposing it in deep sea or geological repositories after due enclosures for safety. Despite immobilization there remains a risk of leaching of radioactivity from the loaded biomass material. This requires to be verified by testing leachability in lab under varying pH conditions that mimic sea water and geological repository environments. Short terms desorption and leachability experiments were conducted at the materials lab at MANIT, Bhopal to determine the leachability of radioactivity from the concrete matrix enclosing the loaded biomass. It was found that leaching was rapid in acidic conditions (0.2 N HCl), with 27.2 % uranium leaching out within 8 days. This indicated the suitability of alkaline sea-waters for safer disposal of uranium laden immobilized biomass. Mathematical models were subsequently developed based on the laboratory data to predict the long-term leachability for assessing the safety of disposal mechanism, and indicated the prominence of diffusion as the leachability mechanism. This paper details the experiments conducted, the materials used, the lab observation results, and the mathematical models developed to determine safety aspects related to possible leaching from immobilized concrete matrix in which radioactive biomass has been loaded.

Understanding smectite to illite transformation at elevated (>100 °C) temperature: Effects of liquid/solid ratio, interlayer cation, solution chemistry and reaction time

Bentonite (mainly smectite) has been proposed as a buffer material in a nuclear waste repository, due to its low permeability, high swelling capability, and high ion exchange capacity. To evaluate the isolation performance of the material, it is important to understand a possible hydrothermal alteration of smectite to illite. We investigated the hydrothermal alteration of smectite under various physical and chemical conditions to identify key controlling factors of the process. The experiments were performed on <2 μm fractions of Na-rich or K-exchanged smectite in DI water or 1 M KCl solution at 200 °C for various liquid/solid ratios over several time periods up to 112 days. Multiple analytical techniques were used to characterize the solid and liquid products to determine the extent of the conversion and the related mineral structural changes. Our results suggest that the conversion could be relatively fast (i.e., weeks) relative to waste disposal isolation at the optimal conditions (high K+ concentration and high liquid/solid ratio), which is much faster than previously postulated for the temperature used here. A high K+ concentration in the solution is required to trigger the structural transition of a low charge smectite layer to a high charge illite layer. This transition appears to be continuous, thus contradicting the dissolution-precipitation mechanism for smectite to illite transformation. Based on the solution chemistry measurements, with respect to silica solubility, the equilibrium constant for the transformation of K-exchanged SWy-2 smectite to illite is estimated. In addition, the overall conversion is limited, to a large extent, by the dissolved silica concentration in the solution. Given a generally low potassium content, a very low liquid/solid ratio, and an extremely slow diffusion in an actual engineered barrier system, the extent of smectite conversion to illite in a repository environment, if happening at all, would be limited. The conversion can be further inhibited by introducing brucite and amorphous silica as chemical additives to the buffer material.

PH and microbial community determine the denitrifying activity in the presence of nitrate-containing radioactive waste.

An important fraction of the currently stored volume of long-lived intermediate-level radioactive waste in Belgium contains large amounts of NaNO3 homogeneously dispersed in a hard bituminous matrix. Geological disposal of this waste form in a water-saturated sedimentary formation such as Boom Clay will result in the leaching of high concentrations of NaNO3, which could cause a geochemical perturbation of the surrounding clay, possibly affecting some of the favorable characteristics of the host formation. In addition, hyper-alkaline conditions are expected for thousands of years, imposed by the cementitious materials used as backfill material. Microbial nitrate reduction is a well-known process and can result in the accumulation of nitrite or nitrogenous gases. This could lead to the oxidation of redox-active Boom Clay components, which could (locally) decrease the reducing capacity of the clay formation. Here, we compared nitrate reduction processes between two microbial communities at different pH related to a geological repository environment and in the presence of a nitrate-containing waste simulate during 1 year in batch experiments. We showed that the microbial community from in Boom Clay borehole water was able to carry out nitrate reduction in the presence of acetate at pH 10.5, although the maximum rate of 1.3 ± 0.2 mM NO3 -/day was much lower compared to that observed at pH 9 (2.9 mM NO3 -/day). However, microbial activity at pH 10.5 was likely limited by a phosphate shortage. This study further confirmed that the Harpur Hill sediment harbors a microbial community adapted to high pH conditions. It reduced twice as much nitrate at pH 10.5 compared to pH 9 and the maximum nitrate reduction rate was higher at pH 10.5 compared to that at pH 9, i.e., 3.4 ± 0.8 mM NO3 -/day versus 2.2 ± 0.4 mM NO3 -/day. Both communities were able to form biofilms on non-radioactive Eurobitum. However, for both microbial communities, pH 12.5 seems to be a limiting condition for microbial activity as no nitrate reduction nor biofilm was observed. Nevertheless, pH alone is not sufficient to eliminate microbial presence, but it can induce a significant shift in the microbial community composition and reduce its nitrate reducing activity. Furthermore, at the interface between the cementitious disposal gallery and the clay host rock, the pH will not be sufficiently high to inhibit microbial nitrate reduction.

The Transition from Used Fuel Container Corrosion Under Oxic Conditions to Corrosion in an Anoxic Environment

The internationally accepted plan for nuclear fuel waste disposal is to store used nuclear fuels within a multi barrier system in a deep geological repository (DGR). Similarly, the DGR concept proposed in Canada involves isolating the used nuclear fuel in copper-coated carbon steel vessels surrounded by bentonite clay buffer boxes and gapfill material. The repository environment is anticipated to evolve from early oxic conditions to later anoxic conditions. Likewise, degradation processes of the UFCs should transition from the initial oxygen-driven processes to those governed by the availability of sulfide as oxidant. This transition may involve the conversion of accumulated (hydr)oxide-type corrosion products produced during the oxic phase to copper sulfide compounds. During this conversion, sulfide species could interact with the UFC surface in several ways depending on the type of (hydr)oxide corrosion products present, the nature and concentration of sulfide species, and other factors: Chemical conversion: in this process substitution reactions replace oxy-anions in the corrosion product layers with sulfide anions. This conversion doesn’t involve redox reactions; therefore, until the conversion is complete, the sulfide species would not drive further corrosion of the Cu surface. Over time, the (hydr)oxide corrosion product layer would be replaced by a sulfide layer.Galvanically-coupled process: in this process sulfides react with Cu metal exposed at the base of pores or cracks in the accumulated corrosion product layer, causing the Cu in those locations to oxidize and copper sulfide to form, while simultaneously expelling oxy-anions and causing cupric or cuprous ions to be reduced elsewhere in the corrosion product layer. Although the conservation of charge in these reactions would mean that no net oxidation of the surface would occur, the spatial separation of the sites where copper is oxidized (on the metal surface) and the locations where reduction occurs (in/on the corrosion product layer) would result in further damage to the Cu layer on the UFC surface, possibly localized at the sites where Cu metal is exposed. Eventually the (hydr)oxide corrosion product layer would be replaced by a sulfide layer and any localization should cease.Direct corrosion: in this process, sulfide species react with the Cu exposed at the base of pores or cracks in the accumulated corrosion product layer, causing further corrosion at these locations without the existing (hydr)oxide species being converted (i.e. process a) or galvanically reduced (i.e. process b). These sulfide species would drive further corrosion of the UFC surface, possibly localized at the sites where Cu metal is exposed. Since this process would not contribute to the removal of the (hydr)oxide corrosion product layer, it is possible that localization, if present, could continue. The research work reported here uses electrochemical and surface characterization methods to evaluate the extent to which each of these possible conversion processes take place, and the timescale/rate of conversion of (hydr)oxide layers grown in different ways. Results so far indicate a fairly rapid and quantitative conversion of oxides according to processes a) and/or b), followed by direct corrosion of the Cu surface by sulfide.

Investigation of soil-water characteristic curves for compacted bentonite considering dry density

Deep geological repositories are often considered for the disposal of high-level radioactive wastes (HLW). HLW should be disposed of safely and permanently using two barrier systems: engineered barrier systems (EBS) and natural barrier systems (NBS). An EBS is an artificial concept comprising various components. A buffer is one of the most important components because it plays a crucial role in the safety of the entire disposal system, and compacted bentonite is considered a highly adequate buffer candidate material. The compacted bentonite is located between a canister with HLW packages and rock-mass. In compacted bentonite, the dry density varies in actual repository environments, and it may be necessary to consider this when evaluating the disposal system safety. As groundwater flows into the rock mass and buffer, the degree of saturation of compacted bentonite increases, with the saturation behavior represented by a soil-water characteristic curve (SWCC). In this study, SWCC tests of Korean compacted bentonites were conducted based on the chilled-mirror dew point temperature condensation (CMDPTC) method, incorporating the dry density variation. The van Genuchten fitting parameters (α, n) were obtained; α decreased exponentially, and n increased linearly according to the dry density. Furthermore, using the obtained SWCC datasets, a simple empirical model was proposed to estimate the van Genuchten fitting parameters.