Copper Canisters Research Articles

Oxic corrosion model for KAERI Reference disposal system via O2 consumption reactions and mixed-potential theory

The durability of copper (Cu) canisters against corrosion is critical for the licensing of deep geological repositories. Assessing oxic corrosion, a primary degradation mechanism, is essential for ensuring the reliability of such repositories. Due to the complex interactions influencing oxic corrosion, a comprehensive model is necessary for evaluating Cu canister corrosion. This study develops a model for the KAERI Reference Disposal System (KRS), incorporating mixed-potential theory with key O2 consumption reactions, including Cu corrosion, Cu(I) oxidation, FeS2 oxidation, aerobic microbial activity, and O2 dissolution and consumption. Simulation of 11 scenarios revealed that the representative KRS case would experience a maximum corrosion depth of 9.3 μm on the Cu canister after 2.3 years due to oxic corrosion, under conditions that are unfavorable for the initiation of pitting corrosion. These results suggest that oxic corrosion is not a threat to the isolation of spent nuclear fuels in KRS.

Corrosive behavior of copper canisters under air and aerobic groundwater at early stages of deep geological disposal

Copper oxidation at low temperatures below 140 °C and its effects on corrosive behavior in aerobic groundwater are investigated to estimate the intactness of canisters at early stages of disposal. The Cu coupon surface is covered by fine particles that form thin oxide layers after 30 d of oxidation; a thin Cu2O layer of thickness <100 nm is formed after oxidation at 40 °C; after oxidation at 140 °C, the Cu2O surface changes to a CuO layer of thickness <500 nm. The thickness of the Cu surface oxidized at 90 °C is between those of the surfaces oxidized at 40 and 140 °C. All Cu coupons exhibit similar current densities ranging from 0.77 to 1.87 μA cm−2, although the corrosion potential of the Cu coupon layered with Cu2O is higher than that of the others. Long-term oxidation tests for 406 d reveal no significant changes in the Cu surface at temperatures below 90 °C, indicating no significant change in the electrochemical behavior. The results of this study suggest that the storage of canisters at temperatures below 90 °C has no significant effect on the degradation of canister performance in long-term disposal.

Microbial influence in Spanish bentonite slurry microcosms: Unveiling a-year long geochemical evolution and early-stage copper corrosion related to nuclear waste repositories

The deep geological repository (DGR) concept consists of storing radioactive waste in metal canisters, surrounded by compacted bentonite, and placed deeply into a geological formation. Here, bentonite slurry microcosms with copper canisters, inoculated with bacterial consortium and amended with acetate, lactate and sulfate were set up to investigate their geochemical evolution over a year under anoxic conditions. The impact of microbial communities on the corrosion of the copper canisters in an early-stage (45 days) was also assessed. The amended bacterial consortium and electron donors/acceptor accelerated the microbial activity, while the heat-shocked process had a retarding effect. The microbial communities partially oxidize lactate to acetate, which is subsequently consumed when the lactate is depleted. Early-stage microbial communities showed that the bacterial consortium reduced microbial diversity with Pseudomonas and Stenotrophomonas dominating the community. However, sulfate-reducing bacteria such as Desulfocurvibacter, Anaerosolibacter, and Desulfosporosinus were enriched coupling oxidation of lactate/acetate with reduction of sulfates. The generated biogenic sulfides, which could mediate the conversion of copper oxides (possibly formed by trapped oxygen molecules on the bentonite or driven by the reduction of H2O) to copper sulfide (Cu2S), were identified by X-ray photoelectron spectroscopy (XPS). Overall, these findings shed light on the ideal geochemical conditions that would affect the stability of DGR barriers, emphasizing the impact of the SRB on the corrosion of the metal canisters, the gas generation, and the interaction with components of the bentonite.

Microbial responses to elevated temperature: Evaluating bentonite mineralogy and copper canister corrosion within the long-term stability of deep geological repositories of nuclear waste

Deep Geological Repositories (DGRs) consist of radioactive waste contained in corrosion-resistant canisters, surrounded by compacted bentonite clay, and buried few hundred meters in a stable geological formation. The effects of bentonite microbial communities on the long-term stability of the repository should be assessed. This study explores the impact of harsh conditions (60 °C, highly-compacted bentonite, low water activity), and acetate:lactate:sulfate addition, on the evolution of microbial communities, and their effect on the bentonite mineralogy, and corrosion of copper material under anoxic conditions. No bentonite illitization was observed in the treatments, confirming its mineralogical stability as an effective barrier for future DGR. Anoxic incubation at 60 °C reduced the microbial diversity, with Pseudomonas as the dominant genus. Culture-dependent methods showed survival and viability at 60 °C of moderate-thermophilic aerobic bacterial isolates (e.g., Aeribacillus). Despite the low presence of sulfate-reducing bacteria in the bentonite blocks, we proved their survival at 30 °C but not at 60 °C. Copper disk's surface remained visually unaltered. However, in the acetate:lactate:sulfate-treated samples, sulfide/sulfate signals were detected, along with microbial-related compounds. These findings offer new insights into the impact of high temperatures (60 °C) on the biogeochemical processes at the compacted bentonite/Cu canister interface post-repository closure.

Investigation of Corrosion Behavior of Oxygen-Free Copper Canisters in Groundwater Chemistry of Deep Geological Repositories.

The disposal of nuclear waste represents a paramount concern for human safety, and the corrosion resistance of containers within the disposal environment stands as a critical factor in ensuring the integrity of such waste containment systems. In this report, the corrosion behavior of copper canisters was monitored in Olkiluoto-simulated/-procured groundwater (South Korea) with different temperatures. The exposure of copper in the procured groundwater at 70 °C revealed a 3.7-fold increase in corrosion vulnerability compared with room temperature conditions, with a current density of 12.7 μA/cm2. During a three-week immersion test in a controlled 70 °C chamber, the canister in the Korean groundwater maintained a constant weight. In contrast, its counterpart in the simulated groundwater revealed continuous weight loss, indicating heightened corrosion. An X-ray diffraction (XRD) analysis identified corrosion byproducts, specifically Cu2Cl3(OH) and calcite (CaCO3), in the simulated groundwater, confirming its corrosive nature. The initial impedance analysis revealed distinct differences: Korean groundwater exhibits high pore resistance and diffusion effects, while the simulated groundwater shows low pore resistance. Consequently, the corrosion of copper canisters in the Korean environment is deemed relatively stable because of significant differences in ion concentrations.

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.

Impact of compacted bentonite microbial community on the clay mineralogy and copper canister corrosion: a multidisciplinary approach in view of a safe Deep Geological Repository of nuclear wastes

Deep Geological Repository (DGR) is the preferred option for the final disposal of high-level radioactive waste. Microorganisms could affect the safety of the DGR by altering the mineralogical properties of the compacted bentonite or inducing the corrosion of the metal canisters. In this work, the impact of physicochemical parameters (bentonite dry density, heat shock, electron donors/acceptors) on the microbial activity, stability of compacted bentonite and corrosion of copper (Cu) discs was investigated after one-year anoxic incubation at 30 ºC. No-illitization in the bentonite was detected confirming its structural stability over 1 year under the experimental conditions. The microbial diversity analysis based on 16 S rRNA gene Next Generation Sequencing showed slight changes between the treatments with an increase of aerobic bacteria belonging to Micrococcaceae and Nocardioides in heat-shock tyndallized bentonites. The survival of sulfate-reducing bacteria (the main source of Cu anoxic corrosion) was demonstrated by the most probable number method. The detection of CuxS precipitates on the surface of Cu metal in the bentonite/Cu metal samples amended with acetate/lactate and sulfate, indicated an early stage of Cu corrosion. Overall, the outputs of this study help to better understand the predominant biogeochemical processes at the bentonite/Cu canister interface upon DGR closure.

Reactive‐transport model for the production, transport, and consumption of sulfide in a spent nuclear fuel deep geological repository in crystalline rock

AbstractA 1‐D reactive‐transport model has been developed to describe the production and transport of sulfide in a deep geological repository in crystalline rock and the subsequent corrosion of the copper canister. The model accounts for various processes, including: (i) the microbial reduction of sulfate by organotrophic and lithotrophic sulfate‐reducing bacteria, (ii) the supply of sulfate from both the ground water and from the dissolution of gypsum present as an accessory mineral in the bentonite buffer, (iii) diffusive transport of reactants and products, and (iv) sequestration of a fraction of the microbially produced sulfide by precipitation as mackinawite. The results of a base case simulation and of sensitivity analyses indicate that the extent of uniform corrosion is approximately 0.2 mm after 1 million years and that the maximum flux of sulfide to the canister surface is below the threshold for localized corrosion or stress corrosion cracking.

Development of a 2D model for rapid estimation of sulfide corrosion of copper canisters in a spent nuclear fuel repository

AbstractReactive‐transport models (RTMs) have been an integral part of the safety case and design optimization for the nuclear waste repository in Finland. Such highly resolved, internally complex RTMs tend to be computationally demanding, thus their design is often a compromise between representativeness and computational efficiency. A modeling strategy is presented, demonstrating how a 2D simplified model can be constructed based upon and calibrated against a geometrically realistic 3D RTM. Key geometrical parameters of the 2D model are consistent with the 3D model, and the 2D model is verified by a series of benchmarks for various designs and conditions of the repository. Overall, the 2D model is proven suitable for fast scoping calculations, as (1) it reduces the mesh elements by 16 times and shortens the run time by about 40 times; (2) it has much more flexible remeshing possibilities; and (3) it provides highly consistent results with the 3D model.

Coupled mixed-potential and thermal-hydraulics model for long-term corrosion of copper canisters in deep geological repository

Canister performance is a major issue for constructing reliable deep geological repositories for storing spent nuclear fuels, and corrosion resistance is main the factor determining canister durability. We developed a 2-D multiphysics model, which can predict both the corrosion rate and potential, for simulating copper canister corrosion in deep geological repositories. We found that canisters corroded through the long-cell action in a deep geological repository, the corrosion was quite limited, and the overall canister durability was around 1.7 million years. Our results demonstrated that copper exhibits sufficient corrosion resistance and that long-cell action-based corrosion cannot severely damage copper canisters.

Radionuclide Transport in Channel Networks with Radial Diffusion in the Porous Rock Matrix

Water flows in only a small fraction of the total area of the fractures in fractured rocks. The width of the “channels” is often in the range of centimeters to tens of centimeters. Nuclides can diffuse into and out of the porous rock matrix, which causes them to be significantly retarded compared to the water velocity. In discrete facture networks, diffusion is modeled to be linear and perpendicular to the fracture surface. From a narrow channel, the diffusion cloud would then be as wide as the channel. When the nuclide has propagated farther than the channel width, the diffusion will become essentially radial, which allows the nuclide flux to increase enormously. For the times of interest for a repository for high-level nuclide waste, this will increase nuclide flux into the matrix by tens to thousands of times, and consequently, the nuclide retardation in the flowing water. Radial diffusion was not invoked in the performance assessment of the Forsmark site, which in January 2022 was chosen by the government to locate Sweden’s high-level waste repository. It is shown, using data from this site, that the effect of radial diffusion from the narrow channels considerably increases the retardation of any escaping radionuclides, potentially allowing for the use of thinner copper canisters.

Hydrogen-induced micro-void formation in copper used for spent nuclear fuel disposal canisters

Spent nuclear fuel is planned to be deposited in the 50 mm thick copper canisters at about 500 m depth in the bedrock. Hydrogen uptake may occur in the disposal conditions in copper due to corrosion reactions and effects of irradiation. Therefore, it is important to know in which conditions the hydrogen uptake results in the formation of pressurized hydrogen bubbles near the copper surface. During the thermal desorption spectroscopy (TDS) measurements the existing hydrogen bubbles grow due to accumulation of dissolved hydrogen and build-up of hydrogen gas pressure inside the voids. The further growth of the voids occurs by plastic deformation and results opening of the voids near the copper surface due to increasing temperature. The friction-stir weld metal samples of phosphorous-doped oxygen-free copper were cathodically charged with hydrogen in a 1.0 N H2SO4 solution with added 10 mg/L thiourea at various electrochemical potentials, -0.95…-1.3 VSCE. A threshold potential of -1.10 VSCE was found for the marked increase of hydrogen uptake and hydrogen-induced void formation near the copper surface.

Laser-Based Characterisation of the Copper Uranyl Sulphate, Johannite

Uranyl sulphate minerals are common alteration phases in uranium mines and uraniferous waste deposits where they occur in conjunction with other products of acidic drainage such as jarosite. Although not persistent in nature due to their high solubility, they may play an important role in governing uranium mobility during the operational and immediate post-closure environment of an engineered radioactive waste repository where oxidising conditions prevail. One such mineral, johannite (Cu(UO2)2(SO4)2(OH)2·8H2O), is of particular interest given the stated intention of several countries to use copper canisters in the disposal of spent nuclear fuel. A museum reference sample of johannite has been characterised by luminescence and multiple-laser Raman spectroscopy, resulting in the first reported luminescence excitation and emission spectra for this mineral. Well-defined Raman features were observed using 785, 633, and 532 nm lasers with the resolved peaks corresponding well to the published spectra. The Raman spectrum measured with the 457 nm laser was mostly masked by a series of repeating doublets attributed to the luminescence emission features, from which band spacing values of 831 and 823 cm−1 were extracted; the former corresponded to both the resolved 785 nm ν1(UO2)2+ peak position and the band spacing value obtained from the first reported luminescence emission spectrum for johannite. Four emission and nine excitation peaks were resolved from the luminescence spectra. The findings indicate that a suite of complementary laser-based techniques offer the potential for real-time characterisation of johannite formed in environments where intrusive sampling, transportation, and ‘off-site’ laboratory analysis are not feasible.

3D modelling of long-term sulfide corrosion of copper canisters in a spent nuclear fuel repository

Copper canisters are a central component in the safety of the Finnish spent fuel repository concept (KBS-3), where the main corrodent potentially affecting the canister integrity is sulfide. In this study, a 3D numerical model is developed to assess the evolution of sulfide fluxes and the spatially resolved canister corrosion depths for the Finnish spent nuclear fuel repository concept. The backfilled tunnel and the disposal hole are implemented using repository geometries, with sulfide being produced at their interface with the rock (excavation damaged zone) by sulfate reducing bacteria (SRB). Recent experimental findings regarding the microbial sulfate reduction process as well as the scavenging of sulfide via iron (oxy)hydroxides are incorporated in the reactive transport model. Long-term simulations are performed, predicting a heterogeneous corrosion of the canister with a max. corrosion depth of 1.3 mm at the bottom corner after one million years. The evolution of sulfide fluxes shows two main phases, depending on the source of sulfate: first sulfate is supplied by the dissolution of gypsum from the bentonite barriers, followed by a steady, low-level supply from the groundwater. Sensitivity cases demonstrate that both the organic carbon and Fe(III) oxide contents in the bentonite are critical to the corrosion evolution, by being the main electron donor for SRB activities and the major sulfide scavenger in the bentonite, respectively. The backfilled tunnel contributes little to the flux of corrosive sulfide to the canister due to the attenuation by Fe(III)-oxides/hydroxides but induces a notable flux of sulfate into the disposal hole.

Analysis of shear mock-up tests in a canister-clay system for spent nuclear fuel isolation

The aim of this paper is to simulate a canister-buffer mock-up test using the finite element computer code CODE_BRIGHT. The response of a copper canister put inside clay barriers that undergo hydration and swelling till complete saturation was investigated. Although the copper is very stiff and resistant and will be emplaced in a clay volume, shear deformations occurring in the clay barrier may have an impact on the canister. A Mock-up test was carried out in a steel cell at a scale of about 1:10 for assessing the consequences of a shear zone for a copper rod that was surrounded by compacted bentonite as a clay barrier. The challenge problem is to perform the simulation of the mock-up tests under the different shear rates that was carried out by Borgesson (1968). The results might contribute to a better understanding of the real shearing response of the system and validate the constitutive models used for the simulations.For this purpose, the mock-up test was simulated with CODE_BRIGHT in hydro-mechanical couplings in three dimensions. For the model validation, the results of the simulations were compared with the test measurements. The constitutive model used for the copper simulation was a viscoplastic model developed by Bodner and Partom. For the bentonite simulation, a viscoplastic model for unsaturated soils based on the Desai and Perzyna theory, using an elliptical yield surface corresponding to the modified Cam-Clay model, was used.

Characterization of vandenbrandeite: A potential alteration product of spent nuclear fuel

AbstractThe use of copper canisters in the Swedish KBS-3 concept for spent nuclear fuel disposal could result in the formation of copper-bearing uranyl phases should a canister suffer from defects or if the containment were to fail before reducing conditions are established in the repository. Most uranyl species would be expected to display higher solubility than the original uranium(IV) dioxide fuel, leading to enhanced release, though this would depend on the phase and prevailing groundwater conditions. Secondary alteration products may also be poorly crystalline or even amorphous, making characterization difficult during the pre-closure period owing to the high radiation field close to the canister. Vandenbrandeite, [CuUO2(OH)4], is a rare mineral in nature but known to form by alteration of primary uraninite through interaction with oxidizing groundwater containing dissolved copper. Consequently, an attempt has been made to characterize two vandenbrandeite specimens of varying crystallinity by luminescence and multiple-laser Raman spectroscopy; techniques amenable to remote, robotic deployment and which have proved useful in discriminating other uranyl oxy-hydroxides, silicates, and phosphates. The first reported luminescence emission and excitation spectra for vandenbrandeite revealed near-negligible luminescence, with a slightly enhanced signal for the specimen displaying poorer crystallinity. This observation agrees well with density functional theory calculations. The simulated projected density of state and band structure show an unlikely transition from the U f-orbitals to Cu d-orbitals, or O states, would be required for luminescence to be detectable; this probably improves for poorly crystalline specimens as the spatial overlap between the orbitals increases. Furthermore, negligible differences in the number of peaks and peak positions were detected in the laser wavelength-dependent Raman spectra although again, variation in background noise and peak shape was observed based on the degree of crystallinity. Good agreement was obtained between experimental and simulated Raman spectra, particularly with the environmentally sensitive axial uranyl stretching modes, validating the crystal system derived in this study. The findings of this study suggest luminescence spectroscopy, when combined with Raman spectroscopy, may be able to both identify vandenbrandeite and distinguish between crystalline and amorphous forms based on their relative luminescence intensity.

A new methodology for thermal analysis of geological disposal of spent nuclear fuel using integrated simulations of gamma heating and finite element modeling

A new methodology is illustrated, where the evolution of temperature in a geological disposal system for spent nuclear fuel is estimated by integrated calculations of a spatially distributed gamma heating source with conventional finite element thermal transport modeling. A case with one canister loaded with fuel assemblies with a cooling time of 30 years in a KBS-3 type repository illustrates the methodology. For this particular case, the effect of including distributed gamma heating rate in the modeling has a small impact on the temperature distribution compared to the conventional case of heat generated locally in the canister, resulting in a small decrease of the maximum temperature in the canister. A large proportion of gamma heating occurs inside the outer boundary of the copper canister for this case. Other potential consequences of radiation escaping the canister are discussed.

Assessing the effect of radioactive waste glass dissolution on early-stage radionuclide migration using simplified geological repository Monte Carlo transport models

The vitrification of radioactive waste within glass and subsequent disposal within a geological disposal facility (GDF) requires a comprehensive understanding of the effect of glass dissolution on GDF performance. This paper aims to analyse the effect of both high-level and intermediate-level waste (HLW and ILW) glass dissolution source terms on radionuclide release into the geosphere just above the disposal vault (the ‘crown’). Radionuclide migration was simulated in GoldSim for HLW in either granite or clay host rocks with a bentonite buffer using carbon steel or copper canisters, whereas ILW simulations considered either granite or clay host rocks, in either bentonite buffer or cement backfill, using concrete or cast-iron canisters. Glass dissolution source terms were varied by coupling GoldSim and MATLAB to modify the initial, residual, and resumption dissolution rates of the glass or by applying the analytical GRAAL model to glass dissolution. HLW glass results indicate no preference of granite over clay host rocks for a given canister type but that a copper canister is preferable to steel. ILW results suggest that a granite–bentonite–cast-iron environment yields lowest crown activities with cast-iron preferable to concrete as the canister, bentonite preferable to cement as the buffer/backfill, and granite preferable to clay as the host rock. Varying glass dissolution source terms (initial, residual, and resumption dissolution rates) had an understood effect on radionuclide migration, although changes were arguably insignificant considering peak crown activity for both HLW and ILW.

On the mechanism of γ-radiation-induced corrosion of copper in water

Radiation-induced corrosion of the copper canister used to contain the spent nuclear fuel in future geological repositories is a process of potential importance for the long-term safety. In recent years, several experimental studies have been conducted to elucidate the mechanism of this process. However, experimental observations and numerical simulations based on the proposed mechanism are not in agreement. In this work we have explored the mechanism further. Based on a series of experimental observations we propose a new mechanism where the corrosion of copper is driven by radiolytically produced O2.

Assessment of Corrosion Lifetime of a Copper Disposal Canister Based on the Finnish Posiva Methodology

In this paper, an approach developed by the Finnish nuclear waste management organization, Posiva, for the construction license of a geological repository was reviewed. Furthermore, a computer program based on the approach was developed. By using the computer program, the lifetime of a copper disposal canister, which was a key engineered barrier of the geological repository, was predicted under the KAERI Underground Research Tunnel (KURT) geologic conditions. The computer program was developed considering the mass transport of corroding agents, such as oxygen and sulfide, through the buffer and backfill. Shortly after the closure of the repository, the corrosion depths of a copper canister due to oxygen in the pores of the buffer and backfill were calculated. Additionally, the long-term corrosion of a copper canister due to sulfide was analyzed in two cases: intact buffer and eroded buffer. Under various conditions of the engineered barrier, the corrosion lifetimes of the copper canister due to sulfide significantly exceeded one million years. Finally, this study shows that it is necessary to carefully characterize the transmissivity of rock and sulfide concentration during site characterization to accurately predict the canister lifetime.