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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]

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]

Within EURAD, targeted collaborative research activities are performed to further deepen understanding regarding the long-term behaviour of key components in the repository near-field, assess specific radionuclide retention processes as well as developing methods for monitoring safety relevant parameters of repository systems. The ambition of the four EURAD Workpackages (WPs) – CONCORD, FUTURE, CORI, MODATS – presented here, is to investigate topics to meet implementation needs and contribute to Safety Cases in Europe at the highest level of scientific excellence. Work is fully integrated into the EURAD concept, emphasizing interactions between different WPs, involvement of End Users, assuring the link to national programmes and contributing to overarching features like Knowledge Management, Training and Education, or European Integration. Comprehensive initial State-of-the-Art reports were prepared by the WPs or currently under development and are available at the EURAD website. The technical/scientific work performed in the four WPs - CONCORD, FUTURE, CORI, MODATS – is discussed in this contribution.

The 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. [ABSTRACT FROM AUTHOR]

The long‐term disposal of nuclear waste in deep geological repositories requires safe containment for up to one million years. To understand how the waste will respond to the gradually evolving environment, researchers can perform sensitive experiments in the laboratory to replicate the repository conditions at specific time points. In Belgium, spent nuclear fuel will be housed within a carbon steel overpack, which is encased within a cement buffer, which may include a stainless steel liner (the "envelope"). The range of corrosion behaviors for carbon steel, and to a lesser extent, stainless steel, has been studied through hydrogen evolution, an end‐product of oxidation under repository conditions. Considerable time may be required for metallic surfaces to approach a steady hydrogen evolution and this is dependent upon the starting condition of the specimen surfaces. The use of nominally‐similar materials can result in significant variation in the reported corrosion rates. The presence of cements, which generate their own hydrogen and influence water chemistry as a function of time, add complexity to even simple experimental configurations that requires careful interpretation. [ABSTRACT FROM AUTHOR]

Copper has been proposed as a corrosion barrier for used fuel containers due to its thermodynamic stability under the anoxic conditions relevant to a deep geological repository. Laboratory simulations of anticipated repository conditions have demonstrated the production of small quantities of hydrogen gas, which have been interpreted as being indicative of an oxidation (corrosion) process. This work reports new corrosion results for repository‐relevant materials: electrodeposited copper, cold spray copper and junction material under Canadian and Swiss simulated ground waters. The attribution of the recorded hydrogen suggests corrosion is limited to the outermost atomic layers of copper cladding. However, the origins of the hydrogen have not been conclusively determined. Three thermal desorption methods have been used to investigate the outgassing of hydrogen from electrodeposited copper which may permit a more accurate interpretation of the hydrogen evolution profile from corrosion cells. [ABSTRACT FROM AUTHOR]

The 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. [ABSTRACT FROM AUTHOR]

In this paper, carbon steel corrosion rates from experiments performed in situ (performed in an underground rock laboratory) and ex situ (performed in a conventional laboratory) test methods in anaerobic saturated bentonite are compared. The results indicate that the long‐term corrosion rate follows a power law decay curve, with a higher initial rate and greater rate of decay at higher temperature. In compacted bentonite blocks, varying the density has no significant effect on the corrosion rate measured during in situ testing, whereas granular bentonite of an equal dry density leads to a higher corrosion rate. Precorrosion of test specimens in an aerated solution at room temperature has a negligible effect on the subsequent anaerobic corrosion rate when tested ex situ for durations up to 11.8 years. The main difference between in situ and ex situ tests is the formation of silicon‐rich corrosion products in the ex situ tests but not in the in situ tests. Despite these differences, the corrosion rates in both tests exhibit the same general temporal evolution and similar magnitudes, suggesting that the main steel degradation processes are maintained in both configurations. [ABSTRACT FROM AUTHOR]

Microscopic and spectroscopic analyses were conducted on steel/bentonite interface samples removed from four in situ experiments that were carried out in three underground research laboratories at different temperatures and under different hydraulic and geochemical conditions. The results provide valuable information about the corrosion processes occurring in high‐level radioactive waste repositories. Systematic patterns can be deduced from the results, irrespective of carbon steel grade, type of bentonite and its degree of compaction, geochemical environment or experimental setup. Thus, a clear dependence of the corrosion rates on temperature and exposure period, as well as on the availability of H2O and O2 provided by the surrounding bentonite buffer, is observed. Furthermore, Fe(II) ions released by corrosion interact with the structural Fe in the clay. Recent developments highlight the usefulness of reactive transport modelling in understanding the coupled corrosion and Fe–clay interaction processes. [ABSTRACT FROM AUTHOR]

Radioactive waste is planned to be disposed in a deep geological repository in the Opalinus Clay (OPA) rock formation in Switzerland. Cu coating of the steel disposal canister is considered as potential a measure to ensure complete waste containment of spent nuclear fuel (SF) and vitrified high-level waste (HLW) or a period of 100,000 years. Sulphide is a potential corroding agent to Cu under reducing redox conditions. Background dissolved sulphide concentrations in pristine OPA are low, likely controlled by equilibrium with pyrite. At such concentrations, sulphide-assisted corrosion of Cu would be negligible. However, the possibility exists that sulphate reducing bacteria (SRB) might thrive at discrete locations of the repository’s near-field. The activity of SRB might then lead to significantly higher dissolved sulphide concentrations. The objective of this work is to employ reactive transport calculations to evaluate sulphide fluxes in the near-field of the SF/HLW repository in the OPA. Cu canister corrosion due to sulphide fluxes is also simplistically evaluated.

Microbial activity has the potential to enhance the corrosion of high-level radioactive waste disposal canisters, which, in the proposed Swiss deep geological repository, will be embedded in bentonite and placed in the Opalinus Clay (OPA) rock formation. A total of 12 stainless steel cylindrical vessels (referred to as modules) containing bentonite were deployed in an anoxic borehole in OPA for up to 5.5 years. Carbon steel coupons were embedded in the bentonite. Individual modules were retrieved after 1, 1.5, 2.5, and 5.5 years. Enumeration of aerobic and anaerobic heterotrophs and sulfate-reducing bacteria (SRB) revealed microbial growth for 1.5 years followed by a decline or stagnation in microbial viability. It was surprising to observe the growth of aerobic heterotrophs followed by their persistent viability in bentonite, despite the nominally anoxic conditions. In contrast, SRB numbers remained at very low levels. DNA-based amplicon sequencing confirmed the persistence of aerobes and the relatively low contribution of anaerobes to the bentonite microbiome. Bentonite dry density, in situ exposure time, and bioavailable trapped oxygen are observed to shape the bentonite microbial community in the clay. [ABSTRACT FROM AUTHOR]

Since 2012, a long‐term in situ corrosion experiment (IC‐A) is being conducted in the Mont Terri Underground Research Laboratory in Switzerland to investigate the corrosion behaviour of candidate canister materials in conditions representative of the Swiss concept for the disposal of high‐level waste and spent nuclear fuel. To date, carbon steel and various types of copper coatings have been retrieved after different exposure periods of up to 3 years, and characterised to establish the composition of the corrosion product, the morphology of the corroded surface, the nature of the interaction between the metal and the surrounding bentonite, and the microbial populations in the bentonite and surrounding porewater. For carbon steel specimens, a complex corrosion product was identified, consisting predominantly of magnetite. Much less alteration on either the metal or the bentonite was observed in the case of copper samples. Low average anaerobic corrosion rates were measured for carbon steel and a very modest amount of alteration was identified on copper. The density and the initial form of the bentonite had a small influence on the rate of corrosion, across all materials. This paper summarises the results of the experimental programme obtained to date and discusses the relationship observed between exposure time and the evolution of the metal–bentonite interface for both carbon steel and copper. [ABSTRACT FROM AUTHOR]

The Swiss waste management programme foresees that low‐ and intermediate‐level radioactive waste will be disposed of in a deep geological repository constructed in Opalinus Clay. Gas generation is expected in the repository due to the decomposition of organic materials and the corrosion of metals, with carbon steel being the primary source. The corrosion behaviour of mild steel under anoxic conditions has been studied over the course of several years to better understand the long‐term hydrogen evolution profile under anticipated repository conditions. Steel, either bare or encased within mortar, was tested in water vapour or immersed in electrolytes representative of aged cement waters at 50°C. The corrosion rate was measured indirectly through the hydrogen analysis using a solid‐state probe. The hydrogen evolution behaviour of grout was also monitored to more accurately determine the hydrogen generating from the corrosion of the embedded steel. For steel in water vapour or in alkaline environments, embedded in cementitious material or immersed in simulated aged cement pore water, corrosion rates were invariably <1 nm/year after several years of analysis. [ABSTRACT FROM AUTHOR]

The materials corrosion test (MaCoTe) is a long‐term, multinational in situ corrosion experiment setup at the Grimsel Test Site, Switzerland. The experiment has been operating since 2014 with a focus on the corrosion behaviour of container materials for the disposal of high‐level waste and spent nuclear fuel under conditions representing a granitic deep geological repository. The experiment consists of eight modules containing metal coupons and bentonite. Two of the modules, each with a different bentonite density, have been retrieved after 394 days of exposure and have been analysed using a range of techniques aimed at studying the corrosion behaviour of the metals and the mineralogical evolution of the bentonite. Weight loss measurements show that carbon steel had a relatively low average corrosion rate (~2 µm year−1). Much lower average corrosion rates were measured for the various types of copper (0.13–0.32 µm year−1). No detectable corrosion was measured on stainless steel coupons. To date, no significant differences were observed in the corrosion behaviour and rate of the test metals in bentonite with different dry densities. [ABSTRACT FROM AUTHOR]

Deep geological repositories for radioactive waste contain metallic materials, either used to construct disposal canisters or as low‐/intermediate‐level waste (L/ILW). The safety relevance of corrosion is linked to canister lifetime in the former case and gas generation in the latter. More specifically, the Belgian "supercontainer" concept envisages mild steel for the used fuel disposal canister, and in the case of the Swiss L/ILW repository, mild steels are the largest metallic waste component due to the decommissioning of civilian power‐generating facilities. For these circumstances, the corrosion environment is dominated by the chemistry of cement, which is used as buffer or backfill material. The corrosion behaviour of mild steel in anoxic environments was studied through the analysis of the hydrogen end‐product. Hydrogen analysis was conducted by periodically purging the cell head‐space and analysing the gas using a solid‐state hydrogen sensor. While this method is limited to providing only uniform corrosion rates averaged over periods of time, ranging from weeks to months, it provides excellent resolution and sensitivity. The test cell environments were matched against the anticipated Belgian high‐level waste and Swiss L/ILW repository environments, and also against experiments that have been conducted by other researchers for comparative purposes. Samples were exposed to synthetic cement pore waters, representing fresh and degraded cement. In young cement waters, the formation of initial corrosion products resulted in steel wire corrosion rates of the order of µm/year, which, at 80°C rapidly declined to ∼10 nm/year. In contrast, SA516 grade 70 steel plate corroded much more slowly under similar conditions. In aged cement waters, initial corrosion rates were higher but declined faster towards a longer‐term rate of ∼10 nm/year. 316L stainless steel, embedded in cementitious material, corroded at a rate of <1 nm/year at 50°C. [ABSTRACT FROM AUTHOR]

Carbon steel (C-steel; E24 and S235 grades) and stainless steel (316L) electrodes were corroded in situ in Opalinus clay under anoxic conditions in a vertical descending borehole. The electrodes were exposed at ambient temperature for two years, and then at 85°C for five years. In situ electrochemical impedance spectroscopy showed that the instantaneous corrosion rate of C-steel decreased over time down to 1 µm year−1, followed by steady state. Microbial and chemical investigations showed that sulphate and thiosulphate reducing prokaryotes were present in the porewater and at the metal surface. Post mortem characterisation revealed contrasting corrosion aspects. The E24 corrosion interface was thick and contained magnetite, mackinawite, hydroxychloride and siderite, together with more oxidised species (goethite, greigite, elemental sulphur). The S235 corrosion interface was thinner and contained siderite and mackinawite. Corrosion damage of the 316L electrode was negligible, and the surface was covered by a thin fringe of pyrite. [ABSTRACT FROM AUTHOR]

Nagra is currently developing design concepts for canisters for the deep geological disposal of spent fuel and high-level waste. A feasibility evaluation study was done to assess a number of candidate canister designs and materials. The potential canister options were assessed with regard to long-term safety by considering mechanical integrity, environmental damage and potential impact on the geological barrier. Manufacturing feasibility, sealing, inspection as well as potential cost were also assessed. A canister concept, based on a thick-walled forged carbon steel substrate coated with copper, was favourably assessed. As a result, development work was undertaken to refine the copper-coated canister design. Different lid designs and closure configurations were considered, using finite element analysis, to determine the limit load by plastic yielding and buckling and to determine safety margins under the design load. In addition, finite element fracture analyses were performed to determine stress intensity factors at the closure weld root. A hemispherical lid with a partially penetrating weld at a depth of 25-32 mm, having a root gap in the axial direction, was found to be fit for purpose while offering the potential to avoid post-weld heat treatment. An evaluation of relevant welding processes for closure welding of the selected joint design was then carried out. The potential welding processes were ranked based on a number of criteria linked to the maturity of the technique, the quality of the resulting weld, the properties of the weld material, and the applicability of the technique to the current canister design and the requirements of deployment in a hot cell. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems. [ABSTRACT FROM AUTHOR]

The anaerobic corrosion of steel was monitored under simulated Swiss low-/intermediate-level waste repository conditions. Steel was studied both as bare rods and cast in cement, in water vapour, simulating the unsaturated anoxic phase, and during immersion in various pore waters, representative of the saturated phase. All tests were performed at 50°C. Hydrogen evolution from grout, comprising commercial cement and sand, was also studied. Grouted steel specimens initially generated hydrogen that was comparable to grout alone, which was attributed to the presence of iron detritus from the ball-milling of cement clinker. The corrosion rate of immersed test specimens peaked during the first 100 days, likely due to the formation of a passive film. Corrosion rates generally declined to <3 nm/year, although there were several notable exceptions where hydrogen generation was erratic. This was attributed to localised corrosion and may be an important factor to understand when modelling hydrogen generation rates. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems. [ABSTRACT FROM AUTHOR]

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

According to the Swiss disposal concept, the safety of a deep geological repository for spent nuclear fuel (SNF) is based on a multi-barrier system. The disposal canister is an important component of the engineered barrier system, aiming to provide containment of the SNF for thousands of years. This study evaluates the criticality safety and shielding of candidate disposal canister concepts, focusing on the fulfilment of the sub-criticality criterion and on limiting radiolysis processes at the outer surface of the canister which can enhance corrosion mechanisms. The effective neutron multiplication factor (k-eff) and the surface dose rates are calculated for three different canister designs and material combinations for boiling water reactor (BWR) canisters, containing 12 spent fuel assemblies (SFA), and pressurized water reactor (PWR) canisters, with 4 SFAs. For each configuration, individual criticality and shielding calculations were carried out. The results show that k-eff falls below the defined upper safety limit (USL) of 0.95 for all BWR configurations, while staying above USL for the PWR ones. Therefore, the application of a burnup credit methodology for the PWR case is required, being currently under development. Relevant is also the influence of canister material and internal geometry on criticality, enabling the identification of safer fuel arrangements. For a final burnup of 55MWd/kgHM and 30y cooling time, the combined photon-neutron surface dose rate is well below the threshold of 1 Gy/h defined to limit radiation-induced corrosion of the canister in all cases. [ABSTRACT FROM AUTHOR]

Abstract An on-line gas monitoring has been conceived and implemented to study the evolution of the composition of the free gas phase in the Full-scale Emplacement (FE) experiment in the Underground Rock Laboratory at Mont Terri (Switzerland). The FE experiment is a trial run for a spent-fuel emplacement drift for a repository according to the Swiss concept for radioactive waste disposal. The monitoring of gas species such as He, Ar, Kr, Xe, N 2 , O 2 , H 2 , CH 4 , and CO 2 was performed successfully over several months. The partial pressures of gases relevant for the operational safety such as H 2 and CH 4 have been found to be below the concentration threshold for ignition. The combination of the on-line monitoring data and conventional noble-gas isotope measurements reveals rapid gas exchange between the pore space of the compacted bentonite granulate material used as backfilling and both the access niche and the host rock surrounding the FE tunnel (Opalinus Clay). Such fast gas exchange partly explains the disappearance of oxygen from the bentonite pore space detected by O 2 sensors even prior to sealing of the drift and the accumulation of a fraction of terrigenic gases such as 4He, 40Ar, CH 4 , and CO 2. Highlights • An on-line gas monitoring has been implemented for the FE experiment at Mont Terri URL. • The monitoring of gas species was performed successfully over several months. • Rapid gas exchange occurs between drift backfilling and FE niche/host rock. • Terrigenic gases (e.g., 4He, 40Ar, CH 4 , CO 2) accumulated in the backfill pore space. • Fast gas exchange partly explains the O 2 removal from the backfill pore space. [ABSTRACT FROM AUTHOR]

Abstract Radioactive waste is planned to be disposed in a deep geological repository in the Opalinus Clay (OPA) rock formation in Switzerland. Copper coating of the steel disposal canister is considered as an option to ensure complete waste containment for a period of 100'000 years. Sulphide is a potential corroding agent to copper. While background sulphide concentrations in the OPA are very low (ca. 10−11 mol/L), it cannot be ruled out that sulphide reducing bacteria (SRB) thriving at discrete locations within the engineered barrier system (EBS) of the repository could generate significantly higher dissolved sulphide concentrations. The aim of this study is to conservatively evaluate the potential for SRB to generate and sustain elevated sulphide concentrations in the repository near-field by considering specific geochemical bottle-necks, to quantify the maximum sulphide fluxes towards the canister, and to assess their potential to corrode the copper canister coating. To address these objectives reactive transport calculations are performed, and main conceptual and parametric uncertainties of the model are assessed. Highlights • A model for sulphide generation, consumption and transport in the near-field of a nuclear waste repository is presented. • The model allows pessimistic estimates of canister copper coating corrosion due to sulphide. • Results suggest that corrosion due to sulphide will not compromise the copper coating during hundreds of thousands of years. [ABSTRACT FROM AUTHOR]