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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 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]

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]

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]

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]

Carbon steel is widely considered as a candidate material for the construction of spent fuel and high-level waste disposal canisters. In order to investigate corrosion processes representative of the long term evolution of deep geological repositories, two in situ experiments are being conducted in the Mont-Terri rock laboratory. The iron corrosion (IC) experiment, aims to measure the evolution of the instantaneous corrosion rate of carbon steel in contact with Opalinus Clay as a function of time, by using electrochemical impedance spectroscopy measurements. The Iron Corrosion in Bentonite (IC-A) experiment intends to determine the evolution of the average corrosion rate of carbon steel in contact with bentonite of different densities, by using gravimetric and surface analysis measurements, post exposure. Both experiments investigate the effect of microbial activity on corrosion. In the IC experiment, carbon steel showed a gradual decrease of the corrosion rate over a period of 7 years, which is consistent with the ongoing formation of protective corrosion products. Corrosion product layers composed of magnetite, mackinawite, hydroxychloride and siderite with some traces of oxidising species such as goethite were identified on the steel surface. Microbial investigations revealed thermophilic bacteria (sulphate and thiosulphate reducing bacteria) at the metal surface in low concentrations. In the IC-A experiment, carbon steel samples in direct contact with bentonite exhibited corrosion rates in the range of 2 µm/year after 20 months of exposure, in agreement with measurements in absence of microbes. Microstructural and chemical characterisation of the samples identified a complex corrosion product consisting mainly of magnetite. Microbial investigations confirmed the limited viability of microbes in highly compacted bentonite. [ABSTRACT FROM AUTHOR]

Opalinus Clay is currently being assessed as the host rock for a deep geological repository for high-level and low- and intermediate-level radioactive wastes in Switzerland. Within this framework, the 'Full-Scale Emplacement' (FE) experiment was initiated at the Mont Terri rock laboratory close to the small town of St-Ursanne in Switzerland. The FE experiment simulates, as realistically as possible, the construction, waste emplacement, backfilling and early post-closure evolution of a spent fuel/vitrified high-level waste disposal tunnel according to the Swiss repository concept. The main aim of this multiple heater test is the investigation of repository-induced thermo-hydro-mechanical (THM) coupled effects on the host rock at this scale and the validation of existing coupled THM models. For this, several hundred sensors were installed in the rock, the tunnel lining, the bentonite buffer, the heaters and the plug. This paper is structured according to the implementation timeline of the FE experiment. It documents relevant details about the instrumentation, the tunnel construction, the production of the bentonite blocks and the highly compacted 'granulated bentonite mixture' (GBM), the development and construction of the prototype 'backfilling machine' (BFM) and its testing for horizontal GBM emplacement. Finally, the plug construction and the start of all 3 heaters (with a thermal output of 1350 Watt each) in February 2015 are briefly described. In this paper, measurement results representative of the different experimental steps are also presented. Tunnel construction aspects are discussed on the basis of tunnel wall displacements, permeability testing and relative humidity measurements around the tunnel. GBM densities achieved with the BFM in the different off-site mock-up tests and, finally, in the FE tunnel are presented. Finally, in situ thermal conductivity and temperature measurements recorded during the first heating months are presented. [ABSTRACT FROM AUTHOR]

Clay rocks are being considered for radioactive waste disposal, but relatively little is known about the impact of microbes on the long-term safety of geological repositories. Thus, a more complete understanding of microbial community structure and function in these environments would provide further detail for the evaluation of the safety of geological disposal of radioactive waste in clay rocks. It would also provide a unique glimpse into a poorly studied deep subsurface microbial ecosystem. Previous studies concluded that microorganisms were present in pristine Opalinus Clay, but inactive. In this work, we describe the microbial community and assess the metabolic activities taking place within borehole water. Metagenomic sequencing and genome-binning of a porewater sample containing suspended clay particles revealed a remarkably simple heterotrophic microbial community, fueled by sedimentary organic carbon, mainly composed of two organisms: a Pseudomonas sp. fermenting bacterium growing on organic macromolecules and releasing organic acids and H2, and a sulfate-reducing Peptococcaceae able to oxidize organic molecules to CO2. In Opalinus Clay, this microbial system likely thrives where pore space allows it. In a repository, this may occur where the clay rock has been locally damaged by excavation or in engineered backfills. [ABSTRACT FROM AUTHOR]

• Anoxic uniform corrosion can often be monitored via analysis of accumulated hydrogen. • Rates as low as 0.01 nm/year can be monitored with periodic analysis. • Several years are required for test systems to reach steady state. • Carbon steel, in anoxic cementitious environments at 50 °C, corrodes <1 nm/year. • This information is of value to organizations responsible for nuclear waste. The safe long-term disposal of radioactive waste arising primarily from power generation is a complex proposition. Safety assessment modelling typically incorporates up to one million years, which is an engineering requirement without precedent. In order to perform this modelling, it is imperative that the behaviour of the materials comprising the repository are well understood over the timescales in question. This article describes a method developed for studying the behaviour of metals under anoxic conditions via the corrosion reaction end product, hydrogen. At its simplest, test cells are left for periods of time, up to several months, to accumulate hydrogen. This gas is then expelled from the cell and analysed using a hydrogen sensor. As such, the method is limited to characterising gas generation and consequently only uniform corrosion, but does permit direct monitoring of corrosion rates as low as 0.01 nm/year, which is necessary in order to accurately monitor the very low long-term corrosion rates expected in a deep geological repository. [ABSTRACT FROM AUTHOR]

In the past decade, TWI has been working together with Nagra (the Swiss national cooperative responsible for the disposal of radioactive waste) to develop design concepts for canisters for the disposal of Spent Fuel (SF) and High Level Waste (HLW) in a deep geological repository. The canisters must provide containment of radionuclides for more than 10,000 years. Several different canister designs have been investigated. A forged carbon steel canister is the current reference design. In the framework of options evaluation and optimisation, this study summarises the work carried out to determine the structural integrity of a closure weld during long-term disposal. An analytical study on the lid and weld design for the closure weld and final materials selection have been undertaken. The lid and closure weld designs were refined to avoid heating the nuclear waste above its allowable temperature during post-weld heat treatment (PWHT). Flaw assessments in accordance with the procedures and guidance given in BS 7910, along with finite element analyses (FEA), were used to identify the most suitable combination of PWHT and lid design to ensure no breach of containment within 10,000 years. • A proposed lid design for a nuclear waste canister is evaluated; the requirement is to ensure integrity for 10,000 years. • BS 7910 is successfully used to assess an atypical application. • To achieve this, FEA and ECA are combined in a complementary approach. [ABSTRACT FROM AUTHOR]

[Display omitted] • Initial anaerobic corrosion rates of steel in bentonite outpace the diffusion of water. • A hydrogen gas phase is temporarily formed close to the steel surface. • Gas and the lack of water desiccate the bentonite resulting in shrinkage microfractures. • Diffusing Fe2+ is oxidized by sorbed oxygen and precipitates in the porespace. • The safety relevant properties of bentonite are not jeopardized by steel corrosion. Experimental evidence related to the interactions between steel corrosion and bentonite in deep geological repositories is reconsidered. The released Fe2+ interacts with the smectite and precipitates as Fe3+ when oxidised by residual immobile oxygen. This leads to the development of coloured fronts in the bentonite. The evolution of these fronts can be reproduced by a reactive transport model that takes account of the competition between diffusion and precipitation. Furthermore, two-phase flow modelling indicates that the consumption of water by the corrosion outpaces the diffusion of water through the bentonite, leading to shrinkage microfractures in bentonite that act as preferential pathways for corrosion products. [ABSTRACT FROM AUTHOR]

In a deep geological repository for radioactive waste in the Opalinus Clay, disposal canisters may be corroded due to sulphide produced by sulphate reducing bacteria (SRB). This paper presents two computational models, a reactive transport model (RTM) and a simplified semi-analytical model (SM), to evaluate the potential of SRB to generate elevated sulphide concentrations, to assess sulphide fluxes to the canister and, in a simplified manner, the resulting canister corrosion. Calculations performed in the context of the repository's safety assessment based on a shared conceptual model reveal that the two computational models are complementary. The reactive transport model incorporates state-of-the-art understanding of the system's geochemistry, but is currently too computationally demanding to be applied in probabilistic safety assessment sensitivity analyses. The simplified model is fast and efficient, but some of its assumptions need to be verified, and some parameter values need to be calibrated using the more complete reactive transport model. Nonetheless, given the same set of assumptions, the two models predict comparable magnitudes of sulphide fluxes to the canister and comparable canister corrosion depths. • Microbial sulphate reduction in a repository for radioactive waste is conceivable • Two models to conservatively evaluate this effect are presented and compared • Both models are complementary with their respective advantages and weaknesses • The models yield comparable results for the same set of assumptions [ABSTRACT FROM AUTHOR]