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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]

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]

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]

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]

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]

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]

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]

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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.)

• 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]