Long-term radionuclide retention in the near field: collaborative R&D studies within EURAD focusing on container optimisation, mobility, mechanisms and monitoring

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

Anaerobic corrosion of carbon steel in compacted bentonite exposed to natural Opalinus clay porewater: Bentonite alteration study

Carbon steel is a potential canister material for the disposal of high-level radioactive waste in deep geological repositories in clays and clay rocks. Bentonite is considered as a potential backfill material for those multi-barrier systems. To predict the long-term performance and for safety assessment the knowledge of canister corrosion behavior is important. The formed corrosion products and mineralogically altered bentonite at the canister/bentonite interface can potentially provide an additional barrier against radionuclide migration. In-situ corrosion experiments were performed at the Mt. Terri underground research laboratory. Coupons of carbon steel were embedded in Volclay MX-80 bentonite with controlled densities, installed in a borehole under simulated repository and anaerobic conditions and exposed to natural Opalinus Clay porewater for a period up to 5.5 years. The bentonite layer at the canister/bentonite interface was characterized by complementary microscopic and spectroscopic techniques (XPS, SEM-EDX, XRD) under anoxic conditions. The interface revealed reddish-brown staining up to 2 mm depth into the bentonite in the zone adjacent to the steel. The SEM-EDX analyses of the interface (embedded crosscut with steel removed) showed calcium and iron enrichment in the bentonite adjacent to the metal. µXRF analysis performed on the bentonite at the interface showed calcium enriched rim up to 100 µm into the bentonite, while µXANES analysis revealed formation of iron silicate compounds in the reacted reddish-brown zone. The steel coupon was removed prior embedding. A line scan from the edge towards bulk bentonite did not indicate any systematic gradient in the Fe2+ / Fe3+ ratio. The formation of mixed Fe2+/3+ silicate compounds appears to be heterogeneous. This work contributes to an increased understanding of steel corrosion mechanisms in clay, which can improve the robustness of canister lifetime predictions. We acknowledge the German Federal Ministry of Education and Research (BMBF) and the Helmholtz association for the financial support. We thank the Mont Terri IC-A Partners and Swisstopo for providing the samples. We acknowledge the provision of the beamtime at the KIT Light Source, KARA, Germany

Unravelling the corrosion processes at steel/bentonite nterfaces in in situ tests

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

A minimalistic microbial food web in an excavated deep subsurface clay rock

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 H-2, 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

Further results on the in situ anaerobic corrosion of carbon steel and copper in compacted bentonite exposed to natural Opalinus Clay porewater containing native microbial populations

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

The anaerobic corrosion of candidate disposal canister materials in compacted bentonite exposed to natural granitic porewater containing native microbial populations

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

Third body effects on friction and wear during the fretting of steel contacts

Frettingwear proceeds through particle detachment from the contacting surfaces which, while trapped in the contact zone, can affect the frictional and wear response. Ball-on-flat fretting experiments were carried out between steel specimens under gross slip regime. A transition in the coefficient of friction was linked to a critical contact pressure. The microstructure and chemical composition of the thirdbody evolve with the applied pressure. The evolution of the friction coefficient is strongly dependent on the thirdbody properties. The wear is controlled by the applied load and thus the real contact area within the wear track

Effect of hydrodynamics on zinc anodizing in silicate-based electrolytes

The widespread use of zinc, both as a sacrificial coating on steel and as a stand-alone material, requires a high level of corrosion resistance in order to increase the product life cycle and to avoid environmental contamination by metal ions. This paper reports the synthesis of corrosion protective conversion layers on zinc by single step anodizing in silicate-based electrolytes. A rotating disc electrode is used to control the mass transport towards the electrode, and to study the effect of hydrodynamics on the structural characteristics and corrosion resistance of the surface layers formed. The dissolution of the zinc substrate is found to be required for the precipitation of silicate-rich surface layers. The correlation between the composition of the anodizing electrolyte and the surface layers indicates that an anodically induced deposition process takes place during anodizing. Electrochemical measurements and humidity chamber corrosion tests show that anodizing in silicate-based electrolytes increases the corrosion resistance of zinc substrates by forming a barrier layer that shields the base material and by inducing a galvanic protection. (c) 2004 Elsevier B.V. All rights reserved.status: publishe

Anodic film formation on zinc in alkaline electrolytes containing silicate and tetraborate ions

The reactivity of zinc in sodium hydroxide electrolytes containing sodium silicate and sodium tetraborate was studied by electrochemical and microstructural examinations. Impedance measurements indicate that a porous surface layer is formed on zinc immersed in sodium hydroxide at its open circuit potential and on polarization in the prepassive region. This surface layer becomes compact at potentials in the passive region. The anodic reactions of a zinc electrode in a sodium hydroxide containing electrolyte are controlled by the mass transport of zincate ions away from the electrode. Silicate and tetraborate ions suppress the anodic dissolution of zinc. In the presence of silicate ions, a compact silicate containing film is formed upon anodic polarization. The electrode rotation affects the microstructure of the surface layer both in silicate-free and silicate-containing sodium hydroxide solutions. On addition of tetraborate ions, the polarization resistance increases and the formation of a surface layer is achieved at increased applied potential. (c) 2007 The Electrochemical Society.status: publishe

In-vivo condition monitoring of metallic implants by electrochemical techniques

The invention relates to a replacement metallic prosthesis to be implanted which contains means monitor its condition during use in order to allow an early detection of failure or insufficient functionality, wherein said means comprise implanted sensors and electronics (3) and a remote device (9), to measure the implant's function and degradation during its life span, wherein said sensors are electro not chemical sensors with electrodes (1). The prosthesis according to the invention may also be used to promote bone growth. The invention also relates to a method using the device of the invention