Microbial impact on corrosion products: the role of hydrogen and magnetite on nuclear waste geological disposal

Document available in extended abstract form only. The generally accepted strategy to manage High Level Nuclear Waste (HLNW) in a number of countries in Europe, Asia and North America is the disposal of waste packages in a deep underground repository (around 500 m) in stable geological environments (e.g. clay or granitic formations). Such strategy involves the use of several natural and/or engineered barriers in order to prevent the radionuclides migration as well as to prevent water circulation around different metallic envelopes (container, overpack and tubing). It is known that the corrosion on long term in water-saturated conditions is the main factor responsible for the alteration of the waste packages. However, understanding the impact of living microorganisms on this repository setup is still an open issue, especially with regard to Microbiologically Influenced Corrosion (MIC) processes. Several studies have indeed demonstrated that bacteria and archaea are present in most of the deep geological formations already investigated. Moreover, the introduction of microorganisms into nuclear waste disposal repositories cannot be excluded due to human activities during the construction and operational phases. It is widely accepted that such microorganisms may affect the corrosion process due to their influence (directly or indirectly) on the chemical (i.e. ions composition, pH, redox potential) and physical characteristics at the metal/environment interface. Such phenomena have been reported as main factor on the economic damages in oil and gas industries, power supply facilities and drinking water industries. Bacterial metabolism may thrive on H2 released by anoxic aqueous corrosion of waste packages, which represents a new energetic source (electron donor) for microbial development, especially in disposal environments with a low amount of biodegradable organic matter. Moreover, the formation of iron oxides as corrosion products, such as magnetite (Fe3O4), will provide Fe(III) (electron acceptor) favouring the development of iron-reducing bacteria (IRB). Consequently, the microbial activity by H2 oxidation coupled to Fe(III) reduction can potentially alter the protective properties of the iron oxides layers, which could reactivate the anoxic corrosion. The main goal of this study is to evaluate the impact of Shewanella oneidensis strain MR-1 (IRB and hydrogeno-trophic bacteria models) on the long term stability of magnetite in presence of H2. A series of experiments have been designed in batch reactors at 30 deg. C under an anaerobic N2 (100%) atmosphere and in abiotic and biotic conditions. Three phases coexist: (i) a liquid phase made of the minimal medium (M1), which supplies the essentials nutrients for the growth of Shewanella oneidensis; (ii) a solid phase made of commercial magnetite powder; and (iii) a N2/CO2/H2 gas phase (30/10/60%, respectively). The microbial activity is evaluated periodically by measurements from the liquid phase (i.e. pH, Fe concentration), gas phase (i.e. H2 consumption by bacteria) and solid characterization. Previously studies have indicated that the combined evolution of Fe and H2 can be attributed to the microbial activity by alteration of the magnetite protective layer, which in turn can reactivate the corrosion process. Moreover, geochemical modelling will be performed in support to the interpretation of the experimental results as well as to determine thermodynamical/kinetics parameters. (authors)