Microbially-induced corrosion of carbon steel in a geological repository environment: Dissertation

Low- and intermediate-level radioactive waste (LLW/ILW) is produced during the operation and decommission of nuclear power plants. At the Olkiluoto power plant, LLW/ILW is disposed of in an underground repository excavated into the bedrock 60-100 m below sea level. The metallic portion of this waste is typically made of carbon steel and stainless steels. In anoxic conditions, such as the groundwater at the Olkiluoto repository site, carbon steel corrosion rate is very slow unless the groundwater is highly acidic or microbial activity is high, altering local conditions to corrosion inducing direction. Microorganisms are able to accelerate general corrosion as well as induce localized corrosion forms and stress corrosion cracking as conditions under the biofilm can differ markedly from those in the adjacent environment. Critically, corrosion of metallic waste can release radioactive nuclides into the groundwater and threaten the long-term integrity of the storage site. The objective of this research was to determine the importance of microbiallyinduced corrosion (MIC) of carbon steel placed in deep geological repository containing LLW/ILW. The structure and function of microbial communities in the deep biosphere are still poorly understood but could have important consequences for the long-term storage of radioactive waste in underground repositories. MIC of carbon steel in anoxic groundwater was studied in the laboratory and in situ in experiments with exposure time ranging from 3 months to 15 years. MIC was examined using gravimetric and electrochemical techniques complemented by molecular biology and surface characterization methods. It was shown that conditions beneath the microbial biofilm accelerated corrosion rate of carbon steel, especially localized corrosion, and that microbial activity in deep groundwater is enhanced by the presence of carbon steel. Naturallyoccurring microorganisms in deep groundwater environments have a great affinity for the surface of carbon steel and rapidly form a biofilm. Phylum proteobacteria, beta- or deltaproteobacteria depending on the experiment, were in the majority in the biofilm forming bacterial community. Archaeal biofilm was formed by phylas Euryarchaeota (DHVE) and Thaumarcheota (MBGB). However, corrosion was inhibited in concrete-encased environments, due to high alkalinity and calcium carbonate concentration in the environment. In many cases, LLW/ILW repositories contain concrete materials, which according to the present results hinders the corrosion at least in the beginning of repository time scale.