• Rajala, Pauliina

Abstract

Low- and intermediate-level radioactive waste (LLW/ILW)is produced during the operation and decommission ofnuclear power plants. At the Olkiluoto power plant,LLW/ILW is disposed of in an underground repositoryexcavated into the bedrock 60-100 m below sea level. Themetallic portion of this waste is typically made ofcarbon steel and stainless steels.In anoxic conditions, such as the groundwater at theOlkiluoto repository site, carbon steel corrosion rate isvery slow unless the groundwater is highly acidic ormicrobial activity is high, altering local conditions tocorrosion inducing direction. Microorganisms are able toaccelerate general corrosion as well as induce localizedcorrosion forms and stress corrosion cracking asconditions under the biofilm can differ markedly fromthose in the adjacent environment. Critically, corrosionof metallic waste can release radioactive nuclides intothe groundwater and threaten the long-term integrity ofthe storage site.The objective of this research was to determine theimportance of microbiallyinduced corrosion (MIC) ofcarbon steel placed in deep geological repositorycontaining LLW/ILW. The structure and function ofmicrobial communities in the deep biosphere are stillpoorly understood but could have important consequencesfor the long-term storage of radioactive waste inunderground repositories. MIC of carbon steel in anoxicgroundwater was studied in the laboratory and in situ inexperiments with exposure time ranging from 3 months to15 years. MIC was examined using gravimetric andelectrochemical techniques complemented by molecularbiology and surface characterization methods.It was shown that conditions beneath the microbialbiofilm accelerated corrosion rate of carbon steel,especially localized corrosion, and that microbialactivity in deep groundwater is enhanced by the presenceof carbon steel. Naturallyoccurring microorganisms indeep groundwater environments have a great affinity forthe surface of carbon steel and rapidly form a biofilm.Phylum proteobacteria, beta- or deltaproteobacteriadepending on the experiment, were in the majority in thebiofilm forming bacterial community. Archaeal biofilm wasformed by phylas Euryarchaeota (DHVE) and Thaumarcheota(MBGB). However, corrosion was inhibited inconcrete-encased environments, due to high alkalinity andcalcium carbonate concentration in the environment. Inmany cases, LLW/ILW repositories contain concretematerials, which according to the present results hindersthe corrosion at least in the beginning of repositorytime scale.

Topics

• impedance spectroscopy
• surface
• Carbon
• stainless steel
• experiment
• forming
• stress corrosion