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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - MIND (Development of the safety case knowledge base about the influence of microbial processes on geological disposal of radioactive wastes)

Teaser

Summary

The MIND project is addressing key technical issues that should be tackled to support the implementation of planned geological disposal projects for higher level radioactive wastes. This is a challenge as social and technical aspects blend and input from social science with regard to the design, implementation and post-construction management of the installation is required. Thus, the impact of the inclusion of microbiology on expert conceptualisation and public perception of geological disposal, have been considered. Microorganisms may have controlling influences on organic waste evolution in situ, multi-barrier integrity and radionuclide migration from the repository. The emphasis has been on quantifying specific measureable impacts of microbial activity on barrier evolution under repository-relevant conditions. The MIND project has contributed to that microbial processes in repositories are now being taken into account to greater extent than previously, leading to refinements of models currently being implemented to evaluate the long-term evolution of radioactive waste repositories.

Work performed

WP1: The materials studied in the MIND project were bitumen, organic ionexchange resins, halogenated polymers (PVC) and cellulose. Irradiation led to an increase in the concentration of dissolved organic carbon, including the alkaline hydrolysis product ISA. Fermentation of the cellulose degradation products at hyperalkaline pH led to the production of H2, and acetate, while CH4 was not detected. A bacterium able to use ISA Anaerobacillus isosaccharinicus was identified. It has been demonstrated that PVC plasticiser and fire retardant additives in the PVC sheet are able to fuel microbial metabolism at pH 10 using nitrate as an electron acceptor. Irradiation of plasticised PVC makes the material less bioavailable. Results of studies of bitumen degradation showed that the nitrate leaching from Eurobitum rapidly stimulated microbial nitrate reduction. Different rates were observed depending on the organic compounds that were used to fuel the nitrate reduction process. Microbes were able to use the Eurobitum as an electron donor to reduce nitrate. Microbial nitrate reduction resulted in a significant production of nitrite, which has the potential to oxidise the Boom Clay and thus affect the mobility of some radionuclides. Irradiation of ionexchange resin may result in formation of bioavailable gas compounds. High resolution electron microscopy studies have been undertaken, along with detailed microbial studies to characterise a new species of bacteria, Stenotrophomonas bentonitica, from bentonites. S. bentonitica is able to reduce selenite. Studies at the Mont Terri Underground Rock Laboratory have examined the potential for methanogenesis to develop in Opalinus Clay as a consequence of reaction of H2 with inorganic carbon. Methanogenesis seems to be controlled by the bicarbonate concentration, which was too low in the in situ and microcosm experiments so methanogenesis remained thermodynamically unfavourable. Through the long term LLW gas generation experiment at Olkiluoto, biogeochemical modelling, simulated the effect of increased methanogenesis as being related to neutralisation of the initial alkaline tank water and delayed consumption of DOC. Consideration of concentrations of sulfide in the tank water also raises the possibility that methanogenesis could be inhibited by sulfide. The model accurately represents the observed rate of CH4 generation during the 18 years of operation, including an observed doubling of gas generation that occurred after 8 years.
WP2: The inventory of reducing gases was compiled in order to address the geochemical constraints of biological activity. The gas phase is dominated by N2 and CH4.The boundary conditions constraining the formation of sulfide in deep geological disposal conditions have been evaluated. Presence of electron donors is one of the most important controlling factors for microbial sulfate reduction. In laboratory experiments with deep groundwater populations, acetate was overall the most efficient activator of the studied microbial communities which indicates acetateâ€™s important role as an electron donor for different Olkiluoto deep subsurface groundwater communities. In compacted clay there are several variables of importance for bacterial life, such as clay type, pH, temperature, transport conditions, water content, pressure, pore space and pore water composition. Significant acetate formation from natural organic matter present in the clays was detected in the studied bentonites. Thus bacterial activity, is possible also at densities where sulfide production could not be detected. The microbial viability appears to be linked to the degree of saturation of the bentonite which may take several years. Sulfide has been found to reduce ferric iron in several bentonites under the formation of elemental sulphur, ferrous iron and iron sulfide. This immobilisation effect can reduce the mass of sulfide that corrode metal canisters over repository life times. A laboratory scale benton

Final results

The findings in the MIND project has contributed to that microbial processes in repository environments are now being considered e.g. when setting the requirements on engineered barriers such as buffer and backfill leading to significant refinements of safety case models currently being implemented