Microbial impacts on gas production in LLW/ILW

Abstract Gas production from low- and intermediate-level nuclear waste (LLW/ILW) is important to the safety of near surface and deep geological nuclear waste repositories through the release of gaseous radionuclides and pressurization that can ultimately affect the transport of radionuclides in groundwater. The main gases generated from LLW/ILW are H2 from anaerobic corrosion of steel and CH4 from organic (cellulose) wastes. Microbes mediate CH4 generation and also can consume H2 as an energy source such as for the reduction of sulfate. A 20-year Gas Generation Experiment at Olkiluoto, Finland has furthered the understanding that microbes play in mediating gas production from LLW. The processes involved in gas generation from the experiment have been elucidated by, interpretation of gas and chemical data, DNA pyrosequencing and biogeochemical modeling. Methanogenesis develops initially mainly by utilizing hydrogen formed by anaerobic corrosion. In contrast, generation of CH4 from cellulose develops more slowly as a consequence of the complexity of the preceding cellulose degradation, fermentation processes, competition with sulfate reduction processes, and inhibition effects related to high pH and aqueous sulfide. The maximum rate of CH4 production develops after 8 years, when concrete buffered high pH regions of the experiment are neutralized by cellulose degradation and fermentation.