The biogeochemistry of gas generation from low-level nuclear waste: Microbiological characterization during 18 years study under in situ conditions

Abstract

Abstract In Finland low level radioactive waste (LLW) contains considerable amounts of cellulose and hemicellulose-based material. Metals are also present in LLW and steel containers are used to store and dispose waste. The microbial degradation of cellulose and hemicellulose, together with the utilization of hydrogen generated by metal corrosion, will result in gas generation under final repository conditions. Microbially mediated LLW degradation and gas generation processes can influence the performance of multi-barrier systems, such as by accelerating corrosion and can affect the mobility of radionuclides from the repository. A large-scale in situ Gas Generation Experiment (GGE) was established in 1997 in Olkiluoto, Finland, to simulate the gas generation from LLW under geological repository conditions. A significant observation from the GGE was that the pH conditions were heterogeneous (pH 11 to 6), providing optimal neutral pH niches for microbial activity from the outset of the experiment. Over the extended time scale of the experiment, chemical conditions were stabilized and differences in the microbial abundances and community structure in various GGE compartments became less significant. The results demonstrate that LLW is converted to methane and carbon dioxide by a succession of anaerobic processes within a complex microbial consortium. Several genes related to cellulose and hemicellulose hydrolysis were detected using bacterial 16S rRNA gene sequencing and PICRUSt bioinformatics software. In addition, microbial groups with potential to metabolise formed saccharides to acetate, hydrogen and volatile fatty acids were detected. Hydrogenotrophic methanogens dominated after one year of operation, which was related to the utilization of hydrogen generated by anaerobic corrosion of steel and metallic waste. Acetoclastic methanogens were detected for the first time in 2005, coinciding with an increase in gas generation rate. Sulphate reducers were the most significant microbial group competing with methanogens for electron donors and their relative ratio compared to methanogens decreased considerably during the operation of the GGE. From the microbiological point of view, the results are consistent with the sequence of microbial processes simulated by previous biogeochemical modelling studies of the experiment.