Identifying the Limits and Impacts of Microbial Metabolism in Geodisposal Scenarios

Abstract

A multi-barrier geological disposal facility (GDF) is the preferred method for the long-term disposal of radioactive waste in the UK. Currently, the UK is restarting the GDF siting process, approaching interested communities and forming working groups. Until a site is selected, no GDF concepts and materials can be ruled out, and illustrative concepts are used to develop the generic safety case in the interim. Despite GDFs being extreme environments, microbiology is an emerging concern across all GDF illustrative concepts. The aim of this thesis is to contribute to the understanding of the limits and impacts of microbial metabolism in three main GDF concepts (bentonite, cement, and salt) using multidisciplinary approaches consisting of microbiological (next generation sequencing, comparative genomics, primer design, stable isotope probing), geochemical (ICP, IC), imaging, and corrosion science (SEM, GI-XRD, FIB-SEM) techniques. For cementitious GDFs, the work in this thesis focuses on the degradation of key organics at high pH (isosaccharinic acid (ISA, produced from the alkaline hydrolysis of cellulose) and gluconate (a cement additive) that can complex with radionuclides and enhance their migration to the wider geosphere. Validation for a proposed ISA degradation pathway was achieved through comparative genomics and culture work, alongside progress in designing a primer set that aims to detect ISA-degrading bacteria within the environment that could have applications in supporting GDF siting and safety case development. For bentonite concepts, the focus was on the impact of salinity (particularly around the brackish-saline boundary) and microbially influenced corrosion resulting from sulfate-reducing bacteria (SRB) activity. Salinity substantially impacted the SRB community enriched from a candidate GDF bentonite buffer, resulting in changes of the dominant SRB genera detected using 16S rRNA gene sequencing. Copper (a candidate waste container material) corrosion increased with salinity (0-28 g L-1) in the presence of active SRB communities. For salt concepts, work was initiated in collaboration with Helmholtz Zentrum Dresden Rossendorf (HZDR, Germany) on measuring slow halophilic metabolism using novel microcalorimetry techniques but was not completed due to travel restrictions (COVID-19). Progress with this work can be found in the appendices. Overall, this thesis has contributed substantial insights into potential microbial impacts on geodisposal scenarios, through extensive review of the literature around extremophilic microorganisms (contributing to a book chapter and review paper), taking the first step for using novel techniques for the characterisation of alkaliphilic microbial communities (including stable isotope probing), and further understanding the impact of SRB microbial diversity and salinity on the corrosion of copper (a GDF-relevant container material). Furthermore, this thesis provides an initial framework for implementing microbiology into a GDF safety case through the development of a Claims-Arguments-Evidence methodology, using a bentonite concept as an example.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Samuel Shaw (Supervisor) & Jonathan Lloyd (Supervisor)