Biogeochemistry of Low Level Radioactive Waste Disposal

Abstract

The UK has a considerable radioactive waste inventory that has accumulated from almost a century of nuclear power and weapons activity. The largest portion of this inventory is low level radioactive waste (LLW), which constitutes operational and decommissioning wastes and contains contaminants including radionuclides, heavy metals and organic ligands. Safe long-term disposal practises are essential to minimise harm to the environment from LLW disposal, and in the UK this currently takes place at the LLW Repository (LLWR) in West Cumbria. Since the 1980s, disposals at the LLWR have involved compacting wastes into steel or Fe drums before grouting and emplacing wastes in engineered concrete vaults. This approach is intended to prevent metal and radionuclide migration from the repository post-closure by promoting the development of alkaline and reducing conditions upon re-saturation. However, organic ligands, such as citrate, are present in LLW in significant quantities from decontamination wastes and their persistence has the potential to enhance radionuclide and metal mobility by chelation. Thus, the biogeochemical fate of organic ligands is a topic of interest in LLW management and relevant environmental safety case documents. Historically, microbial degradation has been assumed to be the dominant removal mechanism for citrate (and other organic ligands) from cementitious LLW repositories. However, the biodegradation of citrate has not been investigated under anaerobic, high pH conditions relevant to a cementitious LLW repository. This includes in the presence of relevant metal contaminants (Ni, U) where complexation and toxicity effects may impact the biogeochemical fate of both citrate and the metal species. In addition, the impact of bacterial metabolites, notably CO2, from citrate/organic degradation on the integrity of cementitious repository features has been relatively unexplored in the context of LLW disposal. This project has focussed on addressing these knowledge gaps and gaining an understanding of anaerobic citrate biodegradation at elevated pH, in the presence of Ni, U and cement. Throughout this project, anaerobic microcosms and enrichment incubation experiments were set up using a well characterised high pH sediment inoculum from a legacy lime working site. Experiments were studied using a variety of analytical techniques including IC, ICP-MS, ESI-MS, electron microscopy with EDS, XAS, XCT and 16S rRNA gene sequencing. In the first study, anaerobic microcosm experiments explored microbial citrate degradation at pH 10, 11 and 12. Results showed citrate was oxidised and removed as a complexant by bacteria using nitrate or Fe(III) as the electron acceptor at > pH 11. In the Fe(III)-reducing experiments, nano-particulate magnetite formed after bioreduction, and interestingly, Fe(II) ingrowth was observed at pH values recorded up to 11.7. In the second study, citrate was supplied as both a metal complexant and electron donor in incubation experiments which contained Ni(II) or U(VI) at three concentration levels, under nitrate- and sulfate-reducing conditions at pH 9-10. The highest metal concentrations (1 mM) were toxic to bacteria, however, at lower levels (0.1-0.01 mM) citrate was fully biodegraded in both nitrate- and 10 sulfate-reducing experiments. In the Ni experiments, a Ni-citrate complex formed and underwent biodegradation. Metals were immobilised in the sulfate-reducing experiments; Ni was precipitated as an insoluble Ni(II)-sulfide phase and U(VI) was bioreduced and precipitated as a mix of uraninite and non-crystalline U(IV)-phosphate. Finally, citrate biodegradation was investigated in the presence of cement pellets under nitrate-reducing conditions. In the presence of cement, citrate - - was fully oxidised, to HCO3 , and removed a potential metal complexant. The metabolic HCO3 2+ reacted with aqueous Ca in the system to form calcite precipitates which coated the cement pellets and sealed cement cracks at the surface-level, blocking potential pathways for aqeuous contaminant migration. Overall, the findings from this project provides insights into the biogeochemical fate and behaviour of citrate in a cementitious LLW repository; including the impacts of citrate biodegradation on metal contaminants and cementitious repository features. Ultimately, these findings will directly inform environmental safety cases for LLW disposal.

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