Microbe - radionuclide interactions in legacy nuclear waste systems (EPSRC iCASE)

The Sellafield site has been in operation since the 1950s and has recently transitioned from power production and fuel reprocessing to decommissioning. Decommissioning involves dismantling facilities, emptying spent nuclear fuel ponds and storage of wastes pending long term disposal. One spent nuclear fuel pond of intense concern at the Sellafield site is the First Generation Magnox Storage Pond (FGMSP). This pond contains a diverse inventory of waste including spent fuel, corroded Magnox sludge and extraneous environmental inputs.

Recent studies have shown the presence of a broad range of microorganisms capable of surviving in the surface waters of the FGMSP. Microorganisms are known to affect the speciation of radionuclides resulting in changes in mobility. Microbes may also affect the mobility of radionuclides due to the precipitation of radionuclide containing biominerals known as colloids. While these processes are well-known, little investigation has been conducted to investigate these processes at the high pH values observed in spent nuclear fuel ponds such as the FGMSP.

This study will aim to identify the microorganisms present in the pond system, from aerobic surface waters into the potentially anoxic sludge environment that has accumulated at the base of the pond. This will be done by collecting samples of the sludge and water environments and carrying out 16S rRNA and 18S rRNA gene sequencing to identify the prokaryotic and eukaryotic organisms present and their relative abundances and use metagenomic techniques as appropriate. The interactions of key radionuclides with microorganisms will also be investigated by setting up microcosm scale laboratory experiments and using a variety of techniques and modelling to identify the impact of microbial processes on key radionuclide speciation. Examination of the formation of radionuclide containing biominerals will be conducted and further investigation into the role of microbes in their formation and stability will be explored using a range of imaging and spectroscopy techniques.

Investigation into these biogeochemical processes under conditions relevant to the FGMSP will provide insight to the survival mechanisms of microorganisms in the ponds and the fate of key radionuclides in the pond system, all of which have implications for pond operation, retrieval and disposal operations at Sellafield.

Aims and objectives
The overall aim of this project is to gain a greater understanding of the biogeochemical processes occurring in the FGMSP.
There are 3 main objectives of this project:
1. To determine what microorganisms colonise the anaerobic environment and sludge in the FGMSP. Water and sludge samples from the FGMSP will be collected. After DNA extraction, both 16S and 18S rRNA sequencing will be carried out to identify the prokaryotic and eukaryotic organisms present, with metagenomic tools applied as appropriate. Microbial profiles will be compared to geochemical measurements to help interpret microbial processes.
2. To determine the fate of specific radionuclides in the presence of these microorganisms. Microcosm experiments inoculated with microbes similar to those found in FGMSP will be set up and incubated under a range of biogeochemical conditions with key radionuclides added. The speciation of a key radionuclides will be examined and the formation of any minerals will be observed (see objective 3). Techniques to be used will include ICP-MS/IC/EXAFS/EM/PHREEQC to model expected reactions.
3. To identify the key biominerals present in the FGMSP and the interactions of these with key radionuclides which may alter radionuclide speciation and mobility. Ideally, we hope to collect water and sludge samples and identify any key minerals present. Given the challenges associated with handling these samples, parallel work will focus on the analysis of mineral phases and radionuclide interactions from microcosm incubations using the techniques above.