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Effect of sediment and groundwater flow heterogeneity on accurately modelling radionuclide transport at UK nuclear sites

  • Full or part time
  • Application Deadline
    Friday, February 28, 2020
  • Funded PhD Project (UK Students Only)
    Funded PhD Project (UK Students Only)

Project Description

The UK Nuclear Decommissioning Authority oversees the removal of buildings and radioactive wastes present at UK nuclear sites. Its purpose is to deliver the decommissioning and clean-up of the UK’s civil nuclear legacy in a safe and cost-effective manner, and where possible to accelerate programmes of work that reduce hazard. Many UK nuclear sites, including Sellafield the largest and most complex site, are located on coastal plains and are underlain by thick fluvial-glacial alluvium deposits. These sediments are highly heterogeneous in nature and consist of layers ranging from clays right up to cobble sized materials. Historically there has been events where unauthorised releases of radioactive fluids have occurred containing radionuclides such as Tc-99 and H-3 that are highly mobile in groundwater, and others such as Sr-90, C-137 and U which display sorption controlled transport. Other chemical contaminants, such as hydrocarbons or acids, have also been released into groundwater. Therefore, extensive radioactive and chemical contamination plumes exist in a very complex unconsolidated aquifer structure relating to different releases from specific waste types and processes over time.
Understanding groundwater flow patterns and reactive transport of contaminants represents a considerable risk to closure plans. Regulatory planning commonly assumes little or no sorption, leading to unrealistically high estimates for the rates of plume migration (such that ‘safe to walk away’ end state conditions cannot be guaranteed). On the other hand, sorption data from experiments using only <2mm sediments may lead to overly optimistic estimates of plume retardation, especially if groundwater flow is mostly occurring through courser sediments with >2mm grainsize. Interaction of plumes with saline influenced groundwater at site boundaries also adds additional complexity to understanding how long term migration of contaminants will occur. This project will use available environmental monitoring data and samples recovered from UK nuclear sites in laboratory experiments designed to discover how sediment and groundwater heterogeneity controls the spread of radioactive contamination in groundwater plumes. Further experimentation will then investigate the effect of soil washing (fines removal) on radionuclide distributions between size fractions and waste volume reduction. The effect of groundwater co-contaminants (such as acid or nitrates) on the preservation of high sorption grain-coatings will also be investigated. The results produced will be used to inform the production of new predictive models for use at affected sites as part of site closure safety cases, and; to help decision making processes involved with choosing waste minimisation and remediation technologies.

Funding Notes

Funding is for Uk and EU nationals who meet the normal three years UK residency requirements. The award will pay tuition fees (£4,500 for 2019/20), a tax-free stipend at the UKRI rate (£15,009 for 2019/20) with a CASE enhancement of £3,000 pa, and a significant research and training grant budget.

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