Assessment of the impact of leak zone processes in the natural attenuation of radionuclides
Monitored Natural Attenuation (MNA) is a contaminated land remedial method which takes advantage of naturally occurring processes. The Nuclear Decommissioning Authority has identified that in some situations the use of NA/MNA may be appropriate in managing radiologically contaminated ground on its sites. In contaminated land assessments NA processes occurring close to the leak point in “leak zones” are often not considered as: 1) These areas are heavily contaminated and difficult to sample; 2) Soil volumes in leak zones are relatively small and hence their contribution to NA could be interpreted to be negligible. As a result when model predictions of contaminated land risk/liabilities are made, these often simply assume that NA processes in leak zones are the same as those occurring further down-gradient. An ongoing study into the fate of C-14 in a historical alkaline leak at Sellafield indicates that in this case NA leak zone processes –i.e. the precipitation of carbonate minerals - are having a profound effect on C-14 attenuation, and are still controlling the site distribution of this radionuclide 35 years after the leak ceased.
This fully funded studentship will investigate the contribution that the leak zone makes to NA in other examples on the NDA estate, including differing soil and groundwater types and leak chemistries, focusing on 90Sr and U behaviour. The study will use archived soil samples from Bradwell, Hunterston, Sellafield and Capenhurst. Leak chemistries vary throughout the NDA estate, with extreme chemistries (e.g. pH, ionic strength, organics) prevalent in some circumstances. Firstly, the project will use batch alteration experiments (in concert with state of the art electron microscopy and X-ray analysis) to assess the change in soil composition (mineralogy, organic content, and surface structures), in a range of leak conditions, in order to predict long term changes in bulk sorption behaviour. Changes in bulk sorption behaviour with respect to Sr and U will then be determined in batch and column experiments, with a focus on directly determining how soil alteration affects radionuclide speciation and fate in the environment (e.g. using synchrotron based XAS analysis). Lastly, the experimental results will be use to inform both thermodynamic models of radionuclide speciation and reactive transport models to assess the extent that near-field chemistry must be considered in MNA programmes at NDA sites (and directly feeding into site closure plans).
1. Identify Natural Attenuation (NA) processes affecting 90Sr, 137Cs and U mobility occurring in historical leak zones on NDA sites via experimentation and modelling.
2. Quantify the significance of any identified NA processes so that the research may be applied to the reduction of uncertainty in numerical models used for the prediction of Risk to human health and non-human biota; Waste volume estimation; success of MNA strategies and any requirement for intervention & Site clean-up costs.
PhD fully-funded by National Nuclear Laboratory for a UK or EU national only. Funding will include tuition fees (£4,100 for 2015/16), tax-free stipend (£14,057 for 2015/16) and research costs. Start date expected to be October 2016.
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FTE Category A staff submitted: 79.20
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