The development of a tool for the determination of mobile radionuclides in contaminated land.

The UK is currently facing significant challenges in the nuclear sector. Demands for low-carbon energy are driving a move towards a new generation of nuclear power stations, yet the legacy of past nuclear activities has still to be fully addressed. Groundwater contamination by long-lived radionuclides is a major concern at several sites around the UK and disposal of radioactive waste requires greater understanding of radionuclide environmental behaviour. The environmental behaviour and fate of radionuclides and their potential for exposing human populations are dependent on their chemical properties. Of particular concern is the capacity for radionuclides to migrate away from the primary source of contamination via groundwater; migration is controlled by the distribution between solid and dissolved phases and the chemical form of the dissolved phase (speciation). However, speciation is difficult to predict, being dependent on complex biogeochemical and hydrological interactions. For example, redox conditions and the presence of organic matter can have significant impacts on radionuclide mobility. The nuclear industry has identified four principal contaminants: Tc-99, Sr-90 and the isotopes of U and Pu. They: (1) occur frequently in contaminated sites; (2) are long-lived (half-lives of 29 and 4500000000 y); (3) are radiologically-significant, resulting in the majority of the dose to critical groups. However they are difficult to measure requiring complex radiochemical separations. Diffusive Gradients in Thin films (DGT) is a well-established in-situ speciation technique for measuring mobile trace metals in water, soil and sediments, however limited work has been conducted with radionuclides. The technique relies on the species of interest (metal, radionuclide, nutrient, etc) diffusing through a thin gel and binding irreversibly to a resin. The mass accumulated during deployment is measured and used to calculate the concentration of the species of interest at the gel / solution interface. The DGT technique offers a number of advantages over conventional sampling and analysis: (1) the radionuclide of interest is selectively bound to the resin, and no further separations are needed, greatly reducing analyst time; (2) by varying gel thickness and pore size, different radionuclide species are able to diffuse through the gel and bind to the resin, allowing dissolved phase speciation to be investigated; and (3) the in-situ nature of the technique avoids post-sampling changes to speciation. Thus, the DGT technique offers a simple and rapid method for the rapid screening and monitoring of contaminated sites, and it can support investigations into the mechanisms underpinning radionuclide migration. Thus, we propose to develop new DGT methods for the analysis of these contaminants. The project has five objectives. Objective 1 is to develop ion-specific resins with the manufacturer. Preliminary work has identified candidate resins, and the student will work on optimising resin chemistry for environmental applications. Objectives 2 and 3 are to optimise the analytical methods for the determination of the candidate radionuclides, maximising the sensitivity of the technique, including the development of a novel liquid scintillation cocktail. Objective 4 is to validate the DGT method for each resin under solution conditions, testing for potential groundwater interferences. Objective 5 is to complete a field validation of the method on contaminated sites, comparing field measurements with laboratory testing of the same groundwaters. Training will be provided to the student with the following outcomes: (1) the student will become a competent radioanalytical chemist, a shortage specialism in the UK; (2) the student will gain awareness of the needs and challenges of both UK SMEs and the nuclear industry; and (3) a new tool will be provided to the nuclear industry for more rapid and detailed assessment of contaminated groundwater.