Radionuclide interactions with materials relevant to a geological disposal facility
thesisposted on 02.06.2017, 11:30 authored by Oliver Preedy
Materials representative of those found in a Geological Disposal Facility (GDF) for the long-term storage of nuclear waste have been investigated for their ability to retard the movement of ionic species found in nuclear waste. Fe1-xO, Fe2O3, Fe3O4 (from steel corrosion) and sandstone (bedrock) used as physical barriers in the GDF have been treated using solutions of pH 7-13 which are representative of the leachate expected from concrete encapsulation of waste in contact with ground water. A mimic of portlandite cement, Ca(OH)2 was also prepared carbonate-free via a sacharate method for use in these leachate experiments. Materials have been characterised using a mixture of techniques such as Powder X-ray Diffraction (PXRD) and Infra-red Spectroscopy which focus on the bulk, short range techniques such as Extended X-ray Absorption Fine Structure (EXAFS), Scanning Electron Microscopy(SEM) and Nuclear Magnetic Resonance(NMR) and physical measurements such as diffusion experiments and fluorescence spectroscopy. Characterisation of the bulk materials before and after treatment using PXRD and SEM indicates that high purity iron oxides are affected differently by the solutions of varying pH. While not detectable by bulk techniques, SEM analysis evidence of the surface of the materials showed that Fe1-xO was deleteriously affected by solutions with pH > 7 more than the more oxidised materials. Initially needle-like crystals formed on the surface of Fe1-xO that are characteristic of goethite which at long aging times up to 168 h, showed transformation to crystal morphologies characteristic of Fe2O3. As the alkalinity increased, the transformation of Fe1-xO to Fe2O3 slowed. Dissolution of the iron surfaces in the solutions of pH 7-13 were determined by measuring the concentration of dissolved iron using ICPMS. While Fe1-xO and Fe3O4 followed first order kinetics, the dissolution kinetics for Fe2O3 appeared more complex. As the alkalinity increased, the rate constant for dissolution decreased in all cases indicating that higher pH is better for containment due to the formation of a passivated surface layer evidenced by SEM. The sorption of uranium to the iron oxide surfaces was investigated as a function of pH (7-13). In all cases, there was evidence of uranium sorption. The greatest sorption was evidenced when Ca(OH)2 was present which is most likely due to the precipitation of the known phase, calcium uranate. In the absence of calcium hydroxide, the sorption of uranium to the iron oxide surfaces decreased as the pH increased, reflecting the increase in formation of the anionic uranium species. In the presence of carbonate, the sorption of uranium onto the surfaces also decreases reflecting the formation of the soluble uranyl carbonate species. NMR spectroscopy of uranyl species in solution indicates that the chemical shift is strongly affected by pH shifting from 163 ppm to 175 ppm as the pH changes from 7 to 13 and allowing the uranium speciation to be used as a pH probe. A much -2- smaller shift in respect of temperature of less than 0.5 ppm was observed in the temperature range studied between 25 and 50°C. The quality of fluorescence spectra has been shown to be strongly affected by complexing species present in solution, the best spectra achieved with non-complexing species such as perchlorate. Migration experiments of the radionuclides uranium, thorium and technetium has been investigated by placing sandstone cores in alkaline solution and analysing both the water itself and the core to examine retention and transport. The results determined that the technetium diffused readily through the sandstone matrix. The uranium and throrium did not achieve breakthrough. This was attributed to the low solubility’s and the formation of stable precipitates.