Analysis of Nuclear Fuel Cycle Materials by X-ray Absorption Spectroscopy
Nuclear energy can be used to reliably generate large quantities of electricity while providing minimal lifetime CO2 emissions. Given the extreme importance of safety in the nuclear industry, it is necessary to have a fundamental understanding of the materials used throughout the nuclear fuel cycle. It is of particular to importance to develop an understanding of these materials at an atomic level. In this thesis, X-ray absorption spectroscopy (XAS), along with several other X-ray based techniques, has been used to study materials that are produced or proposed for use in the nuclear fuel cycle. Uranium mining and milling operations generate large quantities of waste, known as mine and mill tailings. At their McClean Facility in Northern Saskatchewan, AREVA Resources Canada disposes of the tailings waste using the JEB Tailings Management Facility (TMF). AREVA monitors the mineralization of elements of concern (i.e., Ni, As, Fe, Mo, Ra, and U) within the TMF as part of its on-going commitment towards managing the facility’s environmental impact. Molybdenum (Mo) is predicted to mineralize as insoluble powellite (CaMoO4) within the TMF. However, no experimental evidence confirmed the presence of powellite in the TMF. In Chapter 2, the presence of powellite, and other Mo-bearing minerals, was determined using powder X-ray diffraction (XRD), X-ray fluorescence imaging, and Mo K-edge XAS. The results of this study confirmed that powellite was present in the TMF and showed that Mo K-edge XAS was the only effective way to detect the Mo minerals within the TMF. New materials for use as nuclear fuels were also investigated in this thesis. Spent nuclear fuel must be securely stored for long periods of time due to the presence transuranic elements (TRU; i.e., Pu, Am, Np, Cm), and the use of inert matrix fuels (IMF), which consist of actinides embedded in a neutron transparent (inert) material, have been proposed for to “burn-up” or transmute these TRU species. Stabilized ZrO2 materials have been proposed for use in IMF applications, and in Chapter 3 the thermal stability of a series of NdxYyZr1-x-yO2-\delta materials made by a ceramic synthetic route have been studied using powder XRD, scanning electron microscopy (SEM), and X-ray absorption spectroscopy. (Nd was used as a surrogate for Am.) The results of this study showed that the fluorite structure of the NdxYyZr1-x-yO2- \delta materials was stabilized when y >= 0.05, and that the local environment around Zr was independent of composition or annealing temperature. The effect of synthetic method on the thermal stability of the NdxYyZr1-x-yO2-\delta materials was also determined, and this is the subject of Chapter 4. In this study a series of NdxY0.25-xZr0.75O1.88 materials were synthesized using a low-temperature co precipitation synthesis, and these then annealed at 1400 °C and 1500 °C. The as-synthesized and annealed materials were characterized by powder XRD, SEM, and XAS. This study confirmed that the thermal stability of the materials was dependent on synthetic method, and that materials made using a solid-state method were superior to those produced by a solution-based approach. Y-stabilized zirconia has a low thermal conductivity, which is not ideal for a nuclear fuel. The thermal conductivity could be increased if a lighter cation, such as Sc, was used to stabilize the fluorite structure. In Chapter 5, the thermal stability of a series of NdxScyZr1-x-yO2-\delta materials was investigated. The as-synthesized and annealed materials were studied by powder XRD, SEM, and XAS. These results showed that the fluorite structure was only stable in the annealed materials when x+y >= 0.15 and y >= 0.10. The results of this study provided insight into the possible use of scandia-stabilized zirconia for use as an inert matrix fuel. This studies presented in this thesis have used X-ray absorption spectroscopy and a number of other techniques to characterize materials important to the nuclear fuel cycle. The studies presented here were only possible because of the unique information that can be obtained using XAS. This thesis serves to highlight the importance of XAS as a technique and how it can be applied to solve problems related to the material science of the nuclear fuel cycle.
XAS, Nuclear Fuel, X-ray Absorption Spectroscopy, Inert Matrix Fuels, Mill Tailings
Doctor of Philosophy (Ph.D.)