Glancing Angle X-Ray Absorption Spectroscopy to Investigate Changes in the Local Atomic Structure around Uranium in Leached Glasses.

Available from UMI in association with The British Library. Requires signed TDF. The possible transport of radioactive elements from a glassy matrix into the biosphere as a result of water corrosion is one of the main limitations to the long -term storage of nuclear waste in glass. The technique of glancing angle fluorescence x -ray absorption spectroscopy has been developed and applied to investigate changes due to leaching in the local atomic structure surrounding uranium in borosilicate glass. The index of refraction of most materials is less than unity for x-rays. For angles less than the critical angle varphi_{rm c} a well -collimated monochromatic x-ray beam incident on a flat surface will undergo total external reflection. The penetration depth into the second medium is also angle dependent and ranges from several tens of angstroms below varphi _{rm c} to the absorption limit of a fraction of a militetre well above varphi_{rm c}. The geometry of the experiment was tested in the laboratory using thin film samples deposited on flat glass substrates. Conventional EXAFS measurements were carried out on reference samples to provide accurate atomic phase shifts for the glass analysis. Glancing angle EXAFS and XANES measurements were made above and below varphi _{rm c} as a function of leaching time at 100^circ C, giving penetration depths of ~30 A and ~1.4 μm respectively. In all the glasses the uranium is present as U ^{6+} in a uranyl like structure. As leaching proceeds the uranium hydrates and agglomerates. The clustering gives rise to the formation of planar lattices or islands. The effects of hydration can be seen both on the uranyl structure and by comparing wet surfaces with dried surfaces. This is confirmed by XPS and ellipsometry measurements. On the basis of the modified random network model for the glass structure it is suggested that the uranium occupies a modifier site together with sodium. This explains its mobility during the initial corrosion process. After 90^ ' leaching there is evidence for the formation of hydrated uranyl silicates suggesting that the glass former network also starts to dissolve.