In Situ Characterisation of Model UK Nuclear Waste Glasses by X-ray Absorption Spectroscopy Under Process Conditions
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In Situ Characterisation of Model UK Nuclear Waste Glasses by X-ray Absorption Spectroscopy Under Process Conditions Neil C. Hyatt,1* Andrew J. Connelly,1 Martin C. Stennett,1 Francis R. Livens,2 and Robert L. Bilsborrow.3 1
Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD. UK.
2
Centre for Radiochemistry Research, Department of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL. UK.
3
STFC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD. UK.
ABSTRACT The local co-ordination environment of Zr in a model alkali borosilicate glass, of relevance to nuclear waste immobilisation, was studied by in situ X-ray absorption spectroscopy between 25 – 1060 oC. Analysis of Zr K-edge XANES spectra, in comparison with those of well characterised standards, demonstrated, for the first time, the reversible transformation of ZrO6 to ZrO7 co-ordination polyhedra at high temperature. This observation was rationalised on the basis of the combined effects of network modifier cation diffusion and thermal expansion. INTRODUCTION PUREX reprocessing of nuclear fuels, in the UK and France, affords a High Level Waste stream comprising fission products, corrosion species, minor actinides, and traces of U and Pu, that is vitrified in an alkali borosilicate matrix. The choice of this vitreous matrix is due to the considerable flexibility of the glass structure with respect to chemical constitution, permitting vitrification of waste streams of variable composition [1, 2]. These alkali borosilicate glasses may be fabricated at reasonable temperatures, ca 1060oC, and show superior chemical durability with respect to other vitreous wasteforms (e.g. phosphate based glasses) [1, 2]. In Europe, vitrification of high level nuclear waste is currently undertaken at the Sellafield Waste Vitrification Plant (WVP), operated by Sellafield Ltd., and the La Hague vitrification plants, R7 and T7, operated by COGEMA. An understanding of the structure of nuclear waste glasses is essential in order to fully rationalise the relationship between glass composition, structure, and properties such as durability, viscosity and crystallisation potential. The complex chemical nature of such nuclear waste glasses demands an element selective spectroscopic technique, such as X-ray Absorption Spectroscopy (XAS), in order to determine both oxidation state and local chemical environment. Indeed, XAS investigations have been crucial in establishing the structural role of key elements, such as Mo, Zr, Zn, Sr, Tc, Ce, Pu, Pd and Te, in model nuclear waste glasses [3-12]. XAS studies of quenched glasses have provided an insight into important structure-property relationships; for example, the rapid leaching of Mo from simulant waste glasses is rationalised on the basis of the presence of isolated MoO42- species encapsulated by the polymeric glass
network [7, 9, 10]. However, the oxidation state and co-ordination environment determined in quenched materials are not
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