An Investigation into the Oxidation State of Molybdenum in Simplifies High Level Nuclear Waste Glass Compositions
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$Q,QYHVWLJDWLRQLQWRWKH2[LGDWLRQ6WDWHRI0RO\EGHQXPLQ6LPSOLILHG+LJK/HYHO1XFOHDU :DVWH*ODVV&RPSRVLWLRQV R.J. Short, R.J. Hand and N.C. Hyatt, ISL, Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building Mappin Street, Sheffield, S1 3JD, UK The redox state of Mo in glasses containing simplified simulated high level nuclear waste (HLW) streams has been investigated using Electron Spin Resonance (ESR) and X-ray Photoelectron Spectroscopy (XPS). Melts performed in highly oxidising or neutral (air) atmospheres contained Mo6+, but our study indicates that the Mo can be at least partially reduced to Mo4+ or Mo3+ by melting in a reducing atmosphere. The implications for glasses containing a full simulated HLW stream are that the formation of detrimental crystalline phases containing Mo6+ upon heat treatment may be avoided by reducing the oxidation state of Mo during melting. ,1752'8&7,21 Alkali borosilicate glasses are used by British Nuclear Fuels (BNFL) to vitrify HLW raffinates originating from the reprocessing of spent nuclear fuel. Experiments using simulated nuclear waste streams have shown that if the waste loading in these glasses is high (i.e.>20wt%), the glasses may undergo devitrification, which can have detrimental effects on the chemical durability [1]. When Mo is present in concentrations greater than 1wt% the potential for devitrification is increased. The crystalline phases that Mo forms with other fission products are generally termed yellow phase. Yellow phase consists of a mixture of alkali sulphates, chromates and molybdates. Some yellow phase can be highly water soluble and the vitrification process should be optimised in order to minimise its formation. Experiments were performed using a full simulated HLW stream based upon 4:1 ratio of high burn up UO2/mixed oxide (HBU/MOX) fuel reprocessed waste, a composition currently being considered for vitrification in the future. As part of a study into the devitrification of HLW glasses, compositions containing elevated waste loadings of up to 35wt% stimulant waste have been prepared, and it was found that at these waste loadings, these glasses tended to devitrify upon heat treatment (representing the slow cools of the real vitrification product) and the crystalline phase formed was a powellite type molybdate analogous to yellow phase [2]. The Mo in these molybdate phases was hexavalent in the form of (MoO4)2- ions. It was suspected that if the oxidation state of the Mo could be lowered during the melting process, the formation of powellite type molybdates upon heat treatment could be avoided. This paper is a review of ongoing experiments designed to investigate the oxidation state of Mo in glasses containing a simplified version of the simulated HLW stream (in order to simplify analysis of results). Whilst Mo redox in silicate glasses has been investigated before [3,4] the aims of this paper were to investigate Mo redox behaviour under varying redox conditions in an alkali borosilicate glass composition th
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