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Incorporation of molybdenum in nuclear waste glasses.

Short, Rick (2004) Incorporation of molybdenum in nuclear waste glasses. PhD thesis, University of Sheffield.

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Abstract

Alkali borosilicate glasses have been widely used as a matrix for the vitrification of high level radioactive waste (HLW) over the last 40 years. Increasing the waste loading of the HLW glasses could increase the efficiency of the vitrification process, but could also increase the tendency for devitrification, which could have a negative effect on the overall properties of the glass. Molybdenum (found in many HLW compositions) has a low (~1 wt%) solubility in borosilicates, and when present in higher concentrations in HLW melts, tends to combine with other elements from the HLW to form crystalline material phases termed "yellow phase". Yellow phase (a mixture of alkali molybdates, chromates and sulphates) increases the corrosion rate of the Inconel melting furnaces used in HLW vitrification in the UK, and can be water soluble in the solid state. It is therefore undesirable in a product that may eventually come into contact with ground water in an underground storage repository. Simulated (non-radioactive) HLW glasses have been melted in various atmospheres, with a range of waste loadings, to investigate the factors influencing the formation of molybdenum containing crystalline phases. X-ray diffraction and transmission electron microscopy showed that molybdates with a powellite type structure formed upon heat treatment of highly loaded glasses melted in air, and that these molybdates were capable of incorporating the majority of mono, di and trivalent cationic species present in the simulated HLW. The distribution of the cationic species on the A sites of the powellite type molybdate structures was shown to be random using extended x-ray absorption fine structure (EXAFS). Electron spin resonance and x-ray photoelectron spectroscopy were used to show that the oxidation state of the Mo in simplified simulated HLW glasses could be partially reduced by sparging the melt with a reducing (as opposed to a neutral or oxidising) gas. Heat treatments on these compositions showed that powellite molybdates were less likely to form in reduced melts, however sparging glasses containing a full simulated HLW with a reducing gas during melting increased the tendency for powellite type molybdate formation upon heat treatment. EXAFS was used to show that, regardless of the waste loading or sparge applied during melting, the Mo was predominantly present as (Mo04)2- tetrahedra in the glasses, and also in a yellow phase sample taken from a full scale simulated HLW glass.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Other academic unit: Department of Engineering Materials
Identification Number/EthosID: uk.bl.ethos.444469
Depositing User: EThOS Import Sheffield
Date Deposited: 09 Sep 2019 13:35
Last Modified: 10 Sep 2019 10:13
URI: http://etheses.whiterose.ac.uk/id/eprint/14502

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