Incorporation of molybdenum in nuclear waste glasses.

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.

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Awarding institution:	University of Sheffield
Academic Units:	The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
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

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Incorporation of molybdenum in nuclear waste glasses.

Abstract

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Thesis

glass 12(air) as cast

glass 12(air) HT

glass 12(ox) as cast

glass 12(ox) HT

glass 12(red) as cast

glass 12(red) HT

glass 13(air) as cast

glass 13(air) HT

glass 13(ox) as cast

glass 13(ox) HT

glass 13(red) as cast

glass 13(red) HT

glass 14(air) as cast

glass 14(red) as cast

BNES Young Speakers (Cumbria) Abstract

Crystallisation 2003 (Sheffield) Abstract

Glass Odyssey 2002 (Montpellier) Paper

IOM Ceramics Convention (Keele) 2003 Extended Abstract

MRS 2002 (Boston) Paper

MRS 2003 (Kalmar) Paper

SET for Britain (London) 2003 Abstract

SGT 2002 (Durham) Abstract

URA (Sellafield) 2003 Abstract

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