Molybdenum in Nuclear Waste Glasses - Incorporation and Redox State R.J Short, R.J Hand, N.C Hyatt Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK Alkali borosilicate glasses are used in the UK to immobilise high level nuclear waste (HLW) generated by the reprocessing of used fuel rods [1,2] At low waste loadings (~20 wt%) homogeneous glasses are formed upon solidification, that are highly chemically durable in the long term, and are therefore suitable for ultimate disposal in underground repositories To increase the efficiency of the vitrification process it would be desirable to increase the waste loading of these glasses whilst maintaining excellent long-term durability However, increasing the waste loading also increases the tendency of these glasses to devitrify upon cooling This is due to the low solubility levels of certain waste components in the host matrix At high waste loadings, these components tend to combine with other elements from the waste stream and form crystalline compounds which can have a detrimental effect on product durability Molybdenum, which is found in many reprocessed fuel waste streams, has a low solubility in borosilicate glasses and at concentrations above 1wt% tends to form highly water soluble phases with extremely radioactive elements such as Cs137 These compounds are generally termed “yellow phase” as they will usually incorporate some chromium from the HLW which gives them a characteristic yellow colour (see fig.1) [3] It has been suggested that reducing the oxidation state of the molybdenum will Figure A yellow phase “peanut” found increase its solubility in HLW in a simulated HLW glass made on a full glasses [4] and thus allow the scale production line Scale bar = 2cm production of amorphous products with higher waste loadings To characterise the inclusion of Mo, glasses containing a simulated non-active HLW stream at elevated waste loadings (to encourage crystallisation) were melted in air The Mo combined with elements from the simulated waste stream upon heat treatment (to simulate slow cools experienced by actual vitrified products) to form powellite type molybdates that were analogous to the yellow phase found in full scale simulated HLW glasses In glasses such as this, the Mo is in the +6 oxidation state, but it was found that the Mo could be reduced to a +3 oxidation state by melting in a reducing atmosphere of N2/5%H2 (see fig.2) Upon heat treatment of simplified HLW glasses melted in reducing atmospheres, it was found that powellite-type molybdate formation could be avoided (see fig 3) 8 g = 9 (1 ) In t e n s it y ( a r b it r a r y u n its ) Figure Electron Spin Resonance (ESR) trace of a borosilicate glass containing 1wt % Mo melted in a reducing atmosphere The resonance at g = 1.9059 is indicative of Mo3+ [5] (Inset: same glass melted in an oxidising atmosphere – the lack of any resonance implies the Mo is in the +6 oxidation state) ESR was performed at The University of Birmingham School of Chemical Sciences 0 -0 -0 2 3 4 5 0 -2 -4 -6 2 3 4 5 F ie ld ( x 03 G ) Figure X-ray diffraction of heat treated samples of a simplified HLW glass melted in different atmospheres The oxidised glass shows the presence of powellite type molybdates (labelled 1), which are not present in the reduced sample (which has yet to be fully characterised) Conclusions •Highly loaded simulated HLW glasses melted in air contain Mo6+ which forms powellite-type molybdates analogous to yellow phase upon heat treatment •Melting in a reducing atmosphere can reduce Mo6+ to Mo3+ and the formation of powellite-type molybdates upon heat treatment can be avoided [1] Marples, J.A.C., The Preparation, Properties, and Disposal of Vitrified High Level Waste from Nuclear Fuel Reprocessing, Glass Technology, Vol 29, No December 1988, pp 230-247 [2] Donald, I.W., Metcalfe, B.L., Taylor, R.N.J., The Immobilisation of High Level Radioactive Wastes Using Ceramics and Glasses, J Mat Sci., 32, 1997, pp 5851-5887 [3] Lutze, W., Ewing, R.C., Radioactive Waste Forms for the Future (North Holland, Amsterdam) 1988, pp 31-34 [4] Camara, B., Lutze, W., Lux, J., An Investigation on the Valency of Molybdenum in Glasses With and Without Fission Products, Scientific Basis for Nuclear Waste Management 2, (Plenum Press, New York) 1980, pp 93 [5] Horneber, A., Camara, B., Lutze, W., Investigation on the Oxidation State and the Behaviour of Molybdenum in Silicate Glass, Scientific Basis for Nuclear Waste Management 5, (Elsevier Science Publishing Company Inc., New York) 1982, pp 279–288 ... I.W., Metcalfe, B.L., Taylor, R.N.J., The Immobilisation of High Level Radioactive Wastes Using Ceramics and Glasses, J Mat Sci., 32, 1997, pp 5851-5887 [3] Lutze, W., Ewing, R.C., Radioactive