Mike Harrison is a technical specialist with over 15 years experience in the vitrification and thermal treatment of a wide range of radioactive waste streams, including HLW, ILW, plutonium and uranium. He is Senior Technical Lead and NNL Fellow for vitrified wasteform fabrication and performance, and has developed considerable knowledge of glass durability through his interaction with the UK universities and the international community. Mike also has extensive knowledge of glass formulation chemistry, vitrification processes, as well as the properties of vitreous and ceramic wasteforms.

Mike T Harrison

M. T. Harrison*1, T. Taylor1, F. Wright2
1 National Nuclear Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, UK
2 Sellafield Ltd, Sellafield, Seascale, Cumbria, CA20 1PG, UK

In the UK, the Waste Vitrification Plant (WVP) at Sellafield converts the highly active liquid (HAL) waste from the reprocessing of spent nuclear fuel into a glass wasteform suitable for long term storage and disposal. After the completion of reprocessing operations, the highly active storage tanks (HASTs) will be emptied and washed out to remove any accumulated solids. These solids are expected to contain high concentrations of molybdenum. Hence, new glass formulations are being developed specifically for immobilisation of these waste streams.

A new glass formulation containing calcium and zinc has recently been tested on WVP. Although developed specifically for HAST wash-out feeds containing high concentrations of molybdenum, the intention is to use this ‘Ca/Zn’ base glass for all wastes, including standard reprocessing liquors. During the initial trial campaign, the opportunity for improving the performance of the new Ca/Zn base glass by optimising the lithium content was identified. Lithium nitrate is added to the HAL prior to feeding to WVP in order to optimise the vitrification process. Hence, there is a choice in how the lithium is distributed between the waste feed and base glass in order to obtain the desired amount in the final vitrified product. Optimising the lithium partitioning can enhance the operability of WVP by:
• increasing the softening temperature of the base glass to reduce the risk of melter neck blockages,
• ensuring the maximum possible waste loadings are achievable, and
• allowing greater flexibility in Li-dosing of the HAL to minimise the loss of dust to the off-gas system.
Hence, a series of laboratory-scale trials were carried out to investigate the vitrification and product performance of Ca/Zn base glasses with reduced lithium content. As well as the effect of re-distributing the lithium to the waste stream, the study also investigated the envelope of acceptable total Li2O content in the vitrified product as well as the deviation from the ideal Li:Na ratio.

Using the same principle of alkali metal re-distribution from the base glass to the waste feed, a further modified Ca/Zn composition has been investigated for the vitrification of HAL containing high concentrations of sodium. For some of the older HASTs at Sellafield, it may be necessary to use an alkali reagent to chemically dissolve settled beds of the Mo-containing solids. One of the reagents under consideration for this is 2M sodium carbonate (Na2CO3). Because an excess of reagent will have to be employed for the wash-out, the overall waste loading in the glass will be sodium- rather than molybdenum-limited. Hence, minimising the alkali metal content of the base glass will maximise the amount of high-Na waste that can be incorporated in the glass, minimising the number of waste containers that will be produced, with significant operational and storage cost savings.

Ca/Zn glass with high MoO3 content
Ca/Zn glass with 15 wt% Na20