Vapour Phase Hydration of Blended Oxide - Magnox Waste Glasses
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9DSRXU3KDVH+\GUDWLRQRI%OHQGHG2[LGH±0DJQR[:DVWH*ODVVHV Neil C. Hyatt,1* William E. Lee,1 Russell J. Hand,1 Paul K. Abraitis2 and Charlie R. Scales.2 1
Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD. UK.
2
BNFL Technology Centre, Sellafield, Seascale, Cumbria, CA20 1PG. UK.
$%675$&7 Vapour phase hydration studies of a blended Oxide / Magnox simulant high level waste glass were undertaken at 200oC, over a period of 5 – 25 days. The alteration of this simulant waste glass is characterised by a short incubation time of less than 5 days, leading to the formation of an alteration layer several microns thick. Following the incubation period, the alteration proceeds at a constant rate of 0.15(1)µmd-1. The distribution of key glass matrix (Si, Na) and waste (Cs, Zr, Nd, Mo) elements was found to vary significantly across the alteration layer. Vapour phase hydration leads to formation of surface alteration products, identified as smectite, zirconium silicate and alkaline-earth molybdate phases. ,1752'8&7,21 In the UK, high level waste (HLW) arising from the reprocessing of spent Oxide (UO2) and Magnox nuclear fuels is blended and vitrified in a lithium sodium borosilicate glass matrix [1]. The vitrified product is comprised of 25 wt% waste, on an oxides basis, derived from Oxide and Magnox HLW blended in a 75:25 ratio (by weight, on an oxides basis). At present, the vitrified waste, contained in carbon steel containers, is stored in air cooled silos awaiting a decision on final disposal. During interim storage, the initial surface temperature of the waste glass may be as high as 200oC, as a consequence of radiogenic heating. In the event of water becoming trapped in the ullage of the waste glass container (either during production or through a container breach which, subsequently, becomes sealed), vapour phase hydration may be an important mechanism of glass corrosion. Although this scenario is highly unlikely it should, nevertheless, be considered. The vapour phase hydration test also provides a means to compare the relative durability of nuclear waste glasses and is included in the performance criteria applied to the Low Activity Waste (LAW) glasses developed for remediation of the Hanford site [2]. The interaction between HLW glasses and water vapour may also be important in a humid repository environment [3]. The Vapour phase Hydration Test (VHT), recently standardised by Jiricka HWDO [4], provides a method for investigating the interaction between (simulant) nuclear waste glasses and water vapour. Essentially, the VHT methodology involves subjecting a glass specimen to vapour phase hydration in a sealed vessel at elevated temperature. Sufficient water is present in the vessel to allow a thin film of water to condense on the specimen at the reaction temperature, leading to glass hydration, ion-exchange and network dissolution [5, 6]. As a consequence of these corrosion processes, the thin film of water becomes rapidly sa
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