Widening the Envelope of UK HLW Vitrification – Experimental Studies with High Waste Loading Formulations Containing Platinoids

Abstract

Sellafield Ltd operates 3 vitrification lines to convert highly active concentrate liquor arising as a waste product of reprocessing operations into glass for safe interim storage in the Vitrified Product Store (VPS) prior to long term disposal. Highly Active Liquor (HAL) is stored in Highly Active Storage Tanks and transferred to WVP in batches to the liquid stock tank. It is metered in a semi-continuous batch operation to a calciner (rotating tube furnace) where it is converted into an oxide powder (calcine). Glass frit is fed at the lower end of the calciner where it discharges into an Inconel melter vessel controlled at approximately 1100 C. The glass and calcine are melted together and then poured into a container as a batch operation. After two pours the container is allowed to cool, a lid is then fitted to the container, which after further cooling is welded to the container. This container is then cleaned and transferred to the VPS. Platinoid species containing ruthenium, rhodium and palladium present in the HAL form insoluble oxide phases in the glass product. The platinoid concentration in the glass will increase with increasing waste oxide loading to an extent that settling of the platinoids in the glass may occur, leading to heel enrichment, poor melter performance and difficulty in draining the melter. The viscosity will also increase, which may require higher melter temperatures to mix and pour the molten glass and could result in enhanced corrosion of the melter. Inactive laboratory scale experiments with different glass frit formulations have been performed to determine whether product quality could be maintained with higher platinoid concentrations. Operational envelopes with existing formulations were expanded to observe laboratory trial performance and determine any changes to resulting glass qualities. Also, glasses with high waste incorporations have been produced to test process capability and to ascertain any potential phase separation or devitrification issues that could affect either the process or product performance. Physical properties of the different glass formulations were performed to measure changes in viscosity, density and the rates of settling to examine the amount of phase separation that can occur. The results have shown that ruthenium, palladium or rhodium were insoluble in the melt and were not evenly distributed throughout the glass but clustered together. These results will be used as a basis for further development work. This paper presents some findings of these experiments.