The Effect of Increased Waste Loading on the Durability of High Level Waste Glass.
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The Effect of Increased Waste Loading on the Durability of High Level Waste Glass. Chris Brookes1, Mike Harrison2, Andrew Riley1 and Carl Steele1 High Level Waste Plants, Sellafield Ltd, Seascale, Cumbria, United Kingdom; 2 National Nuclear Laboratory, Seascale, Cumbria, United Kingdom. 1
ABSTRACT This paper describes the results from static leach tests using the ASTM International standard Materials Characterisation Centre (MCC-1) and Product Consistency Test (PCT) protocols for inactive High Level Waste (HLW) glasses fabricated at full scale on the Sellafield Vitrification Test Rig. The samples comprised monoliths and powders of a 75:25 Oxide:Magnox Blend glass with 31 wt% waste incorporation and a Magnox-only glass with 35 wt% waste incorporation. The tests were carried out in de-ionized water at 90 °C for durations up to 42 days and normalized mass losses calculated. The results of MCC-1 and PCT tests on both 31 wt% Blend and 35 wt% Magnox glasses, showing measurable differences to the corresponding standard 25 wt% waste incorporation glasses, are presented. A series of Scanning Electron Microscopy (SEM) investigations were also undertaken. The variation in composition and thickness of the alteration layer with sample type and duration is reported. INTRODUCTION The Sellafield Waste Vitrification Plant (WVP) is tasked with converting into glass highly radioactive liquors arising from the reprocessing of spent nuclear fuel to provide a stable and durable waste form suitable for safe long term storage and ultimate disposal. WVP processes liquors from the reprocessing of both Magnox and Oxide spent nuclear fuel. Magnox feed is relatively low in fission products but contains significant amounts of Al and Mg from the fuel cladding. Oxide feed is of higher burnup and contains more fission products, along with Gd and other process additives. Oxide feed is mixed with Magnox waste in order to yield a Blend product. WVP product glass is manufactured to a Quality Assured Process Specification which limits the incorporation of waste oxides in glass to less than 31 wt% for both UK and overseas customers. While the limit is 31 wt%, the standard target incorporation rate was, until recently, 25 wt% for both Blend and Magnox product. However, there is scope within UK waste acceptance criteria to allow up to 35 wt% waste incorporation per container. By amending the Process Specification and targeting higher incorporations where possible, WVP will reduce the total number of containers produced over the lifetime of the plant requiring eventual disposal. This paper highlights recent research to understand the effect of increased waste loading on the durability of glass. EXPERIMENTAL Static leach tests were performed on two simulated HLW glasses with high waste loading fabricated at full scale on the Sellafield Vitrification Test Rig (VTR), i.e. 1. Blend (75:25 Oxide:Magnox) with 31 wt% waste oxide incorporation, and 2. Magnox with 35 wt% waste oxide incorporation. The compositions (based on analysis of dissolved glass
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