Immobilisation of Simulated Plutonium-Contaminated Material in Phosphate Glass: An Initial Scoping Study
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Immobilisation of Simulated Plutonium-Contaminated Material in Phosphate Glass: An Initial Scoping Study Paul A. Bingham, Russell J. Hand, Charlie R. Scales1 Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK. 1 Nexia Solutions, Building B170, Sellafield, Seascale, Cumbria CA20 1PG, UK.
ABSTRACT
Vitrification is a potential route for the immobilisation of Plutonium Contaminated Material (PCM). This is an Intermediate Level Waste (ILW) arising from operations in which there is contact with Pu isotopes. PCM consists of low levels of Pu combined with metals, masonry, glass, ceramics, polymers and other carbonaceous materials. Simulated PCM containing CeO2 as a PuO2 surrogate was mixed with a phosphate precursor and vitrified. Preoxidation of PCM simulant prior to vitrification minimised the violence of batch reactions. No pre-oxidation produced inhomogeneous slag-like materials with high residual metals and particulates. Pre-oxidation at 600°C in air and at 1200°C in an O2-rich atmosphere produced more favourable results, with increasingly vitreous products resulting from more oxidised PCM simulant. The most oxidised PCM simulant produced phosphate glasses with low levels of particulate inclusions, as confirmed by x-ray diffraction and scanning electron microscopy. Particulates included iron-rich metallics and aluminous oxides. Increased melting times and temperatures may have reduced the number of inclusions slightly, but O2 bubbling during melting resulted in little additional benefit. Waste loading equivalent to ~60 weight % of untreated waste may be possible. There was little evidence of Ce partitioning, indicating that it was immobilised within the glass matrix and had little preference for metallic or crystalline phases. These results demonstrate the potential feasibility for vitrification of PCM in phosphate glass, justifying further investigation into this potentially novel solution.
INTRODUCTION
Plutonium-contaminated materials (PCM) arise from the handling of Pu in a variety of operations [1]. They may consist of polymers (PVC or PE), soil, masonry, ceramics, aluminium and steel, which have been contaminated by low Pu levels ( 99 % NH4H2PO4, followed by melting. Samples were prepared thus: Set A) Untreated PCMS + NH4H2PO4 Set B1) (PCMS pre-treated at 600°C in air for 24h) + NH4H2PO4 Set B2) (PCMS pre-treated at 1200°C in O2 for 24h) + NH4H2PO4 Batches were calculated by weight, therefore higher levels of Fe and Al and lower levels of C were present in the heat-treated PCMS than in the untreated PCMS. Therefore, per unit weight of PCMS batch addition, the (Fe + Al) content increased in the order Set A < Set B1 < Set B2. Batch compositions and sample details are shown in table II. Batches of 50 - 200g material were weighed, mixed and placed in mullite crucibles in an electric furnace, heated for 9 hours to 1030°C and held at this temperature for 7 hours. Samples 1-4, 7-10 and 12 were then heated at 1150°C for 2 hours. Sample
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