A Comparison of Alumino and Calcium Phosphate Sintering Aids for Consolidation of Halide Containing Wastes
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A Comparison of Alumino and Calcium Phosphate Sintering Aids for Consolidation of Halide Containing Wastes Shirley K. Fong and Brian Metcalfe Materials Science Research Division, AWE, Aldermaston, Berkshire RG7 4PR, UK ABSTRACT A process has been developed at AWE for the immobilisation of halide containing wastes arising from the reprocessing of plutonium. Initially, the wastes are calcined with a calcium phosphate to form a number of target host phases, E-tricalcium phosphate (E-TCP) and apatite for the immobilisation of actinides, and apatite and spodiosite for halide. These mineral phases are then mixed with a glass binder, cold pressed and sintered to form a monolithic waste form. Two glass binders GTI/168, a sodium alumino phosphate glass and GTI/206, a sodium calcium phosphate glass were compared to optimise the halide retention in the waste-form. Analysis from powder X-ray Diffraction (PXRD) and scanning electron microscopy (SEM) showed that neither glass stabilises spodiosite. However, GTI/206 glass retains 10 wt.% more apatite and results in a much smaller proportion of the non-chloride bearing whitlockite phase than GTI/168. In all compositions where GTI/168 glass was used as a sintering aid, the sodium deficient and calcium enriched glass was present as an amorphous matrix phase. INTRODUCTION A two stage process immobilisation route has been developed to immobilise a number of halide and plutonium containing wastes arising from the pyrochemical reprocessing of plutonium at AWE. Initially the wastes are reacted with calcium hydrogen phosphate to produce E-TCP, spodiosite and apatite type phases, which have been shown to be resistant to radiation damage [1-3] and to accommodate a range of actinides [4-5]. The waste is then sintered with a glass to convert into a monolithic waste-form. Two glass binders (GTI/168), a sodium alumino phosphate and (GTI/206) a sodium calcium phosphate glass are compared in order to determine which glass optimises halide retention in the waste-form. The reaction of both of these glasses with the halide containing host phases, chlorapatite, spodiosite and fluorapatite is compared by XRD and SEM. EXPERIMENTAL Fluorapatite (Ca5F(PO4)3), chlorapatite (Ca5Cl(PO4)3) and spodiosite (Ca2ClPO4) were prepared by calcining appropriate quantities of CaHPO4, CaCO3 CaCl2 and CaF2. These were then mixed with either GTI/168 (40Na2O.19Al2O3.9P2O5.2B2O3) or GTI/206 (Na2O.CaO.P2O5) glass in a 75:25 waste:glass ratio, cold pressed and sintered at 750 °C for 4 hrs. Phase identification was carried out using PXRD and SEM. Phase analysis was carried out using Bruker Diffrac Plus Evaluation software [6] with the ICDD PDF-4 2010 database [7]. Rietveld refinements, including weight percentage calculations, were carried out using General Structure
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Analysis System (GSAS) [8]. SEM was carried out using Zeiss EVO 40 equipped with an Oxford Instruments Xmax 80mm2 EDX detector using 10 - 15 kV on both secondary and backscattered modes. Powder X-ray diffraction was carried out using a Bruker D8 Advance wi
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