A Comparison of Consolidation Routes for Halide Containing Wastes
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A Comparison of Consolidation Routes for Halide Containing Wastes Phillip M. Mallinson1, Shirley K. Fong1, Eric R. Vance2 and James D. Phillips3 1
Materials Science Research Division, AWE, Aldermaston, Berkshire RG7 4PR. UK ANSTO, Kirrawee DC, NSW 2232. Australia 3 Department of Materials, Imperial College, London SW7 2AZ. UK 2
ABSTRACT Research has been carried out to optimize the consolidation stage for the immobilization of pyrochemical wastes with a sodium aluminophosphate glass. The alternative techniques of hot pressing and hot isostatic pressing of the calcined wastes with the glass have been investigated. This has been performed on simulant waste material and the products investigated by scanning electron microscopy and X-ray diffraction. The consolidation techniques were compared to each other and to the original process for suitability as a waste-form. INTRODUCTION The pyrochemical reprocessing of plutonium has produced a number of waste materials that need to be stored and disposed of safely. The wastes contain both radioactive actinides and varying quantities of other cations as oxide, chloride and fluoride compounds. A two stage process has been developed to immobilize these wastes [1-5]; in the first stage the waste is calcined with CaHPO4 to produce apatite type phases and cation substituted β-Ca3(PO4)2 (β-TCP) which have been shown to be able to accommodate a range of actinides and be resistant to radiation damage [4,6-8]. The calcined powder is then mixed with a sodium aluminophosphate glass and sintered to produce a monolithic waste-form. Hot pressing (HP) has been previously reported as a technique suitable for the immobilization of radioactive wastes in glass-ceramic systems [9,10]. The primary advantage of hot pressing over pressure-less sintering is the lower temperature requirements needed to produce the final waste-form; this is a particularly important for halide containing wastes due to the volatility and corrosive nature of these species. Hot isostatic pressing (HIPing) offers further advantages in that as consolidation takes place within a sealed can no off-gassing is possible and there is no secondary waste produced, e.g. crucibles. HIPing has been reported as a viable route to a quality waste-form for a number of different types of waste [11-13]. EXPERIMENTAL Non-radioactive simulant waste was produced by mixing the powders using a ball mill in the correct proportions as shown in Table I. This was then mixed with CaHPO4 with the ratio of one part waste to three parts CaHPO4 and calcined at 750 °C for 4 hrs as a powder.
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Table I. Composition of the simulant waste. Component Concentration (wt.%) HfO2 20.8 Sm2O3 4.5 Ga2O3 28.0 Al2O3 9.8 MgO 6.3 Fe2O3 1.3 Ta2O3 1.3 NiO 1.3 CaF2 10.4 KCl 16.3 The calcined material was mixed with the sodium aluminophosphate glass (composition – NaO-22.9 wt.%, Al2O3-24.8 wt.%, P2O5-51.0 wt.%, B2O3-1.3 wt.%) in the ratio 3:1. Consolidation was performed either by simple unpressed firing, hot pressing or hot isostatic processing, see Table II.
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