Effect of Gallium Oxide on Phase Assemblage in Apatite and Whitlockite Hosts for Waste Immobilization
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Effect of Gallium Oxide on Phase Assemblage in Apatite and Whitlockite Hosts for Waste Immobilization Lee. A. Gerrard, Shirley. K. Fong, Brian. L. Metcalfe and Ian. W. Donald Materials Science Research Division, AWE, Aldermaston, Berkshire, UK ABSTRACT To immobilize halide and actinide ions present in specific ILW waste a process has been developed that uses mineral phases as the host material. The mechanism of substitution of gallium into these phases will have a large effect on the phase assemblage. This will inevitably affect the total amount of halide that can be immobilized in to total phase mixture. The full simulated waste stream composition containing varying concentrations (1-40 wt.%) of gallium oxide was studied. Also nominal compositions for gallium doped fluorapatites (Ca10-1.5xGax)F2(PO4)6 (x = 0, 0.25, 0.5, 0.75, 1.0) and gallium doped whitlockites Ca9Gay(PO4)6+y (x = 0.2, 0.4, 0.6, 0.8, 1.0) were prepared at 750-1050 ºC. These were studied by powder x-ray diffraction (XRD) to determine the phase assemblage and solid solution limits of gallium in the apatite and whitlockite phases. It was found that a complete solid solution was formed between whitlockite, Ca3(PO4)2, and Ca9Gay(PO4)6+y. In the nominal apatite compositions it was found that gallium did not substitute into the apatite structure but was instead partitioned over Ca9Gay(PO4)6+y, gallium phosphate, and unreacted gallium oxide. At higher temperatures gallium suppressed the formation of the apatite phase and was largely partitioned into the Ca9Gay(PO4)6+y phase whereas at lower temperature the majority was present as unreacted Ga2O3. In the full DCHP compositions it was found that gallium is likely to be partitioned over a number of phases including apatite, cationdoped whitlockite and gallium phosphate. INTRODUCTION A number of fluoride, chloride, plutonium and americium containing wastes arising from pyrochemical reprocessing of plutonium require to be safely immobilized at AWE [1-3]. The relatively high proportion of chlorides and fluorides make conventional routes such as vitrification and cementation unsuitable. Consequently, a two stage process has been developed where the actinides and halides are initially immobilized in phosphate mineral phases by a solid state reaction with calcium hydrogen phosphate. The resulting immobilized waste is a fine powder, so is not considered passively safe. The second stage is then to sinter this powder with a sodium aluminophosphate glass to convert it into a solid wasteform. Apatite and whitlockite were chosen as the main host phases for immobilization due to a number of reasons. Firstly, studies of natural apatites have shown that this mineral phase is highly resistant to radiation damage. This property has also been confirmed in accelerated ageing studies, using the short lived 238Pu isotope, where no radiation damage was detectable by power x-ray diffraction (XRD) after the equivalent of 400 years ageing [4]. Secondly, both apatites and whitlockites are known to accommodate a range of actinides. Fin
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