We have developed a calcium phosphate ceramic based on the mineral phases chlorapatite, Ca5(PO4)3Cl, and spodiosite, Ca2(PO4)Cl, which, on the basis of evidence from non-active simulant studies, is expected to provide an extremely effective host for immobilizing the chloride constituents resulting from the pyrochemical reprocessing of Pu. The immobilized product is in the form of a free-flowing, non-hygroscopic powder in which the chlorides are chemically combined within the mineral phases. In order to provide a monolithic wasteform suitable for long term storage, the feasibility of encapsulating this product in a compatible glass is being assessed. The final wasteform will be manufactured by a pressureless sintering route with sodium aluminophosphate based glasses currently being developed for this purpose. These glasses are of particular interest due to a combination of useful properties which include good sintering characteristics, together with excellent chemical durability. We have noted, however, that during sintering the Na present in the glass reacts with the chlorapatite and spodiosite phases to form a dispersion of halite crystals within the final wasteform. The formation of free halite within the wasteform is clearly undesirable, but can be minimized by careful control over the processing parameters, in particular the sintering temperature, or by reducing the chloride loading in the wasteform. The implications of these findings are highlighted and discussed.
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Donald, I.W., Metcalfe, B.L. & Greedharee, R.S. A Glass-Encapsulated Ceramic Wasteform for the Immobilization of Chloride-Containing ILW: formation of Halite Crystals by Reaction Between the Glass Encapsulant and Ceramic Host. MRS Online Proceedings Library 713, 21 (2001). https://doi.org/10.1557/PROC-713-JJ2.1