Immobilization of Simulant ILW Actinide Wastes Containing Halides: Effect of Process Parameters on Wasteform Properties
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Immobilization Of Simulant ILW Actinide Wastes Containing Halides: Effect Of Process Parameters On Wasteform Properties. Brian L. Metcalfe, Shirley K. Fong and Ian W. Donald Materials Science Research Division, Atomic Weapons Establishment, Aldermaston, Berkshire, RG7 4PR, U.K. ABSTRACT The applicability of a process, originally developed to immobilize a chloride-rich intermediate level waste (ILW) arising from the pyrochemical reprocessing of plutonium, to alternative ILW waste-streams containing a large variety of cations, present as oxides together with KCl and CaF2, has been investigated. Non-radioactive studies, using simulated wastes containing hafnium and samarium as surrogates for the actinides, have been performed to establish the optimum processing parameters for the manufacture of wasteforms suitable for long-term storage/disposal. It has been demonstrated that the wastes can be immobilized by calcining with either Ca3(PO4)2 or CaHPO4 as the host. In excess of 97.5 % of the chloride present in calcined wastes, containing up to 3.2 wt% halides as a mixture of F- and Cl-, is immobilized when using this process. Whereas in the original calcined waste, the Cl- (about 12 wt%) was immobilized as chlorapatite, Ca5(PO4)3Cl, and spodiosite, Ca2(PO4)Cl, in these current wastes fluorapatite is the predominant halide containing phase. Conversion of the immobilized waste powder into a monolithic wasteform was achieved by sintering the powder with the addition of a sodium aluminium phosphate based glass. Both the calcined powder and the sintered monolithic wasteforms showed excellent durability in aqueous solution. INTRODUCTION Aqueous reprocessing of three waste-streams arising from the pyrochemical reprocessing of plutonium is expected to convert the bulk of the waste into low level waste, suitable for safe disposal, and to concentrate the actinide contaminants into three distinct ILW waste-streams (Types II-IV). Conversion of these ILW wastes into a form acceptable for long-term storage/disposal requires the actinides and residual halides present to be chemically immobilized in a monolithic form. A two stage process had been developed at AWE [1, 2] to immobilize actinide ILW waste containing a high proportion of chloride (Type I waste) whereby the waste was calcined with a Ca3(PO4)2 host resulting in the waste being immobilized in the newly formed chlorapatite and spodiosite phases. To convert the immobilized waste powder into a monolithic wasteform suitable for long-term storage, a sodium aluminium phosphate glass, in the form of a finely ground powder, was added and the mixture sintered. This process appeared to offer the potential [3, 4] to immobilize these three additional types of ILW wastes although they consisted mainly of oxides, unlike the Type I which was chloride based. Non-radioactive studies to confirm the suitability of the process and establish the optimum processing parameters were performed at AWE prior to active studies being undertaken at Pacific Northwest National Laboratory. In the non-active st
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