Calcium Phosphate: A potential host for halide contaminated plutonium wastes
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1124-Q04-02
Calcium Phosphate: A potential host for halide contaminated plutonium wastes. Brian L. Metcalfe1, Ian W. Donald1, Shirley K. Fong1, Lee A. Gerrard1, Denis M. Strachan2 and Randall D Scheele2 1 Atomic Weapons Establishment, Aldermaston, Reading, RG7 4PR, UK 2 Pacific Northwest National Laboratories, Richland, WA, USA ABSTRACT The presence of significant quantities of fluoride and chloride in four types of legacy wastes from plutonium pyrochemical reprocessing required the development of a new wasteform which could adequately immobilize the halides in addition to the Pu and Am. Using a simulant chloride-based waste (Type I waste) and Sm as the surrogate for the Pu3+ and Am3+ present in the waste, AWE developed a process which utilised Ca3(PO4)2 as the host material. The waste was successfully incorporated into two crystalline phases, chlorapatite, [Ca5(PO4)3Cl], and spodiosite, [Ca2(PO4)Cl]. Radioactive studies performed at PNNL with 239Pu and 241Am confirmed the process. A slightly modified version of the process in which CaHPO4 was used as the host was successful in immobilizing a more complex multi-cation oxide–based waste (Type II) which contained significant concentrations of Cl and F in addition to 239Pu and 241Am. This waste resulted in the formation of cation-doped whitlockite, Ca3-xMgx(PO4)2, β-calcium phosphate, β-Ca2P2O7 and chlor-fluorapatite rather than the chlorapatite and spodiosite formed with Type I waste. INTRODUCTION The presence of significant concentrations of chloride and fluoride ions in the intermediate level legacy wastes derived from pyrochemical reprocessing of Pu prevents the waste from being successfully vitrified in borosilicate glass. The approach taken at AWE was to react the waste with a host material to produce a durable ceramic which would immobilize both the actinides and the halides. Of the ceramic phases, which can contain significant proportions of chloride and fluoride, apatite and spodiosite were chosen for further investigation. Apatite is a naturally occurring mineral of general formula, A5(BO4)3(OH, F, Cl), where A can be a variety of 1 to 3 valent cations and B is commonly P, V or As [1]. The ability to incorporate a variety of elements into the apatite structure and its good aqueous durability at pH >5 [2] offered potential as a waste-form. Calcium phosphate [Ca3(PO4)2] was chosen as the host leading to the formation of chlorapatite [Ca5(PO4)3Cl] and spodiosite [Ca2(PO4)Cl]. The initial waste investigated (Type I) was chloride based with the Pu being present as Pu(III) whereas the second waste (Types II) was oxide based but containing varying quantities of Cl- and F-. Pu is present as Pu(IV) in this waste which necessitated different surrogates for the Pu when performing the initial non-radioactive studies. For Type I waste Sm was used as the surrogate for both Pu and Am but Hf was used as the surrogate for Pu in the other waste-stream. A simple process was developed for Type I waste whereby waste was mixed with the host and calcined at 750°C for 2h during whi
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