Development of A Glass-Encapsulated Ceramic Wasteform For The Immobilization Of Chloride-Containing ILW
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'HYHORSPHQW2I$*ODVV(QFDSVXODWHG&HUDPLF:DVWHIRUP)RU7KH,PPRELOL]DWLRQ2I &KORULGH&RQWDLQLQJ,/: Brian L. Metcalfe, Shirley K. Fong and Ian W. Donald Materials Science Research Division, Atomic Weapons Establishment, Aldermaston, Berkshire, RG7 4PR, U.K. $%675$&7 Pyrochemical reprocessing of plutonium generates several ILW waste-streams and a simple process has been developed at AWE to immobilize the chloride and actinide ions present in one of these waste-streams (Type 1) by calcining the waste with calcium phosphate. We have now investigated the possibility of using this process as the basis for treating a more complex wastestream (Type 2) and have determined that with some minor modification the original process can effectively immobilize the greater range of cations present in Type 2 waste to again produce a free-flowing non-hygroscopic powder. Weight for weight replacement of the Type 1 waste by Type 2 produced a powder which was approximately 20% deficient in PO43- when compared with stoichiometric powder. Addition of phosphorus pentoxide to the simulant waste/calcium phosphate powder mixture prior to calcination to produce stoichiometric powder significantly improved the chloride ion uptake. In order to comply with safety requirements it is necessary to convert the free-flowing powder into a monolithic wasteform suitable for long term storage. Conversion of the powder to a monolithic wasteform by sintering using a sodium aluminophosphate glass as a binder has been investigated and has proved successful, but to achieve high densities it has been necessary to cold press the samples prior to sintering. ,1752'8&7,21 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 including Ca, Ba, Na, Pb, Sr, La, Ce and B is commonly P. The ability to substitute such a wide range of ions into the structure offered promise as the host material for immobilizing Type 1 waste which consists primarily of actinide contaminated calcium chloride. When this waste is mixed with calcium phosphate at elevated temperature the solid state reaction with the chloride present in the waste leads to the formation of the mineral phases chlorapatite [Ca5(PO4)3Cl] and spodiosite [Ca2(PO4)Cl] whilst the actinide species are incorporated into the crystal structure of these phases through substitution of calcium ions. The overall result is the formation of a free-flowing, non-hygroscopic powder which is highly durable in aqueous environments. Radioactive and non-radioactive studies to date [1-7] have demonstrated these phases to be effective hosts for immobilizing both the actinide and chloride constituents of the waste. This system also offers potential for the immobilization of a second more complex waste-stream (Type 2) which, in addition to the actinides and chloride present in Type 1 waste, also contains fluoride and a variety of different valent cations; for example, K+, Mg2+, Ga3+, Al3+, Pu3+, Am3+, Pu4+. Whilst the cations would be expected to substit
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