Development of a Phosphate Ceramic as a Host for Halide-contaminated Plutonium Pyrochemical Reprocessing Wastes
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Development of a Phosphate Ceramic as a Host for Halide-contaminated Plutonium Pyrochemical Reprocessing Wastes Brian Metcalfe1, Shirley Fong1, Lee Gerrard1, Ian Donald1, Denis Strachan2, and Randall Scheele2 1 MSRD, AWE plc, AWE Aldermaston, Reading, RG7 4PR, United Kingdom 2 PNNL, Richland, WA, 99352 ABSTRACT The presence of halide anions in four types of wastes arising from the pyrochemical reprocessing of plutonium required an immobilization process to be developed in which not only the actinide cations but also the halide anions were immobilized in a durable, leach resistant form. AWE has developed such a process using Ca3(PO4)2 as the host material. Successful trials of the process using actinide-doped Type I waste (essentially a chloride-based waste) were carried out at PNNL where the immobilization of the waste in a form resistant to aqueous leaching was confirmed. Normalized mass losses determined using a modified MCC-1 test at 40°C/28 days were 12 x 10-6 g.m-2 and 2.7 x 10-3 g.m-2 for Pu and Cl, respectively. Accelerated radiation-induced damage effects are being determined with specimens containing 238Pu. No changes in the crystalline lattice have been detected with XRD after the 239Pu equivalent of 400 years ageing. Confirmation of the process for Type II waste (an oxyhydroxide-based waste) is currently underway at PNNL. Differences in the ionic state of plutonium in the four types of waste have required different surrogates to be used. Samarium chloride was used successfully as a surrogate for both Pu(III) and Am(III) chlorides. Early investigations into the use of HfO2 as the surrogate for Pu(IV) oxide in Type II waste showed some apparent differences in the phase assemblages of the surrogate and actinide-based products. However XRD examination of the products at higher resolution has demonstrated there is no significant difference and that for this work HfO2 is a suitable surrogate for PuO2. INTRODUCTION Pyrochemical reprocessing of plutonium gives rise to a variety of actinide bearing wastestreams containing significant chloride concentrations. As a result of the Cl- content these wastes are not amenable to established methods of immobilization e.g. vitrification in alkali-borosilicate glass or high-fired ceramics. A method was developed at AWE for one of the waste-streams (Type I) [1] whereby the waste was calcined at 750°C with Ca3(PO4)2 resulting in the actinides and Cl- ions being incorporated into chlorapatite (Ca5(PO4)3Cl) and spodiosite (Ca2PO4Cl). Subsequently the ceramic process was adapted to the immobilization of Types II and III wastes [2] that are oxide based but still containing some F and Cl ions and Type IV waste. Development work was performed with representative waste compositions in which Sm and Hf were substituted for the actinide components of the waste. Trials with non-radioactive materials were carried out at Pacific Northwest National Laboratories (PNNL) to confirm the process prior to performing trials using actinide-bearing wastes. The product of the calcinati
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