Aging Studies of Pu-238 and -239 Containing Calcium Phosphate Ceramic Waste-forms
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© British Crown Owned Copyright 2012/AWE Aging Studies of Pu-238 and -239 Containing Calcium Phosphate Ceramic Waste-forms Shirley K. Fong1, Brian L. Metcalfe1, Randall D. Scheele2, Denis M. Strachan2 1 Atomic Weapons Establishment, Aldermaston, Berkshire RG7 4PR, UK 2 Pacific Northwest National Laboratories, Richland, WA, USA ABSTRACT A calcium phosphate ceramic waste-form has been developed at AWE for the immobilisation of chloride containing wastes arising from the pyrochemical reprocessing of plutonium. In order to determine the long term durability of the waste-form, aging trials have been carried out at PNNL. Ceramics were prepared using Pu-239 and -238, these were characterised by PXRD at regular intervals and Single Pass Flow Through (SPFT) tests after approximately 5 yrs. While XRD indicated some loss of crystallinity in the Pu-238 samples after exposure to 2.8 x 1018 α decays, SPFT tests indicated that accelerated aging had not had a detrimental effect on the durability of Pu-238 samples compared to Pu-239 waste-forms. INTRODUCTION A two stage process immobilisation route has been developed to immobilise a number of halide and plutonium containing wastes arising from the pyrochemical reprocessing of plutonium at AWE. Initially the wastes are reacted with calcium hydrogen phosphate to produce a substituted β-tricalcium phosphate (β-TCP) and apatite type phases, which have been shown to be resistant to radiation damage [1-3] and to accommodate a range of actinides [4-5]. The waste is then sintered with an aluminophosphate glass to convert into a monolithic waste-form. Development work was carried out at AWE using Hf and Sm as surrogates for Pu and Am. Trials with non-radioactive materials were carried out at Pacific Northwest National Laboratories (PNNL) before active waste-forms were synthesised. Samples were characterised by XRD at regular intervals and Single Pass Flow Through (SPFT) tests after aging for approximately 5 yrs to determine suitability for long term waste disposal. Waste-forms were prepared using both Pu239 and Pu-238 to accelerate aging by a factor of 274.9. However, in order to reduce operator dose, the majority of the Am was replaced by Sm in these waste-forms. EXPERIMENTAL Ceramics were prepared containing (a) 94% Pu-239, 6% Pu-240 and (b) 78% Pu-238, 22% Pu-239, these are referred to as Pu-239 and Pu-238 samples. Simulant waste materials were prepared using HfO2 in the correct mol% ratio. Pu-239 containing ceramics was prepared by milling Pu(IV) oxalate and Am(II) oxalate with the remaining waste-form constituents (Table 1) and CaHPO4 reagent. Oxalates were added
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in the correct proportions to yield the oxides compositions below. Pu-238 loaded ceramics were prepared in a similar manner, however a Pu(III) oxalate precursor was used. Mixtures were ball milled with zirconia grinding media for over 12 hrs. Table 1 Waste composition Constituent PuO2 Am2O3 Ga2O3 Al2O3 Sm2O3 MgO Fe2O3 Ta2O5 NiO CaF2 KCl CaHPO4
Mass % 4.85 0.05 6.45 2.26 1.00 1.46 0.31 0.30 0.30 2.40 3.76 76.86
Ceramic w
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