Rietveld Analysis of Phase Separation in Annealed and Leach Tested Cm-Doped Perovskite
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RIETVELD ANALYSIS OF PHASE SEPARATION IN ANNEALED AND LEACH TESTED CM-DOPED PEROVSKITE T.J. WHITE*, H. MITAMURA** AND T. TSUBOI**
*Ian Wark Research Institute, The University of South Australia, Warrendi Road, The Levels SA 5095 Australia "**Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki, 319-11 Japan. ABSTRACT
A quantitative powder X-ray diffraction study was made of actinide doped perovskite of bulk composition Ca., 9An.001oTio. •,Al 9 00,0 8,O, where An corresponded to approximately equimolar proportions of Cm-244 and Pu-240. Sections of this sample accumulated irradiation doses up to 7 7.51 x 10' alpha decay events per gram (acg-1). The damaged samples were treated in two ways. First, to establish the critical temperature for structural recovery under the reducing conditions of geological repositories, isochronal annealing was carried out at 600, 800, 1000 and 1I00T for 12 hours in graphite crucibles. Two groups of perovskites that had previously sustained doses of 4.5 x 1017 and 7.4 x 1017 ot g-' were tested in this way. For the former group, these conditions resulted in up to 9 weight percent (wt%) of available actinide separating as a fluorite-type dioxide near the perovskite surface. In the latter group, calcium was reduced to the metal which vaporised, leaving an excess of refractory titanium that crystalised as rutile. Second, material which had sustained doses of 1.6-4.0 x 1017 alpha decays per gram was subjected to an MCC-1 leach test for two months at 90T using a pH - 2 solution. Under these conditions surficial perovskite dissolved congruently to release calcium into solution while the titanium reprecipitated as anatase. The implications of these results for the ultimate disposal of perovskite-bearing polyphase nuclear waste ceramics are considered. INTRODUCTION
Perovskite is a major host for transuranic radionuclides intitanate ceramic waste forms. It has been recognised that natural occurrences of this mineral can be in thermodynamic disequilibrium in the presence of some groundwaters [1], prompting many detailed investigations of its chemical stability over a range of conditions [2]. Generally, such studies have used fully crystalline perovskite. In reality, perovskite will be subject to alpha-decay events which may render its crystal structure aperiodic. Therefore, accurate models to predict long term behaviour must take into account the enhanced dissolution that can accompany metamictization, and the possibility of phase separation which may be promoted by self annealing. In this paper, we describe for the first time a quantitative crystallographic and solid state chemical investigation of the radiation and chemical durability of transuranic doped perovskite subjected to accelerated self-annealing and leaching tests. The primary characterisation tool was powder X-ray diffraction (XRD) that allowed not only the examination of as-damaged material, but also permitted in-situ observation of surficial alteration that accompanied high tempera
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