Electron Microprobe Investigation of Ti-pyrochlore Doped with Pu-238
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Electron Microprobe Investigation of Ti-pyrochlore Doped with Pu-238 Maria V. Zamoryanskaya and Boris E. Burakov Laboratory of Applied Mineralogy and Radiogeochemistry, the V.G. Khlopin Radium Institute, 28, 2-nd Murinskiy ave., St. Petersburg, 194021, Russia, e-mail: [email protected] and [email protected]
ABSTRACT A polycrystalline sample of Ti-pyrochlore, (Ca,Gd,Hf,U,Pu)2Ti2O7, doped with approximately 8.7 wt.% 238Pu and 20.0 wt.% 238U, has been studied at different cumulative doses of alpha induced radiation damage using scanning electron microscopy (SEM) and quantitative electron probe microanalysis (EMPA). All analyses of 238Pu-doped Ti-pyrochlore were done for comparison with 239Pu-doped pyrochlore synthesized under similar conditions. Our results suggest that accelerated alpha-induced radiation damage affects not only the crystalline structure of 238Pu-doped pyrochlore, but also causes changes in the chemical composition of the pyrochlore, and as proposed, destruction of pyrochlore-U-Pu solid solution. Numerous inclusions of a separate U-oxide phase were found in the 238Pu-doped pyrochlore crystalline matrix. The inclusions were observed visually using SEM and increased as a function of the cumulative dose. Also, after a cumulative dose of 1.1 x 1025 alpha decays/m3 the pyrochlore became nearly amorphous and inclusions of Hf-Ti-Ca-O phase were found in the crystalline matrix. In all cases separate phases of U-oxide and Hf-Ti-Ca-O were localized in chemically inhomogeneous areas. The results obtained allow us to assume that under self-irradiation the amorphization of Ti-pyrochlore might be accompanied with destruction of single solid solution, (Ca,Gd,Hf,U,Pu)2Ti2O7, into several pyrochlore phases of different chemical composition and precipitation of some amount of Pu and U into separate oxide phases. INTRODUCTION Ceramic materials based on the Ti-pyrochlore structure, (Ca,Gd,Hf,U,Pu)2Ti2O7, have been proposed as a durable host phase for the immobilization of weapons-grade plutonium and other actinides [1]. To investigate the resistance of Ti-pyrochlore to accelerated radiation damage, which simulates the effects of long-term storage of 239Pu waste forms, samples doped with 238Pu have been recently synthesized and studied by different laboratories [2-4]. The main emphasis of the previous investigations, however, should be considered as qualitative because of the difficulty to precisely and accurately analyze highly radioactive alpha-emitting samples that may contaminate and destroy analytical equipment. We have carried out for the first time ever quantitative electron probe microanalysis (EMPA) analyses, accompanied by scanning electron microscopy (SEM) imaging, of different Ti-pyrochlore samples doped with 238Pu and 239Pu. The main objective of this paper is to summarize new quantitative EMPA and SEM results for synthetic Ti-pyrochlore affected by self-irradiation from 238Pu. All results described in this paper have been obtained at the Laboratory of Applied Mineralogy and Radiogeochemistry
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