Investigation of Valence State of U, Pu, and Am in Crystalline Host-Phases Using in situ Cathodoluminescence Spectroscop
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Maria V. Zamoryanskaya, Boris E. Burakov and Vladimir M. Garbuzov
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] $%675$&7 Ceramics compounds based on the pyrochlore, (Ca,Gd,Hf,An)2TiO7; zircon, (Zr,An)SiO4, and garnet, (Y,Gd,An..)3(Al,Ga,An...)5O12, where An = U, Am, Pu, Np, Cm, structures have been investigated to determine their use as durable nuclear waste forms for the immobilization of weapons-grade plutonium and other actinides. U, Pu, and Am were incorporated into synthetic zircon, pyrochlore, and garnet and were investigated using LQVLWX cathodoluminescence (CL) and compared with Ce3+, Ce4+, Eu2+ and Eu3+ in order to determine the valence state of the actinide dopants. The results obtained allow for the identification of Am3+ ions in synthetic zircon and garnet. The CL emission of Am3+ is characterized by narrow bands in the orange-red spectral region and is similar to CL emission of Eu3+ in zircon and garnet. No characteristic CL emission of Pu4+ was observed in zircon, pyrochlore or garnet. This is similar to CL emission of Ce4+, which does not have characteristic CL emission in the visible region of the spectrum. Although ions of Ce3+ were characterized by intense CL emission, no characteristic CL emission of Pu3+ was identified in zircon, pyrochlore, or garnet. The CL study of U-doped garnet and pyrochlore allow for the identification of the presence of two forms of uranium: the uranyl ion, (UO2)2+, and tetrahedral complex, (UO4)2-. Ions of U4+ did not have characteristic CL emission in the visible region. The study of pyrochlore samples doped simultaneously with 238Pu and 238U revealed that CL intensity of the uranyl ion increases with increasing dose of self irradiation from the 238Pu. It was assumed that self-irradiation might cause the change of actinide valence state. ,1752'8&7,21 The development of ceramic materials for the immobilization of waste actinides is based on the investigation of durable host phases [1-5]. Some of these crystalline materials doped with actinides were obtained from previous studies. These are: pyrochlore, (Ca,Gd,Hf,U,Pu,Am)2Ti2O7 [4,7]; zircon, (Zr,Pu,Am)SiO4 [7,8,12]; and garnet, (Y,Gd,Am..)3(Al,Ga,Pu,...)5O12 [6]. The study of valence states of U, Pu and Am incorporated into zircon, pyrochlore and garnet in comparison with actinide surrogates such as: Ce3+, Ce4+, Eu2+ and Eu3+ is important in order to understand the incorporation of actinides into the structures of host phases under particular conditions of ceramic synthesis with different oxygen fugacities. The actinide loading capacity of host phases for different actinides depends on their valence state which in turn is related to structure and crystal chemistry of the host material. Therefore, in some cases the change of actinide valen
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