Investigation of Pu Incorporation into Zircon Single Crystal
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Investigation of Pu Incorporation into Zircon Single Crystal John M. Hanchar1, Boris E. Burakov2, Maria V. Zamoryanskaya2, Vladimir M. Garbuzov2, Alexander A. Kitsay2 and Vladimir A. Zirlin2 1 Department of Earth and Environmental Sciences ,The George Washington University, Washington D.C., 20006 USA, e-mail: [email protected] 2 Laboratory of Applied Mineralogy and Radiogeochemistry, V.G.Khlopin Radium Institute, 28, 2-nd Murinskiy ave., St.Petersburg, 194021, Russia, e-mail: [email protected]; ABSTRACT In order to obtain Pu-doped zircon, (Zr,Pu)SiO4, with a maximum Pu content in the form of solid solution, zircon single crystals have been grown using the flux method from starting materials overloaded with Pu. The crystals obtained ranged from 0.2-0.5 to 3.5-4.5 mm in size, are transparent, and characterized by deep pink-brown color. No inclusions of separate Pu phases were observed in the crystals. The distribution of Pu in crystals is zoned and the Pu content varying from approximately 5 to 14 wt.% el. The zircon unit cell parameters calculated from XRD data of bulk powdered sample were: a=6.620(1); c=5.989(2). The results obtained allow us to conclude that the capacity of the zircon structure to incorporate Pu exceeds 10 wt.% el.; however, additional research is required to study the extent of solid solution, (Zr,Pu)SiO4, at higher Pu contents. INTRODUCTION Zircon, (Zr, An..)SiO4 (where An = U, Pu, Np, Am, Cm) is well-known durable actinide host phase. Ceramics based on the zircon structure have been suggested for the immobilization of actinides, including weapons-grade plutonium [1-4]. Actinide-doped zircon is also attractive for other unusual applications, such as the refractory and chemically inert source of alpha irradiation for use in space propulsion or in chemically aggressive media (e.g., strong acidic or alkaline solutions, etc.). There are still, however, no data concerning the extent of solid solution in the system of ZrSiO4-PuSiO4, although a broad range of solubility has been proposed [4]. Results obtained from previous studies on the synthesis of polycrystalline Pu doped zircon crystals demonstrate that the highest Pu content achieved in the zircon structure did not exceed 6.9 wt. % el. [5]. Higher Pu loading in starting precursor caused the formation of separate crystalline PuO2 in the matrix of the final zircon-based ceramic. Although incorporation of more than 8 wt.% 238Pu into the polycrystalline zircon under reducing conditions has been reported, there were no electron microprobe (EMPA) or other in situ analyses provided [6]. The use of polycrystalline zircon samples to determine the highest level of Pu incorporation is characterized with some uncertainties. For example, it is difficult to control segregation of Pu during precursor preparation or ceramic synthesis. The flux method allows for growth of relatively large (i.e., up to several mm), well formed, zircon crystals, and as such, we chose this method for our current investigation. It has previously been demonstrated that a lithiu
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