A Cathodoluminescence Investigation of Pyrochlore, (Ca,Gd,Hf,U,Pu) 2 Ti 2 O 7 , Doped with 238 Pu and 239 Pu
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A Cathodoluminescence Investigation of Pyrochlore, (Ca,Gd,Hf,U,Pu)2Ti2O7, Doped with 238Pu and 239Pu Maria V. Zamoryanskaya1, Boris E. Burakov1, Roman V. Bogdanov2, Andrey S. Sergeev3 1 V.G. Khlopin Radium Institute, 28, 2-nd Murinskiy ave., St.Petersburg, 194021, Russia, fax: (7)-(812)-346-1129; e-mail: [email protected] or [email protected]; 2 Department of Chemistry, Saint-Petersburg State University, St.Petersburg, Russia; 3 Department of Geology, Saint-Petersburg State University, St.Petersburg, Russia ABSTRACT Crystalline ceramic based on the pyrochlore structure [(Ca,Gd,Hf,U,Pu)2Ti2O7], has been proposed in the U.S. as a candidate waste form for the immobilization of weapons grade plutonium. Several samples of synthetic polycrystalline pyrochlore doped separately with 238Pu and 239Pu were studied using cathodoluminescence (CL) and electron probe microanalysis (EPMA) immediately after synthesis and then after 175, 245 and 405 days. The CL spectra of pyrochlore doped with 239Pu and 238Pu obtained immediately after ceramic synthesis and 175 days later were nearly the same and characterized by a typical broad band emission with a maximum peak centered at 2.5 eV. New CL peaks with maximum intensities at 2.0 and 2.3 eV in the emission spectra of 238Pu- and 239Pu-doped pyrochlore were observed 245 days later. It was determined that this newly formed CL peak at 2.3 eV is similar to one in CL spectrum of artificial standard glass doped with the uranyl ion, (UO2)2+. It was suggested that CL band with maximum at 2.0 eV is caused by the U tetrahedral complex (UO4)2-. Also, the same peaks were observed in the CL spectrum of natural U-rich pyrochlore containing approximately 23 wt.% U (mainly in the form of uranyl ion). The results obtained allow us to conclude that the radiation damage of pyrochlore-based ceramic waste form is accompanied with conversion of tetravalent uranium incorporated into pyrochlore structure to the mobile uranyl ion, (UO2)2+, and complex (UO4)2-. INTRODUCTION Ceramics based on the pyrochlore [(Ca,Pu,Gd,Hf,U)2Ti2O7] structure have been proposed as a durable host phase for the immobilization of weapons grade Pu and other actinides [1-6]. Recently, samples of pyrochlore-based ceramics doped with 239Pu and 238Pu were obtained and studied [7-9]. Cathodoluminescence (CL) allows for the observation and identification of defects in crystalline materials and to determine in some cases the valence state of different ions (e.g., some transition metals, lanthanides, and actinides). This technique was used in combination with the electron probe microanalysis (EPMA) method to characterize pyrochlore samples doped with 239Pu and 238Pu in comparison with natural U-containing pyrochlore. EXPERIMENTAL The following samples obtained from previous research were characterized using CL and EPMA methods: - Ceramic based on pyrochlore [(Ca,Hf,Gd,Pu,U)2Ti2O7] doped with approximately 10 wt.% 239 Pu and 21 wt.% 238U [7-8]; JJ11.29.1
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Ceramic based on pyrochlore [(Ca,Hf,Gd,Pu,U)2Ti2O7] doped with 12.6 wt.% Pu (8.7
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