Self-Irradiation of Monazite Ceramics: Contrasting Behavior of PuPO 4 and (La,Pu)PO 4 Doped with Pu-238
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Self-Irradiation of Monazite Ceramics: Contrasting Behavior of PuPO4 and (La,Pu)PO4 Doped with Pu-238 Boris E. Burakov, Maria A. Yagovkina, Vladimir M. Garbuzov, Alexander A. Kitsay and Vladimir A. Zirlin 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] ABSTRACT To investigate the behavior of monazite during accelerated radiation damage, which simulates effects of long term storage, 238Pu-doped polycrystalline samples of (La,Pu)PO4 and PuPO4 were synthesized for the first time ever and studied using powder X-ray diffraction (XRD) analysis and optical microscopy. The starting precursor materials were obtained by precipitation of La and (or) Pu from their aqueous nitrate solutions followed by calcination in air at 700°C for 1 hour, cold pressing, and sintering in air at 1200-1250°C for 2 hours. The 238Pu contents in ceramic samples measured using gamma spectrometry were (in wt.% el.): 8.1 for (La,Pu)PO4 and 7.2 for PuPO4. The (La,Pu)PO4 monazite remained crystalline at ambient temperature up to a cumulative dose of 1.19 x 1025 alpha decays/m3. In contrast, the PuPO4 monazite became nearly completely amorphous at a relatively low dose of 4.2 x 1024 alpha decays/m3. Swelling and crack formation due to the alpha decay damage was observed in the PuPO4 ceramic. Also, under self-irradiation this sample completely changed color from initial deep blue to black. The (La,Pu)PO4 monazite was characterized by a similar change in color from initial light blue to gray, however, no swelling or crack formation have so far been observed. The results of this study allow us to conclude that the radiation damage behavior of monazite strictly depends on the chemical composition. The justification of monazite-based ceramics as actinide waste forms requires additional investigation. INTRODUCTION Monazite, (Ln,Act,Y,Ca,Sr)(P,Si)O4, in where Ln = lanthanides and Act = actinides, is a well known durable actinide host phase suggested for the immobilization of weapons grade plutonium and other actinides [1-3,9,10]. It has been known for some time that natural monazite samples may contain substantial amounts of U and Th but remain highly crystalline [4]. This observation, together with the excellent aqueous durability, suggests that monazite based ceramics may be suitable has waste forms for Act and fission products. Investigations of synthetic monazite after heavy-ion bombardment [5,7,8] show that it is possible to amorphize monazite at a relatively low fluence of approximately 1014 ions/cm2, but the critical amorphization temperature (Tc) above which the material remains crystalline increases with the mass of the Ln cation. The crystallinity of monazite can be easily restored by annealing at 227°C [2] or by electron irradiation [6]. Although, some features of La-monazite single crystals doped with 239Pu; 237Np; 241Am and 246Cm were summarized earlier [2] and ceramic samples based on 241 Am-doped La-monazite ha
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