To investigate the behavior of monazite during accelerated radiation damage, which simulates effects of long term storage, 238 Pu-doped polycrystalline samples of (La,Pu)PO 4 and PuPO 4 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 238 Pu contents in ceramic samples measured using gamma spectrometry were (in wt.% el.): 8.1 for (La,Pu)PO 4 and 7.2 for PuPO 4 . The (La,Pu)PO 4 monazite remained crystalline at ambient temperature up to a cumulative dose of 1.19 x 10 25 alpha decays/m 3 . In contrast, the PuPO 4 monazite became nearly completely amorphous at a relatively low dose of 4.2 x 10 24 alpha decays/m 3 . Swelling and crack formation due to the alpha decay damage was observed in the PuPO 4 ceramic. Also, under self-irradiation this sample completely changed color from initial deep blue to black. The (La,Pu)PO 4 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.
Single crystals of zircon doped with 239 Pu, up to 2 mm in size, were grown for the first time ever, using a Li-Mo flux. The crystals were transparent and free of inclusions of separated Pu phases. The zircon crystals were studied using optical and scanning electron microscopy, the electron microprobe, and cathodoluminescence imaging and spectroscopy. The incorporation of 239 Pu ranged from 0.1 to 1.4 wt. % el. The intensity of the CL emission in the Pu-doped crystals is correlated with the Pu content.
Zircon, ZrSiO4, is a prospective durable host material for the immobilization of excess weapons plutonium. Using cerium as a chemical analogue for plutonium, the experiments on the synthesis of Ce-doped zircon were conducted by sintering of sol-gel precursors in air and vacuum. The results showed that cerium substantially promotes zircon formation from sol-gel precursors and sintering in air is preferable for cerium incorporation in zircon structure. Based on measured lattice constants, solid solution compositions (Zr0.96Ce0.04)SiO4 and (Zr0.98Ce0.02)SiO4 were formed in samples sintered in air at 1400°C and 1600°C, respectively. The solubility limits of cerium and actinides in zircon and mechanism of zircon formation are discussed.
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