XAS and XRD study of annealed 238Pu- and 239Pu-substituted zircons (Zr0.92Pu0.08SiO4)

An annealing study has been completed on two compositionally identical Pu-substituted zircons (Zr0.92Pu0.08SiO4) prepared 18 yr ago with different Pu-238/Pu-239 isotopic ratios. The activity and accumulated dose for each material varies greatly due to the very different half-lives of Pu-238 (87.7 yr) and Pu-239 (24100 yr). The predominantly Pu-238-substituted zircon is in a highly amorphous state after accumulating a dose of 2.8 x 10(19) alpha-decays/g, which is a factor of two higher than the dose previously determined necessary to amorphize this material. The Pu-239-substituted zircon has remained highly crystalline after an accumulated dose of 1.2 x 10(17) alpha-decays/g. The Pu in both samples is trivalent due to sintering under reducing conditions. The short-range and long-range structures of each sample have been characterized by detailed X-ray absorption spectroscopy and X-ray diffraction (XRD) methods. The oxidation of Pu3+ to Pu4+ the crystalline Pu-239-substituted zircon during annealing in air resulted in a decrease in lattice distortion due to the decrease in ionic radius of Pu3+ to Pu4+ On the Zr4+ site. A significant amount of PuO2 exsolved from the zircon structure after annealing at 1200 degrees C in air. Detailed characterization of the amorphous Pu-238-substituted zircon demonstrated that while devoid of long-range order it still retained a distorted zircon structure and composition over a length scale of 0.5 nm. The recrystallization of the amorphous Pu-238-substituted zircon could be achieved directly at temperatures as low as 1200 degrees C when annealed for 12 h in air; however, annealing at 1000 degrees C resulted in decomposition into constituent oxides. (C) 2000 Elsevier Science B.V. All rights reserved.