Abstract
We report X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra for the plutonium LIII and uranium LIII edges in titanate pyrochlore ceramic. The titanate ceramics studied are of the type proposed to serve as a matrix for the immobilization of surplus fissile materials. The samples studied contain approximately 10 wt% fissile plutonium and 20 wt% natural uranium, and are representative of material within the planned production envelope. Based upon natural analogue models, it had been previously assumed that both uranium and plutonium would occupy the calcium site in the pyrochlore crystal structure. While the XANES and EXAFS signals from the plutonium LIII are consistent with this substitution into the calcium site within pyrochlore, the uranium XANES is characteristic of pentavalent uranium. Furthermore, the EXAFS signal from the uranium has a distinct oxygen coordination shell at 2.07 Å and a total oxygen coordination of about 6, which is inconsistent with the calcium site. These combined EXAFS and XANES results provide the first evidence of substantial pentavalent uranium in an octahedral site in pyrochlore. This may also explain the copious nucleation of rutile (TiO2) precipitates commonly observed in these materials as uranium displaces titanium from the octahedral sites.
| Original language | English |
|---|---|
| Pages (from-to) | 56-62 |
| Number of pages | 7 |
| Journal | Journal of Nuclear Materials |
| Volume | 304 |
| Issue number | 1 |
| DOIs | |
| State | Published - Jul 2002 |
| Externally published | Yes |
Funding
This work was supported by the US Department of Energy Office of Fissile Materials Disposition under Contract no. W-31-109-Eng-38. Use of the Advanced Photon Source was supported by the US Department of Energy, Basic Energy Sciences, Office of Energy Research (DOE-BES-OER), under Contract no. W-31-109-Eng-38. The MRCAT beamlines are supported by the member institutions and the US DOE-BES-OER under contracts DE-FG02-94ER45525 and DE-FG02-96ER45589. This work was supported by the US Department of Energy Office of Fissile Materials Disposition. Use of the Advanced Photon Source was supported by the US Department of Energy, Basic Energy Sciences, Office of Energy Research (DOE-BES-OER), under Contract no. W-31-109-Eng-38. Work performed at MRCAT is supported, in part, by funding from the Department of Energy under grant number DEFG0200ER45811. The authors also thank Scott Aase and Don Reed of Argonne for valuable assistance with sample preparation and beamline operations, Allen Bakel and Edgar Buck of Argonne and E.R. `Lou' Vance of ANSTO for useful discussions of the ceramic microstructure, and Henry Shaw (Lawrence Livermore) for encouraging the pursuit of this work.