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Computational Investigation of the Formation of Hyperstoichiometric Uranium Dioxide (UO2+x)

Published online by Cambridge University Press:  19 October 2011

Frances Skomurski ,
Udo Becker  and
Rodney Ewing
Frances Skomurski
Affiliation:
fskomurs@umich.edu, University of Michigan, Geological Sciences, 2534 C.C. Little Building, 1100 North University Ave., Ann Arbor, MI, 48109, United States, 734--615-0656, 734-763-4690
Udo Becker
Affiliation:
ubecker@umich.edu, University of Michigan, Geological Sciences, 2534 C.C. Little Building, 1100 North University Ave., Ann Arbor, MI, 48109, United States
Rodney Ewing
Affiliation:
rodewing@umich.edu, University of Michigan, Geological Sciences, 2534 C.C. Little Building, 1100 North University Ave., Ann Arbor, MI, 48109, United States
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Abstract

The stability of spent nuclear fuel as a waste form largely depends upon the oxidation behavior of uranium dioxide (UO2) under repository conditions. Because U6+ phases are more soluble than U4+ phases, and changes in oxidation state can lead to volume changes which increase the susceptibility of spent fuel to corrosion, understanding the mechanisms behind the formation of more highly oxidized UO2 phases is important. While a variety of diffraction and spectroscopic studies have been used to investigate the oxidation behavior of UO2 to U4O9, questions still remain as to the effect of interstitial oxygen on the oxidation state of uranium in hyperstoichiometric UO2. In this study, computational techniques were used to investigate questions such as: Is the formation of U5+ or U6+ more energetically favorable upon the oxidation of UO2

A density functional theory (DFT) approach was used to investigate energetically favorable locations of interstitial oxygen atoms and to determine their effect on the charge distribution of uranium in U4O9, specifically. A single unit cell of U4O9 is used as a starting model, and the optimized geometry and charge distribution is tested by using starting models with different U charge assignments. Results from our calculations suggest that the formation of one U5+ per addition of interstitial oxygen at a perpendicular bisector site is favorable. Deflection of two lattice oxygen atoms along the body diagonal of the cubic sites is also observed upon the addition of one interstitial oxygen atom. Structural and bond length data are compared with experimental data whenever possible.

Keywords

Uoxidationactinide
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 985: Symposium NN – Scientific Basis for Nuclear Waste Management XXX , 2006 , 0985-NN05-05
Copyright
Copyright © Materials Research Society 2007

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