Paper No. 16-7
Presentation Time: 9:55 AM
REDOX RESPONSE OF ACTINIDE OXIDES AND OXYHYDROXIDES TO HIGHLY IONIZING RADIATION
TRACY, Cameron L.1, LANG, Maik K.
2, PRAY, John M.
3, PALOMARES, Raul I.
4, ZHANG, Fuxiang
3, PARK, Changyong
5, TRAUTMANN, Christina
6, SKURATOV, Vladimir A.
7 and EWING, Rodney C.
1, (1)Department of Geological Sciences, Stanford University, Stanford, CA 94305, (2)Department of Nuclear Engineering, University of Tennessee, Knoxville, Knoxvolle, TN 37996, (3)Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Ann Arbor, MI 48109, (4)Department of Nuclear Engineering, University of Tennessee, Knoxville, TN 37996, (5)High Pressure Collaborative Access Team (HPCACT), Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439, (6)GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, 64291, Germany; Technische Universität Darmstadt, Darmstadt, 64287, Germany, (7)Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, 141980, Russia, cltracy@umich.edu
Actinide oxides are the primary components of nuclear fuels and wastes. They form oxyhydroxides in the presence of water, as is typical of geological disposal and reactor accident conditions. All such materials undergo irradiation by particles of high specific energy due to nuclear decay (alpha particles) and fission (fission fragments). Radiation in this energy regime interacts with matter through the dense excitation of electrons to the conduction band, yielding a hot electron-hole plasma prior to recombination. In this process, electronic defects are generated and energy is transferred to atoms in the form of phonons. This energy deposition can dramatically modify the crystal chemistry of these materials, influencing their stability and actinide transport.
The effects of high-energy heavy ion irradiation on CeO2 (an actinide oxide analogue), ThO2, UO2, UO3, (UO2)(OH)2 (α-uranyl hydroxide) and (UO2)8O2(OH)12(H2O)10 (metaschoepite) were investigated [1]. X-ray diffraction (XRD) and Raman spectroscopy were used to characterize irradiation-induced structural modifications, while valence changes were monitored by x-ray absorption spectroscopy (XAS). Irradiation yielded cation valence reduction, indicating that additional electrons localized on their f-orbitals following ionization. Coupling of this valence reduction to structural distortions and phase transformations was observed, as cation oxidation state changes results in modified bonding. The increase in ionic radius that accompanies valence reduction caused unit cell expansion and heterogeneous microstrain in the dioxides. Reduction in the U6+ compounds to U4+ yielded instability of the uranyl structural units. This caused amorphization and the growth of a nanocrystalline UO2+x phase, consistent with the phase morphology observed in samples recovered from the natural nuclear fission reactor at Oklo, Gabon [2].
[1] C.L. Tracy et al., Nat. Commun. 6, 6133 (2015)
[2] S. Utsunomiya et al., Earth Planet. Sc. Lett. 240, 521 (2005)
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