The reaction of water on polycrystalline UO 2 : Pathways to surface and bulk oxidation S.D. Senanayake a , R. Rousseau b , D. Colegrave b , H. Idriss a, * a Materials Chemistry, Department of Chemistry, University of Auckland, Rm.527A, Private Bag 92019, Auckland, New Zealand b Present address: Institut des Sciences de l’Inge ´nieur de Toulon et du Var, BP56, 83162 La Valette du Var cedex, France Received 8 February 2005; accepted 14 April 2005 Abstract The reaction of polycrystalline uranium dioxide with H 2 O is studied by in situ Raman spectroscopy, temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The difference between UO 2 and U 3 O 8 canbeseeninthebulkbyXRDandRamanandonthesurfacebyboththecorelevelandvalencebandregions. In the valence band region the U 5f line was far more pronounced for UO 2 than for U 3 O 8 . It was possible to monitor the near surface oxidation of UO 2 toU 3 O 8 byH 2 O at 300 K using Raman spectroscopy while oxygen deposition was quantified using XPS. TPD of D 2 OonH 2 -reduced U 3 O 8 (UO 2 ) showed desorption of D 2 .D 2 desorption occurred in two temperature domains (at 410Kand 570K).IncreasingthesurfaceexposuretoD 2 O affected the distribution of bothD 2 peaks. The first desorption-peak populated first while at relatively higher D 2 O exposure the second desorption peak increased considerably in intensity. The second desorption of D 2 during D 2 O-TPD can be tracked down to oxi- dation of deeper layers. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Thesurfacechemistryofuraniumoxideshasbeenthe focus of several studies in the last few years [1–15]. The rich chemistry associated with the uranium oxides stems from their numerous complex phases, with the most thermodynamically stable phases being UO 2 ,U 3 O 8 and UO 3 . Uranium dioxide as a major component of nuclear fuel has been under close chemical scrutiny; in particularwithrelationtoaspectsofthestorageofspent and excess nuclear materials over long periods of time for safe radioactive decontamination. The oxidation of UO 2 surface is known to result in a bulk transformation firsttoU 3 O 7 thentoU 3 O 8 and this process is associated with a volume expansion of 35% [16,17]. This requires careful design and engineering for permanent fuel stor- age to avoid further complications that may arise over long periods of time. The abundance of water in nature and its influence on all aspects of chemistry is clear and promotes great interest with relation to its chemical interactions with solid surfaces [18]. The reactions of water on uranium oxides have been undertaken in sev- eral works over single crystal [3,11,12], thin film [19], and polycrystalline surfaces [13–15] with observation of thermally activated evolution of hydrogen over 0022-3115/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2005.04.060 * Corresponding author. Tel.: +64 9 373 7599; fax: +64 9 373 7422. E-mail address: h.idriss@auckland.ac.nz (H. Idriss). Journal of Nuclear Materials 342 (2005) 179–187 www.elsevier.com/locate/jnucmat