Interaction of PuO₂thin films with water

Abstract

Plutonium dioxide / Water / Reduction / Photochemistry / Photoelectron spectroscopySummary. The surface adsorption and reaction of water with PuO 2 thin films was investigated by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS, respectively). Initial motivation of the work was to further investigate the potential role of water in promoting the surface oxidation of PuO 2 to Pu(IV)/Pu(V) mixed oxides formerly discussed in literature which may seriously impede the stability of spent nuclear fuel. Water may act as oxidant, as catalyst for the oxidation by O 2 , or as reactant leading to formation of hydrous oxide being oxidized by O 2 instead of PuO 2 (cr). In order to obtain high water coverage under the experimentally required ultra-high vacuum conditions, water was adsorbed at low temperature (77 K) as thick ice film. Results were compared to thin water layers adsorbed at room temperature.When adsorbed at 298 K, water dissociates forming a thin hydroxyl (OH − ) layer with small amounts of molecularly adsorbed water but no further reaction (in the sense of oxidation or reduction) is detected. At 77 K, water condenses as ice film. Here, a mainly non-dissociative adsorption of water is observed for layers ∼ 1 ML while for higher dosages, only molecular water/ice is observed and no significant contribution of OH (in the water layers) is detected. When exposing the sample to UV light while warming it up, the ice layer thaws and desorbs leaving behind a Pu 2 O 3 surface. This surprising reduction of PuO 2 stands in sharp contrast to the radiolytically driven oxidation of spent fuel in presence of water. It is discussed in terms of a photochemically driven interface reaction.

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

“…Furthermore, the plutonium 4f 7/2 and 5/2 transitions could be fit with two species (Figure 3e), with binding energies of 424.8-424.9 eV and 426.5-426.6 eV for the 4f 7/2 transitions and 437.6-437.7 eV and 439.3-439.4 eV for the 4f 5/2 transitions (Table S2). These binding energies correspond very well to previously reported binding energies for Pu(III) (4f 7/2: 424.4 eV; 4f 5/2: 437.2-437.6 eV) and Pu(IV) (4f 7/2: 425.8-427.0 eV; 4f 5/2: 438.5-439.3 eV) [54,57,66,68,72,73] ( Table 2). The observation of Pu(III) and Pu(IV) on the surface of the magnetite crystals is compatible with previous observations of either Pu(III) or Pu(IV) on the surface of magnetite [13,17], while more focused experimental investigations are essential to determine the influence of HCO 3…”

“…Any satellite peaks were fitted using a symmetrical Gaussian peak shape (GL(0)). The doublet separation between the actinide 4f 7/2 and 5/2 transitions was kept constant at 10.85, 11.8 and 12.875 eV for uranium, neptunium and plutonium, respectively [54][55][56][57] and the ratio of the peak area of the 4f 7/2 and 5/2 transitions was fixed at 0.714 [54][55][56][57]. Figure 1 shows the percentage of uranium and neptunium removed from solution in the MOPS and NaHCO 3 buffered experiments.…”

Effect of hydration and thermal treatment on ceria surface using non-intrusive techniques

“…The kinetics and extent of H 2 generation may depend on the long term surface alteration by water and its radiolysis products. Surface alteration of plutonium dioxide has been investigated using intrusive techniques (microbalance, X-ray photoelectron spectroscopy, UV photoelectron spectroscopy, X-ray diffraction, X-ray absorption fine structure) [3,[10][11][12][13][14][15][16]. The effect of water adsorption on the surface state is supposed to be minor; it requires to find techniques that modify the least possible reaction equilibria.…”

Water on Actinide Dioxide Surfaces: A Review of Recent Progress

“…These experiments demonstrate that the interaction of H 2 O with AnO 2 surfaces is dependent on the surface oxygen vacancies, and oxygen vacancies prompt the dissociation of H 2 O and the production of H 2 on AnO 2 surfaces. The interaction of H 2 O with PuO 2 films has been studied using XPS and ultra-violet photoelectron spectroscopies (UPS) [60,61]. It was found that H 2 O dissociates and forms a thin hydroxyl (OH -) layer with small amounts of molecularly adsorbed water at 298 K. While under 80-120 K, H 2 O is adsorbed as thick ice multilayers and no significant OHis detected.…”

“…For UO 2 , the interaction of water with both perfect and reduced single crystalline (111) films and polycrystalline films has been carried out [58,59]. On PuO 2 surfaces, several experimental studies have been performed on polycrystalline films [1,[60][61][62][63].…”