Abstract
We studied in-situ the interaction of sodium metal with UO2+x (0 ≤ x ≤ 1) using thin films prepared by sputter deposition on Au substrate. X-ray Photoelectron Spectroscopy (XPS) and Grazing Incidence X-Ray Diffraction (GIXRD) characterized films before and after interaction. The results show that sodium does not reduce stoichiometric UO2 at room temperature. Plasmon loss peaks, observed at the Na1s photoemission (PE) line, are characteristic of metallic sodium particles, and point to a weak interaction between sodium and UO2. The oxidation of the sodium at room temperature takes place only on hyper-stoichiometric uranium dioxide films. Indeed, sodium deposition on UO2+x (0 < x ≤ 1), eventually results in the complete reduction of U(VI) down to U(IV). Molecular and atomic oxygen affect differently the oxidation of uranium and sodium. The wetting of the gold by the sodium is much more important when sodium is oxidised by atomic oxygen. This leads to a shift of the Na1s core level peak, while molecular oxygen gives a peak broadening without shift. Atomic oxygen seems to play the same role as the interstitial oxygen present in the UO2+x film. The oxygen dissociation may be the limiting step of sodium oxidation on gold and on UO2. Heat treatment of sodium on UO3 at about 773 K leads to the formation of the NaUO3 film as demonstrated by a quantitative analysis by XPS and GIXRD. The results show also that once U(V) is formed, it stays stable up to at least 973 K, the temperature at which shrinkage of the film is observed.
Highlights
In response to the difficulties in achieving sustainability, with sufficient high degree of safety and more competitive economic bases of nuclear power plants, Generation IV reactors have been considered and amongst selected options, Sodium cooled Fast Reactor (SFR) possesses one of the most attractive designs [1]
A slight interdiffusion between sodium and gold at this temperature may explain the presence of a weak Na1s intensity peak: It could be attributed to a near surface alloy formed by Na-Au interdiffusion and stable at T = 673 K [13]
The results demonstrate that Na does not reduce stoichiometric UO2 at room temperature
Summary
In response to the difficulties in achieving sustainability, with sufficient high degree of safety and more competitive economic bases of nuclear power plants, Generation IV reactors have been considered and amongst selected options, Sodium cooled Fast Reactor (SFR) possesses one of the most attractive designs [1]. To comply with high safety standards, SFRs of Generation IV foresee implementation of an internal storage of spent and failed fuel pins, omitting the need of an external sodium pool to avoid potential risks of interaction with air and water. Understanding of the corrosion mechanism becomes of prime importance for establishing feasibility of an internal storage of failed fuel pins, and may impose strict limits on their management. The Na-U-O system contains a wide range of sodium uranium oxides, as reported, such as Na3UO4, Na4UO5, Na2UO4, Na2U2O7, Na6U7O24, NaUO3. During the interaction of UO2 with liquid sodium containing dissolved oxygen, an incoherent surface layer is formed [5]. Under ultra-high vacuum (UHV) we want to understand the interaction of metallic sodium with uranium oxides, if Na enables to stabilize U(V) or U(VI) oxidation state
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