UO 2 surface oxidation by mixtures of water vapor and hydrogen as a function of temperature

Abstract

In the present work, X-Ray Photoelectron Spectroscopy (XPS) was used to study the effect of water vapor on the UO2 surface as a function of temperature. The experiments were performed in situ inside a high pressure chamber attached to the XPS instrument. UO2 samples were put in contact with either hydrogen or argon streams, saturated with water at room temperature, and the sample surface evolution was analyzed by XPS. In the case of the water vapor/argon experiments, one experiment at 350ºC was performed and, in the case of the water vapor/hydrogen experiments, the temperatures used inside the reactor were 60, 120, 200 and 350ºC. On one hand, in presence of argon, the results obtained showed that the water vapor in the argon stream oxidized 93% of the U(IV) in the sample surface. On the other hand, the degree of UO2 surface oxidation showed a different dependence on the temperature in the experiments performed in the presence of hydrogen: the maximum surface oxidation occurred at 120ºC, where 65.4% of U(IV) in the sample surface was oxidized, while at higher temperatures, the surface oxidation decreased. This observation is attributed to the increase of hydrogen reducing effect when temperature increases which prevents part of the oxidation of the UO2 surface by the water vapor.

Oxidation by H2O(g) in the presence of H2(g) of UO2 doped with Pd nanoparticles

“…This behavior was very different than the observed by Espriu-Gascon et al [9] in the same experiments with H2(g) and water vapor but without doping UO2 with Pd nanoparticles.…”

“…U(IV) percentage at 100ºC, 200ºC, and 350ºC were 57%, 42% and 7%, respectively. These results confirmed previous observations that temperature favored the extent of the oxidation of UO2 by water vapor [5,9,[21][22][23].…”

“…For example, the consumption of hydrogen peroxide in contact with hydrogen was observed at hydrogen partial pressures between 10 and 48 bar [8]. In addition, previous results of the extent of UO2 oxidized by water vapor showed that in the presence of hydrogen the percentage of U(IV) in the surface of UO2 subjected to oxidation by water vapor increased from 7% to 64% at 350 ºC [9].…”

“…Initial UO2 samples were determined to be partially oxidized due to their contact with atmospheric air and, in order to start the experiments with a stoichiometric UO2, a prereduction process was needed. A previous study on the temperature needed to reduce the oxidized phases on non-doped UO2 samples showed that the reduction was completed after 20 minutes of contact with H2(g) (15 ml•min -1 of hydrogen from Messer (Germany) with 99.999% of purity) at 500ºC [9]. However, the necessary temperature to reduce uranium in the doped samples was 350ºC.…”

“…U 4f7/2 band was used to determine the uranium oxidation state. The band was deconvoluted into three Journal of Radioanalytical and Nuclear Chemistry different contributions, associated to U(IV), U(V) and U(VI), respectively, following the deconvolution methodology previously reported in Espriu-Gascon et al [9].…”