Impact of Stoichiometry on the Mechanism and Kinetics of Oxidative Dissolution of UO₂ Induced by H₂O₂ and γ-Irradiation

Abstract

Radiation-induced oxidative dissolution of uranium dioxide (UO2) is one of the most important chemical processes of U driven by redox reactions. We have examined the effect of UO2 stoichiometry on the oxidative dissolution of UO2 in aqueous sodium bicarbonate solution induced by hydrogen peroxide (H2O2) and γ-ray irradiation. By comparing the reaction kinetics of H2O2 between stoichiometric UO2.0 and hyper-stoichiometric UO2.3, we observed a significant difference in reaction speed and U dissolution kinetics. The stoichiometric UO2.0 reacted with H2O2 much faster than the hyper-stoichiometric UO2.3. The U dissolution from UO2.0 was initially much lower than that from UO2.3 but gradually increased as the oxidation by H2O2 proceeded. Increase in the initial H2O2 concentration caused decrease in the U dissolution yield with respect to the H2O2 consumption both for UO2.0 and UO2.3. This decrease in the U dissolution yield is attributed to the catalytic decomposition of H2O2 on the surface of UO2. The γ-ray irradiation induced the U dissolution that is analogous to the kinetics by the exposure to a low concentration (2 × 10–4 mol dm–3) of H2O2. The exposure to higher H2O2 concentrations caused lower U dissolution and resulted in deviation from the U dissolution behavior by γ-ray irradiation.

On the change in UO2 redox reactivity as a function of H2O2 exposure

“…This is in line with the change in kinetics observed in the powder experiments discussed above. 18,22 Since the dissolution yields are considerably higher for powders, a change in the redox reactivity also has a significant impact on the overall reactivity.…”

“…In this work, we have studied the oxidative dissolution of UO 2 pellets at H 2 O 2 exposures ranging from 0.33 mol m −2 to 1.36 mol m −2 . Based on the experimental results of this and previous studies, 18,20,22 the effect of H 2 O 2 exposure on the UO 2 surface reactivity is analysed and discussed.…”

“…It was recently observed, that the reaction between H 2 O 2 and UO 2 powder does not follow strict first order kinetics. 18,22 Instead, it turned out that the rate for H 2 O 2 consumption at a given H 2 O 2 concentration depends on the initial H 2 O 2 concentration. In other words, the overall kinetics changes with turnover of H 2 O 2 .…”

X-Ray and ultraviolet photoelectron spectroscopy studies of Uranium(iv),(v) and(vi) exposed to H2O-plasma under UHV conditions

“…They varied from hot-cell studies on leaching of real spent nuclear fuel [2][3][4] to lab experiments using UO 2 -powder, 5 UO 2 -pellets doped with non-radioactive isotopes of fission products [6][7][8] or alpha-emitters 9 suspended in aqueous solutions with or without H 2 O 2 5 or exposed to external gamma-fields in aqueous systems. [10][11][12][13] Due to the complexity of even the simplest of the model systems, many mechanistic aspects have only been indirectly confirmed from experiments. The reaction between the molecular radiolysis product H 2 O 2 and UO 2 is one example.…”

Time-dependent surface modification of uranium oxides exposed to water plasma

“…5 Initially, the oxidants will dominate the surface reactions for kinetic reasons. 6 Assessment of the relative impact of radiolytic oxidants under repository conditions revealed that H 2 O 2 is the oxidant of main importance. 7 In addition to oxidizing UO 2 , H 2 O 2 can also undergo catalytic decomposition on the UO 2 surface.…”