The role of surface-bound hydroxyl radicals in the reaction between H₂O₂ and UO₂

Abstract

In this work, we have studied the reaction between H 2 O 2 and UO 2 with particular focus on the nature of the hydroxyl radical formed as an intermediate. Experiments were performed to study the kinetics of H 2 O 2 consumption and uranium dissolution at different initial H 2 O 2 concentrations. The results show that the consumption rates at a given H 2 O 2 concentration are different depending on the initial H 2 O 2 concentration. This is attributed to an alteration of the reactive interface, likely caused by blocking of surface sites by oxidized U/surface-bound hydroxyl radicals. The dissolution yield given by the amount of dissolved uranium divided by the amount of consumed hydrogen peroxide was used to compare the different cases. For all initial H 2 O 2 concentrations, the dissolution yield increases with reaction time. The final dissolution yield decreases with increasing initial H 2 O 2 concentration. This is expected from the mechanism of catalytic decomposition of H 2 O 2 on oxide surfaces. As the experiments were performed in solutions containing 10 mM HCO − 3 and a strong concentration dependence was observed in the 0.2-2.0 mM H 2 O 2 concentration range, we conclude that the intermediate hydroxyl radical is surface bound rather than free.

Confirming the Formation of Hydroxyl Radicals in the Catalytic Decomposition of H2O2 on Metal Oxides Using Coumarin as a Probe

“…The nature of surface‐bound hydroxyl radicals has been investigated fairly extensively using DFT . Quite recently, the surface bound hydroxyl radical formed in the first step of the catalytic decomposition of H 2 O 2 on oxide surfaces has also been found to play a major role in the oxidative dissolution of used UO 2 ‐based nuclear fuel under waste repository conditions . In general, catalytic decomposition of H 2 O 2 on metal oxide surfaces is an important process in nuclear technology (to understand radiation induced corrosion) and it can also be used as an advanced oxidation technique (AOT) to purify water from organic pollutants.…”

Pd‐Catalyzed Surface Reactions of Importance in Radiation Induced Dissolution of Spent Nuclear Fuel Involving H2

“…The dissolution yield has been found to depend on the H 2 O 2 concentration. [6] When spent nuclear fuel is discarded from a nuclear reactor it consists of around 95 % UO 2 , the remaining 5 % being fission products and heavier actinides. [7] Fission products are categorized based on their appearance in spent nuclear fuel according to: 1) Fission gases and volatile fission products, 2) Fission products forming metallic precipitates (ɛ-particles), 3) fission products forming oxide precipitates as well as 4) fission products which substitute U in the fuel matrix.…”

Dynamics of Spent Nuclear Fuel Dissolution : Impact of Catalysis, Matrix Composition and Time Evolution

“…The high H 2 O 2 exposures were reached, after repeating the experiment three times for a given pellet. In comparison, the typical surface area over solution volume ratios for previously published experiments are around 22 m -1 for pellets [100,128] and between 2700 m -1 to 10800 m -1 for powders [113]. H 2 O 2 exposures for these experiments are around 0.08 mol m -2 for pellets, whereas for powders exposures range from 1.8 x 10 -5 mol m -2 to 7.4 x 10 -4 mol m -2 .…”

“…Due to generally higher dissolution yields in the powder experiments described above, a change in redox reactivity also results in a change of the overall kinetics for H 2 O 2 consumption. When combining the results from figure 9 with the high surface area over solution volume ratio experiments on powders [113] and a pellet [128], figure 10a is obtained. While experiments on UO 2 powders imply high surface area over solution volume ratios and therefore low accumulated H 2 O 2 exposures per surface area, experiments on pellets P1 to P5 represent the other extreme in term of surface area over solution volume ratio or accumulated exposure.…”

“…It is important to note, that the lifetime as well as the reactivity of surface bound hydroxyl radicals are not the same as for free hydroxyl radicals [111][112][113].…”