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Journal Articles

Impact of stoichiometry on the mechanism and kinetics of oxidative dissolution of UO$$_{2}$$ induced by H$$_{2}$$O$$_{2}$$ and $$gamma$$-irradiation

Kumagai, Yuta; Fidalgo, A. B.*; Jonsson, M.*

Journal of Physical Chemistry C, 123(15), p.9919 - 9925, 2019/04

https://doi.org/10.1021/acs.jpcc.9b00862
 Times Cited Count：19 Percentile：62.54(Chemistry, Physical)

Radiation-induced oxidative dissolution of uranium dioxide (UO$$_{2}$$) is one of the most important chemical processes of U driven by redox reactions. We have examined the effect of UO$$_{2}$$ stoichiometry on the oxidative dissolution of UO$$_{2}$$ induced by hydrogen peroxide (H$$_{2}$$O$$_{2}$$) and $$gamma$$-ray irradiation. By comparing the reaction kinetics of H$$_{2}$$O$$_{2}$$ between stoichiometric UO$$_{2.0}$$ and hyper-stoichiometric UO$$_{2.3}$$, we observed a significant difference in reaction speed and U dissolution kinetics. The stoichiometric UO$$_{2.0}$$ reacted with H$$_{2}$$O$$_{2}$$ much faster than the hyper-stoichiometric UO$$_{2.3}$$. The U dissolution from UO$$_{2.0}$$ was initially much lower than that from UO$$_{2.3}$$, but gradually increased as the oxidation by H$$_{2}$$O$$_{2}$$ proceeded. The $$gamma$$-ray irradiation induced the U dissolution that is analogous to the kinetics by the exposure to a low concentration (0.2 mM) of H$$_{2}$$O$$_{2}$$. The exposure to higher H$$_{2}$$O$$_{2}$$ concentrations caused lower U dissolution and resulted in deviation from the U dissolution behavior by $$gamma$$-ray irradiation.

Journal Articles

The Role of surface-bound hydroxyl radicals in the reaction between H$$_{2}$$O$$_{2}$$ and UO$$_{2}$$

Fidalgo, A. B.*; Kumagai, Yuta; Jonsson, M.*

Journal of Coordination Chemistry, 71(11-13), p.1799 - 1807, 2018/07

https://doi.org/10.1080/00958972.2018.1466287
 Times Cited Count：29 Percentile：88.5(Chemistry, Inorganic & Nuclear)

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 the reaction 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 U 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 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.

Oral presentation

Effects of hyperstoichiometry and adsorption of phthalic acid on oxidative dissolution of uranium oxide

Kumagai, Yuta; Fidalgo, A. B.*; Jonsson, M.*

no journal, , 

Radiation-induced oxidative dissolution of nuclear fuel is anticipated in the geological repository of spent fuel and after severe accidents, where the fuel would be in direct contact with water. Therefore, understanding of the oxidative dissolution of UO$$_{2}$$ is indispensable to estimate radioactive release. This study has addressed effects of hyperstoichiometry of UO$$_{2}$$ and adsorption of organic compounds on the process. Comparison between hyperstoichiometric UO$$_{2.3}$$ and stoichiometric UO$$_{2.0}$$ demonstrated that the reactivity of UO$$_{2.0}$$ approaches that of UO$$_{2.3}$$ by the H$$_{2}$$O$$_{2}$$ reaction. The result indicates formation of hyperstoichiometric surface layer during the oxidative dissolution. Phthalic acid, which was used as a model compound, suppressed the U dissolution by $$gamma$$-ray irradiation but had little involvement in the H$$_{2}$$O$$_{2}$$ reaction, in spite of adsorption exceeding 1 molecule/nm$$^{2}$$. The results suggest an involvement of radical intermediate derived from phthalic acid in the surface reaction.

Oral presentation

H$$_{2}$$O$$_{2}$$ and $$gamma$$-ray induced oxidative dissolution of stoichiometric UO$$_{2.0}$$

Kumagai, Yuta; Fidalgo, A. B.*; Jonsson, M.*

no journal, , 

We have investigated the effect of UO$$_{2}$$-hyper-stoichiometry on the oxidative dissolution upon H$$_{2}$$O$$_{2}$$ exposure and by water radiolysis. The oxidative dissolution of UO$$_{2}$$ is anticipated in the concept of geological disposal of spent nuclear fuel and also in the direct contact with coolant water after a severe accident. However, our understanding on the dissolution kinetics has so far not taken the hyper-stoichiometric nature of UO$$_{2}$$ into account. Thus, we have studied the oxidative dissolution of hyper-stoichiometric UO$$_{2.3}$$ and stoichiometric UO$$_{2.0}$$ and compared the results. The results show that the UO$$_{2}$$ stoichiometry largely affects the kinetics of oxidative dissolution. The oxidation of stoichiometric UO$$_{2.0}$$ in water proceeded fast accompanied with low initial U dissolution, but gradually the reaction became slower and U dissolution became more significant.

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