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Journal Articles

Radiation chemistry provides nanoscopic insights into the role of intermediate phases in CeO $_{2}$ mesocrystal formation

Li, Z.*; Piankova, D.*; Yang, Y.*; Kumagai, Yuta; Zschiesche, H.*; Jonsson, M.*; Tarakina, N. V.*; Soroka, I. L.*

Angewandte Chemie; International Edition, 61(6), p.e202112204_1 - e202112204_9, 2022/02

https://doi.org/10.1002/anie.202112204

Times Cited Count：4 Percentile：30.82(Chemistry, Multidisciplinary)

The role of intermediate phases in CeO $_{2}$ mesocrystal formation from aqueous Ce(III) solutions subjected to -radiation was studied. Radiolytically formed hydroxyl radicals convert soluble Ce(III) into less soluble Ce(IV). Transmission electron microscopy and X-ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO $_{2}$ mesocrystal evolution following the oxidation to Ce(IV): (1) formation of hydrated Ce(IV)-hydroxides, serving as intermediates in the liquid-to-solid phase transformation; (2) CeO $_{2}$ primary particle growth inside the intermediate phase; (3) alignment of the primary particles into "pre-mesocrystals" and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of mineral bridges. Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures.

Journal Articles

-radiation and H $_{2}$ O $_{2}$ induced oxidative dissolution of uranium(IV) oxide in aqueous solution containing phthalic acid

Kumagai, Yuta; Jonsson, M.*

Dalton Transactions (Internet), 49(6), p.1907 - 1914, 2020/02

https://doi.org/10.1039/c9dt03952j

Times Cited Count：0 Percentile：0.01(Chemistry, Inorganic & Nuclear)

This study aims to reveal possible involvements of organic acids in the oxidative dissolution of UO $_{2}$ . Using phthalic acid as a model compound, we have measured adsorption on UO $_{2}$ and investigated effects on the reaction between H $_{2}$ O $_{2}$ and UO $_{2}$ and on oxidative dissolution induced by -irradiation. Significant adsorption of phthalic acid was observed even at neutral pH. However, the reaction between H $_{2}$ O $_{2}$ and UO $_{2}$ in phthalic acid solution induced oxidative dissolution of U(VI) similarly to aqueous bicarbonate solution. These results indicate that even though phthalic acid adsorbs on the UO $_{2}$ surface, it is not involved in the interfacial reaction by H $_{2}$ O $_{2}$ . In contrast, the dissolution of U by irradiation was inhibited in aqueous phthalic acid solution, whereas H $_{2}$ O $_{2}$ generated by radiolysis was consumed by UO $_{2}$ . The inhibition suggests that radical species derived from phthalic acid was involved in the redox reaction process of UO $_{2}$ .

Journal Articles

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

Journal Articles

Overall approaches and experiences of first-time participants in the Nuclear Forensics International Technical Working Group's Fourth Collaborative Material Exercise (CMX-4)

Ho, D. M. L.*; Nelwamondo, A. N.*; Okubo, Ayako; Ramebck, H.*; Song, K.*; Han, S.-H.*; Hancke, J. J.*; Holmgren, S.*; Jonsson, S.*; Kataoka, Osamu; et al.

Journal of Radioanalytical and Nuclear Chemistry, 315(2), p.353 - 363, 2018/02

https://doi.org/10.1007/s10967-017-5677-0

Times Cited Count：2 Percentile：20.93(Chemistry, Analytical)

The Fourth Collaborative Material Exercise (CMX-4) of the Nuclear Forensics International Technical Working Group (ITWG) registered the largest participation for this exercise in nuclear forensics, with seven of the 17 laboratories participating for the first time. In this paper, participants from five of the first-time laboratories shared their individual experience in this exercise, from preparation to analysis of samples. The exercise proved to be highly useful for testing procedures, repurposing established methods, exercising skills, and improving the understanding of nuclear forensic signatures and their interpretation trough the post-exercise review meeting.

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 -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 -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.

Oral presentation

Oxidative uranium dissolution from UO $_{2}$ in the presence of adsorbed phthalic acid

Kumagai, Yuta; Jonsson, M.*

no journal, ,

Contact of water with spent nuclear fuel is anticipated in scenarios of failure of the repository system for the direct disposal of spent fuel. Upon the direct contact of water, the UO $_{2}$ matrix of the fuel is expected to gradually dissolve due to oxidation of uranium by the action of ionizing radiation. In this study, we examined effects of organic acid on the UO $_{2}$ dissolution by using phthalic acid as a model compound. We investigated oxidation of UO $_{2}$ by exposure to H $_{2}$ O $_{2}$ in aqueous solution containing phthalic acid. Significant adsorption of phthalic acid on UO $_{2}$ was observed. The coverage of the surface was estimated to reach 80 %. The H $_{2}$ O $_{2}$ -exposure experiments revealed that adsorbed phthalic acid has no significant effect on the redox reaction by H $_{2}$ O $_{2}$ on the UO $_{2}$ surface, despite the high surface density. H $_{2}$ O $_{2}$ oxidation of UO $_{2}$ with adsorbed phthalic acid resulted in U dissolution to similar extents with the U dissolution measured in aqueous bicarbonate solutions.