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Journal Articles

Durability of U-Zr oxide solid solution in aqueous H$$_2$$O$$_2$$ solution

Kumagai, Yuta; Kusaka, Ryoji; Takano, Masahide; Watanabe, Masayuki

Journal of Nuclear Materials, 625, p.156553_1 - 156553_7, 2026/04

https://doi.org/10.1016/j.jnucmat.2026.156553
 Times Cited Count：0 Percentile：0.00(Materials Science, Multidisciplinary)

Uranium-zirconium oxide solid solution, (U, Zr)O$$_2$$, is a representative matrix phase found in fuel debris formed during severe nuclear reactor accidents. Understanding its chemical behavior in oxidative aqueous environments is important for evaluating the potential release of radionuclides during water contact. In this study, we investigated the reactivity of (U, Zr)O$$_2$$ with hydrogen peroxide (H$$_2$$O$$_2$$) in pure water to assess its resistance to oxidative dissolution, because H$$_2$$O$$_2$$ is the dominant oxidant produced by water radiolysis. The dissolution behavior of uranium and zirconium was monitored through repeated H$$_2$$O$$_2$$ exposure experiments, and the solid phases were characterized using Raman micro-spectroscopy and X-ray diffraction. Kinetic modeling was performed to interpret experimental data. The results showed that uranium dissolution occurred initially but decreased significantly upon repeated H$$_2$$O$$_2$$ exposure, while zirconium dissolution proceeded more slowly. Raman analysis revealed only minor surface changes, with limited formation of uranyl peroxide phases. The kinetic simulation reproduced the experimental trends by assuming a small fraction of redox-active surface sites. These findings suggest that the observed durability of (U, Zr)O$$_2$$ against H$$_2$$O$$_2$$-induced oxidative dissolution is not due to the formation of a protective surface layer, but rather reflects the limited redox reactivity of most of the surface. This study provides a quantitative basis for understanding the H$$_2$$O$$_2$$-induced oxidation of (U, Zr)O$$_2$$ in water, relevant to the long-term behavior of fuel debris.

Journal Articles

Study on the relation between the crystal structure and thermal stability of FeUO$$_{4}$$ and CrUO$$_{4}$$

Akiyama, Daisuke*; Kusaka, Ryoji; Kumagai, Yuta; Nakada, Masami; Watanabe, Masayuki; Okamoto, Yoshihiro; Nagai, Takayuki; Sato, Nobuaki*; Kirishima, Akira*

Journal of Nuclear Materials, 568, p.153847_1 - 153847_10, 2022/09

https://doi.org/10.1016/j.jnucmat.2022.153847
 Times Cited Count：7 Percentile：61.11(Materials Science, Multidisciplinary)

FeUO$$_{4}$$, CrUO$$_{4}$$, and Fe$$_{x}$$Cr$$_{1-x}$$UO$$_{4}$$ are monouranates containing pentavalent U. Even though these compounds have similar crystal structures, their formation conditions and thermal stability are significantly different. To determine the factors causing the difference in thermal stability between FeUO$$_{4}$$ and CrUO$$_{4}$$, their crystal structures were evaluated in detail. A Raman band was observed at 700 cm$$^{-1}$$ in all the samples. This Raman band was derived from the stretching vibration of the O-U-O axis band, indicating that Fe$$_{x}$$Cr$$_{1-x}$$UO$$_{4}$$ was composed of a uranyl-like structure in its lattice regardless of its "x"' value. M$"o$ssbauer measurements indicated that the Fe in FeUO$$_{4}$$ and Fe$$_{x}$$Cr$$_{1-x}$$UO$$_{4}$$ were trivalent. Furthermore, Fe$$_{x}$$Cr$$_{1-x}$$UO$$_{4}$$ lost its symmetry around Fe$$^{mathrm{III}}$$ with increasing electron densities around Fe$$^{mathrm{III}}$$, as the abundance of Cr increased. These results suggested no significant structural differences between FeUO$$_{4}$$ and CrUO$$_{4}$$. Thermogravimetric measurements for UO$$_{2}$$, FeUO$$_{4}$$, and CrUO$$_{4}$$ showed that the temperature at which FeUO$$_{4}$$ decomposed under an oxidizing condition (approximately 800 $$^{circ}$$C) was significantly lower than the temperature at which the decomposition of CrUO$$_{4}$$ started (approximately 1250 $$^{circ}$$C). Based on these results, we concluded that the decomposition of FeUO$$_{4}$$ was triggered by an "in-crystal" redox reaction, i.e., Fe$$^{mathrm{III}}$$ $${+}$$ U$$^{mathrm{V}}$$ $$rightarrow$$ Fe$$^{mathrm{II}}$$ $${+}$$ U$$^{mathrm{VI}}$$, which would not occur in the CrUO$$_{4}$$ lattice because Cr$$^{mathrm{III}}$$ could never be reduced under the investigated condition. Finally, the existence of Cr$$^{mathrm{III}}$$ in FexCr$$_{1-x}$$UO$$_{4}$$ effectively suppressed the decomposition of the Fe$$_{x}$$Cr$$_{1-x}$$UO$$_{4}$$ crystal, even at a very low Cr content.

Journal Articles

Oxygen potentials of(Th,U)O$$_{2}$$$$_{+}$$$$_{x}$$ solid solutions

Ugajin, Mitsuhiro

J. Nucl. Mater., 110, p.140 - 146, 1982/00

https://doi.org/10.1016/0022-3115(82)90138-6
 Times Cited Count：25 Percentile：88.90(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Stoichiometry of fully oxidized PuO$$_{2}$$-NdO$$_{1}$$$$_{.}$$$$_{5}$$ and PuO$$_{2}$$-YO$$_{1}$$$$_{.}$$$$_{5}$$ solid solutions

; ; ;

Journal of Nuclear Science and Technology, 19(8), p.681 - 683, 1982/00

https://doi.org/10.3327/jnst.19.681
 Times Cited Count：2 Percentile：43.07(Nuclear Science & Technology)

no abstracts in English

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