Durability of U-Zr oxide solid solution in aqueous HO 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

Uranium-zirconium oxide solid solution, (U, Zr)O, 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 with hydrogen peroxide (HO) in pure water to assess its resistance to oxidative dissolution, because HO is the dominant oxidant produced by water radiolysis. The dissolution behavior of uranium and zirconium was monitored through repeated HO 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 HO 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 against HO-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 HO-induced oxidation of (U, Zr)O in water, relevant to the long-term behavior of fuel debris.

Mutual separation of Ru, Rh, and Pd via reflux-assisted extraction and reverse-extraction using ion-pair and solvation with S- and amino-N-donor reagents

Sasaki, Yuji; Kaneko, Masashi; Matsumiya, Masahiko*; Kumagai, Yuta

Journal of Molecular Liquids, 441, p.129013_1 - 129013_10, 2026/01

https://doi.org/10.1016/j.molliq.2025.129013

This study determined extraction and back-extraction conditions for the mutual separation of three light PGMs, Ru, Rh, and Pd. Results revealed that reagents containing soft N and S donor atoms efficiently extract and strip Pd through solvation. In comparison, Ru and Rh undergo ion-pair extraction, requiring both anionic metal species and cationic extractants. These essential chlorinated PGM anions and protonated extractants having amino N atoms are present in HCl media. D(Ru) and D(Rh) values of approximately 100 and 10, respectively, were obtained using nitrilo-triacet-amide (NTAamide), which exhibits tetradentate coordination. Refluxing in 3-6 M HCl at 250 C, a condition that promotes the formation of highly chlorinated PGM anionic species, increased D(Ru) and D(Rh). Based on these findings, a flow diagram for the mutual separation of three PGMs was developed.

Extraction, separation and isolation of MA from Ln using two extractants (TODGA and ADAAM) and a masking agent (DTBA)

Sasaki, Yuji; Kaneko, Masashi; Kumagai, Yuta; Ban, Yasutoshi

Progress in Nuclear Science and Technology (Internet), 8, p.202 - 204, 2025/09

https://doi.org/10.15669/pnst.8.202

Two extractants and a masking agent of TODGA (TetraOctyl-DiGlycolAmide), ADAAM (AlkylDiAmideAMine), and DTBA (DiethyleneTriamine-triacetic-BisAmide) were developed in JAEA. TODGA can extract both trivalent actinides (An) and lanthanides (Ln), DTBA may separate An from Ln, and ADAAM has high separation factor (SF: 6) for Am/Cm. The suitable conditions for the extraction, separation and isolations of An from Ln are investigated using these reagents. In this work, we show the basic information on extraction behavior of An and Ln using TODGA, DTBA and ADAAM and propose the suitable aqueous and the organic conditions for An+Ln extraction, An/Ln separation and Am/Cm separation.

Separation of Am from lanthanides and Cm using the ADAAM(EH)-DTBA extraction system and elucidation of its complexation with DTBA through DFT calculations

Sasaki, Yuji; Kaneko, Masashi; Kumagai, Yuta; Ban, Yasutoshi

Solvent Extraction and Ion Exchange, 43(5-6), p.768 - 784, 2025/06

https://doi.org/10.1080/07366299.2025.2513982

We developed a method called the alkyl-diamide amine (ADAAM(EH))-diethylenetriamine-triacetic acid-bis(diethylacetamide) (DTBA) technique for separating actinide (An) and lanthanide (Ln) elements from high-level liquid waste. ADAAM(EH) exhibits a high separation factor (SF) not only for Am/Cm but also for light/heavy Ln under acidic conditions (1.5 M HNO). Consequently, Am can be separated from Cm as well as from middle Ln (Sm, Eu, and Gd) using ADAAM(EH) as an extractant. DTBA enables the stripping of Am and potentially separates it from Ln in the organic phase at low pH ( 2). The lowest SF among the light Ln, observed for Nd/Am, exceeded 10, indicating that Am can be separated from light Ln by stripping using DTBA from the ADAAM(EH) extraction solvent. The density functional theory calculations for understanding the coordination bond properties of metal-DTBA complexes are performed, which supported higher affinity of DTBA toward Am than Nd.

Consideration of the dielectric response for radiation chemistry simulations

Toigawa, Tomohiro; Kai, Takeshi; Kumagai, Yuta; Yokoya, Akinari*

Journal of Chemical Physics, 160(21), p.214119_1 - 214119_9, 2024/06

https://doi.org/10.1063/5.0211089

The spur reaction is crucial for determining radiolysis or photolysis in liquid, but the spur expansion process has yet to be elucidated. One reason is the need to understand the role of the dielectric response of the solvating molecules surrounding the charged species generated by ionization. The dielectric response corresponds to the time evolution of the permittivity and might affect the chemical reaction-diffusion of the species in a spur expansion process. This study examined the competitive relationship between reaction-diffusion kinetics and the dielectric response by solving the Debye-Smoluchowski equation while considering the dielectric response. The Coulomb force between the charged species gradually decreases with the dielectric response. Our calculation results found a condition where fast recombination occurs before the dielectric response is complete. Although it has been reported that the primary G-values of free electrons depend on the static dielectric constant under low-linear-energy transfer radiation-induced ionization, we propose that considering the dielectric response can provide a deeper insight into fast recombination reactions under high-linear-energy transfer radiation- or photo-induced ionization. Our simulation method enables the understanding of fast radiation-induced phenomena in liquids.

A Raman spectroscopy study of bicarbonate effects on UO

McGrady, J.; Kumagai, Yuta; Watanabe, Masayuki; Kirishima, Akira*; Akiyama, Daisuke*; Kimuro, Shingo; Ishidera, Takamitsu

Journal of Nuclear Science and Technology, 60(12), p.1586 - 1594, 2023/12

https://doi.org/10.1080/00223131.2023.2236104

UO dissolution in bicarbonate solution with HO; The Effect of temperature

McGrady, J.; Kumagai, Yuta; Kitatsuji, Yoshihiro; Kirishima, Akira*; Akiyama, Daisuke*; Watanabe, Masayuki

RSC Advances (Internet), 13(40), p.28021 - 28029, 2023/09

https://doi.org/10.1039/D2RA08131H

Upon nuclear waste canister failure and contact of spent nuclear fuel with groundwater, the UO matrix of spent fuel will interact with oxidants in the groundwater generated by water radiolysis. Bicarbonate (HCO) is often found in groundwater, and the HO induced oxidative dissolution of UO in bicarbonate solution has previously been studied under various conditions. Temperatures in the repository at the time of canister failure will differ depending on the location, yet the effect of temperature on oxidative dissolution is unknown. To investigate, the decomposition rate of HO at the UO surface and dissolution of U in bicarbonate solution (0.1, 1, 10 and 50 mM) was analysed at various temperatures (10, 25, 45 and 60 C). At [HCO] 1 mM, the apparent equilibrium concentration of U decreased with increasing temperature. This was attributed to the formation of U-bicarbonate species at the surface and a change in the mechanism of HO decomposition from oxidative to catalytic. At 0.1 mM, no obvious correlation between temperature and U dissolution was observed, and thermodynamic calculations indicated this was due to a change in the surface species. A pathway to explain the observed dissolution behaviour of UO in bicarbonate solution as a function of temperature was proposed.

Radiation-induced dissolution of uranium oxide materials in water

Kumagai, Yuta

Hoshasen Kagaku (Internet), (115), p.43 - 49, 2023/04

Oxidation and dissolution of uranium oxide materials has been a subject of numerous studies as a basis of the geological disposal technology for spent nuclear fuel. The understandings obtained by these studies provide useful suggestions for research and development regarding the retrieval and storage of nuclear fuel debris generated by a nuclear severe accident. Here, these research backgrounds of oxidative dissolution of uranium oxides are briefly reviewed and some studies relating to radiation-induced reactions will be introduced.

Application of high-energy-resolution X-ray absorption spectroscopy at the U L-edge to assess the U(V) electronic structure in FeUO

Yomogida, Takumi; Akiyama, Daisuke*; Ouchi, Kazuki; Kumagai, Yuta; Higashi, Kotaro*; Kitatsuji, Yoshihiro; Kirishima, Akira*; Kawamura, Naomi*; Takahashi, Yoshio*

Inorganic Chemistry, 61(50), p.20206 - 20210, 2022/12

https://doi.org/10.1021/acs.inorgchem.2c03208

FeUO was studied to clarify the electronic structure of U(V) in a metal monouranate compound. We obtained the peak splitting of HERFD-XANES spectra utilizing high-energy-resolution fluorescence detection-X-ray absorption near edge structure (HERFD-XANES) spectroscopy at the U L-edge, which is a novel technique in the U(V) compounds. Theoretical calculations revealed that the peak splitting was caused by splitting the 6d orbital of U(V). Such distinctive electronic states are of major interest to researchers and engineers working in various fields, from fundamental physics to the nuclear industry and environmental sciences for actinide elements.

Structure, stability, and actinide leaching of simulated nuclear fuel debris synthesized from UO, Zr, and stainless-steel

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

Journal of Nuclear Materials, 567, p.153842_1 - 153842_15, 2022/08

https://doi.org/10.1016/j.jnucmat.2022.153842

To understand the chemical structure and stability of nuclear fuel debris consisting of UO, Zr, and Stainless Steel (SUS) generated by the Fukushima Daiichi Nuclear Power Plant accident in Japan in 2011, simulated debris of the UO-SUS-Zr system and other fundamental component systems were synthesized and characterized. The simulated debris were synthesized by heat treatment for 1 to 12 h at 1600C, in inert (Ar) or oxidative (Ar + 2% O) atmospheres. Np and Am tracers were doped for the leaching tests of these elements and U from the simulated debris. The characterization of the simulated debris was conducted by XRD, SEM-EDX, Raman spectroscopy, and Mssbauer spectroscopy, which provided the major uranium phase of the UO -SUS-Zr debris was the solid solution of UO (s.s.) with Zr(IV) and Fe(II) regardless of the treatment atmosphere. The long-term immersion test of the simulated debris in pure water and that in seawater revealed the macro scale crystal structure of the simulated debris was chemically very stable in the wet condition for a year or more. Furthermore, the leaching test results showed that the actinide leaching ratios of U, Np, Am from the UO-SUS-Zr debris were very limited and less than 0.08 % for all the experiments in this study.