Investigation of chemical state of uranium included in simulated waste glass

Katsuoka, Nanako; Akiyama, Daisuke*; Kirishima, Akira*; Nagai, Takayuki; Okamoto, Yoshihiro; Sato, Nobuaki*

2023-Nendo "Busshitsu, Debaisu Ryoiki Kyodo Kenkyu Kyoten" Kenkyu Seika, Katsudo Hokokusho (Internet), 1 Pages, 2024/07

no abstracts in English

Dissolution behavior and aging of iron-uranium oxide

Tonna, Ryutaro*; Sasaki, Takayuki*; Okamoto, Yoshihiro; Kobayashi, Taishi*; Akiyama, Daisuke*; Kirishima, Akira*; Sato, Nobuaki*

Journal of Nuclear Materials, 589, p.154862_1 - 154862_10, 2024/02

https://doi.org/10.1016/j.jnucmat.2023.154862

The dissolution behavior of FeUO compounds formed by a high-temperature reaction of UO with iron, a stainless-steel component of reactor structural materials, was investigated under atmospheric conditions. The compounds were prepared in an electric furnace using UO and FeO as starting materials, and their solid states were analyzed using X-ray diffraction, scanning electron microscopy energy dispersive X-ray spectroscopy, and X-ray absorption fine structure spectroscopy. The concentration of nuclides dissolved in water was examined by performing static leaching tests of FeUO compounds for up to three months. A redox reaction was proposed to occur between trivalent Fe and pentavalent U ions in the early stage of FeUO dissolution. It was thermodynamically deduced that the reduced divalent Fe ion was finally oxidized into a trivalent ion in the presence of dissolved oxygen, and iron hydroxide limited the solubility of Fe. Meanwhile, the concentration of hexavalent U (i.e., uranyl ion) was limited owing to the presence of secondary minerals such as metaschoepite and sodium uranate and subsequently decreased, possibly owing to sorption on Fe oxides, for example. The concentrations of multivalent ions of fission products, such as Ru and Ce, also decreased, likely for the reason above. By contrast, the concentration of soluble Cs ions did not decrease. The validity of this interpretation was supported by comparing the results with the dissolution behavior of a reference sample (Fe-free UO).

Summary report in FY2022 of subsidy program for "the Project of Decommissioning and Contaminated Water Management (Development of Analysis and Estimation Technology for Characterization of Fuel Debris (Development of Technologies for Enhanced Analysis Accuracy, Thermal behavior Estimation, and Simplified Analysis of Fuel Debris)" started in FY2011

Koyama, Shinichi; Ikeuchi, Hirotomo; Mitsugi, Takeshi; Maeda, Koji; Sasaki, Shinji; Onishi, Takashi; Tsai, T.-H.; Takano, Masahide; Fukaya, Hiroyuki; Nakamura, Satoshi; et al.

Hairo, Osensui, Shorisui Taisaku Jigyo Jimukyoku Homu Peji (Internet), 216 Pages, 2023/11

https://en.dccc-program.jp/wp-content/uploads/20231117_JAEA.pdf

In FY 2021 and 2022, JAEA perfomed the subsidy program for "the Project of Decommissioning and Contaminated Water Management (Development of Analysis and Estimation Technology for Characterization of Fuel Debris (Development of Technologies for Enhanced Analysis Accuracy, Thermal Bahavior Estimation, and Simplified Analysis of Fuel Debris)" started in FY 2021. This presentation material summarized the results of the project, which will be available shortly on the website of Management Office for the Project of Decommissioning, Contaminated Water and Treated Water Management.

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.

Dry synthesis of brannerite (UTiO) by mechanochemical treatment

Akiyama, Daisuke*; Mishima, Tomoki*; Okamoto, Yoshihiro; Kirishima, Akira*

High Temperature Materials and Processes, 42(1), p.20220268_1 - 20220268_9, 2023/04

https://www.degruyter.com/document/doi/10.1515/htmp-2022-0268/html

A powder mixture of UO and TiO was mechanochemically treated in a planetary ball mill under Ar atmosphere for 1 h using a tungsten carbide vial and balls as the milling medium. Such mechanochemical (MC) treatment reduced the crystallinity of UO and TiO. The mechanochemically treated powder mixture was heated at 973-1573K for 6 h under Ar atmosphere and analyzed by X-ray diffraction analysis, scanning electron microscopy-energy-dispersive X-ray spectroscopy, and X-ray absorption fine structure analysis. UTiO did not form below 1373K without MC treatment and only the starting materials were observed. At 1473 and 1573K, a small amount of UTiO and equal amounts of UTiO and UO were formed, respectively. The mechanochemically treated sample produced nearly pure UTiO containing small amounts of UO impurities when heated above 1173K for 6 h. UTiO was highly crystalline and uniform regardless of the synthesis temperature.

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.

Study on the relation between the crystal structure and thermal stability of FeUO and CrUO

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

FeUO, CrUO, and FeCrUO 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 and CrUO, their crystal structures were evaluated in detail. A Raman band was observed at 700 cm in all the samples. This Raman band was derived from the stretching vibration of the O-U-O axis band, indicating that FeCrUO was composed of a uranyl-like structure in its lattice regardless of its "x"' value. Mssbauer measurements indicated that the Fe in FeUO and FeCrUO were trivalent. Furthermore, FeCrUO lost its symmetry around Fe with increasing electron densities around Fe, as the abundance of Cr increased. These results suggested no significant structural differences between FeUO and CrUO. Thermogravimetric measurements for UO, FeUO, and CrUO showed that the temperature at which FeUO decomposed under an oxidizing condition (approximately 800 C) was significantly lower than the temperature at which the decomposition of CrUO started (approximately 1250 C). Based on these results, we concluded that the decomposition of FeUO was triggered by an "in-crystal" redox reaction, i.e., Fe U Fe U, which would not occur in the CrUO lattice because Cr could never be reduced under the investigated condition. Finally, the existence of Cr in FexCrUO effectively suppressed the decomposition of the FeCrUO crystal, even at a very low Cr content.

Uranium dissolution and uranyl peroxide formation by immersion of simulated fuel debris in aqueous HO solution

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

Journal of Nuclear Science and Technology, 59(8), p.961 - 971, 2022/08

https://doi.org/10.1080/00223131.2021.2023055

We investigated potential degradation of fuel debris caused by HO, which is the oxidant of major impact from water radiolysis. We performed leaching experiments on different kinds of simulated debris comprising U, Fe, Cr, Ni, and Zr in an aqueous HO solution. Chemical analysis of the leaching solution showed that U dissolution was induced by HO. Raman analysis after the leaching revealed that uranyl peroxides were formed on the surface of the simulated debris. These results demonstrate that uranyl peroxides are possible alteration products of fuel debris from HO reaction. However, the sample in which the main uranium-containing phase was a U-Zr oxide solid solution showed much less uranium dissolution and no Raman signal of uranyl peroxides. Comparison of these results indicates that formation of an oxide solid solution of Zr with UO improves the stability of fuel debris against HO reaction.

A Study of HO-induced oxidative degradation of simulated fuel debris

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

Hoshasen Kagaku (Internet), (113), p.61 - 64, 2022/04

The severe accident at TEPCO's Fukushima Daiichi Nuclear Power Station resulted in generation of fuel debris. The fuel debris is in contact with water and the radiolysis of water can accelerate degradation of the debris. The analysis of particles sampled from inside or near the damaged reactors indicates the complicated compositions of the fuel debris. It is challenging to estimate the effect of water radiolysis on such a complicated material. Therefore, in this study, we investigated the potential degradation process by leaching experiments of simulated fuel debris in aqueous HO solution. The results show that the reaction of HO induced uranium dissolution from most of the samples and then formation of uranyl peroxides. In contrast, a sample that had U-Zr oxide solid solution as the major phase exhibited remarkable resistance to HO. These findings revealed that the degradation of the simulated debris reflects the reactivity and stability of the uranium phase in the matrices.

Star-polymer-DNA gels showing highly predictable and tunable mechanical responses

Ohira, Masashi*; Katashima, Takuya*; Naito, Mitsuru*; Aoki, Daisuke*; Yoshikawa, Yusuke*; Iwase, Hiroki*; Takata, Shinichi; Miyata, Kanjiro*; Chung, U.-I.*; Sakai, Takamasa*; et al.

Advanced Materials, 34(13), p.2108818_1 - 2108818_9, 2022/01

https://doi.org/10.1002/adma.202108818