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Journal Articles

Oxygen chemical diffusion in fast neutron reactor fuel U $_{0.698}$ Pu $_{0.289}$ Am $_{0.013}$ O $_{2-x}$

Vauchy, R.; Horii, Yuta; Hirooka, Shun; Akashi, Masatoshi; Sunaoshi, Takeo*; Nakamichi, Shinya; Saito, Kosuke

Proceedings of 34th Nuclear Energy for New Europe (NENE2025) , p.232 - 238, 2026/01

Journal Articles

Oxygen potential and oxygen diffusion data for guiding the manufacture of MOX fuel for fast neutron reactors

Vauchy, R.; Horii, Yuta; Hirooka, Shun; Akashi, Masatoshi; Sunaoshi, Takeo*; Nakamichi, Shinya; Saito, Kosuke

Journal of Nuclear Materials, 616, p.156115_1 - 156115_16, 2025/10

https://doi.org/10.1016/j.jnucmat.2025.156115

Times Cited Count：1 Percentile：37.36(Materials Science, Multidisciplinary)

Journal Articles

Fabrication of low-O/M fast reactor MOX fuel and analysis on its oxygen potential behaviors

Hirooka, Shun; Vauchy, R.; Horii, Yuta; Sunaoshi, Takeo*; Saito, Kosuke

Proceedings of TopFuel 2025; Nuclear Reactor Fuel Performance Conference (Internet), 10 Pages, 2025/10

Reducing the oxygen-to-metal (O/M) ratio in MOX fuels plays an important role in suppressing the corrosion depth in the cladding due to fuel-cladding chemical interaction (FCCI), which is the key to determining the lifetime of fast reactor MOX fuels. Owing to a number of irradiation and post-irradiation experiments, a clear decreasing trend in the corrosion depth with lower O/M ratio in the as-fabricated MOX pellet was reported, whereas a significant redistribution of the O/M ratio in a pellet driven by the radial temperature gradient during irradiation could supply a higher oxygen potential near the pellet periphery where the FCCI should occur. The reduction of the O/M ratio in the MOX pellet fabrication processes can be achieved by heat treatment by taking high temperature, longer time, and lower oxygen partial pressure in the gas into account; the properties governing the reduction are not sufficiently studied. This study demonstrated the variation of O/M ratio in MOX pellets and the in-situ O/M ratio during the heat treatment was analyzed by using a thermogravimeter, which revealed a decreasing behavior during heating and dwell as well as an increasing behavior in the O/M ratio during the cooling step. Furthermore, the redistribution of O/M ratio, analyzed by Sari's model, was discussed to investigate the O/M ratio and the oxygen potential near the pellet periphery which is likely to have a more direct influence on the FCCI than the as-fabricated O/M ratio. By using the recent oxygen potential data on MOX, it is found that the oxygen potential profile in the radial direction is especially drastic near the pellet periphery and is sensitive to the as-fabricated O/M ratio.

Journal Articles

Control and irradiation behaviors of oxygen potential of MOX fuel

Hirooka, Shun; Vauchy, R.; Horii, Yuta; Sunaoshi, Takeo*; Saito, Kosuke; Ozawa, Takayuki

Proceedings of Workshop on Fuel Performance Assessment and Behaviour for Liquid Metal Cooled Fast Reactors (Internet), 8 Pages, 2025/07

no abstracts in English

Journal Articles

Reduction and phase transformation of Ce-doped zirconolites

Hayashizaki, Kohei; Hirooka, Shun; Yamada, Tadahisa*; Sunaoshi, Takeo*; Murakami, Tatsutoshi; Saito, Kosuke

Ceramics (Internet), 8(1), p.24_1 - 24_12, 2025/03

https://doi.org/10.3390/ceramics8010024

Journal Articles

Uranium-plutonium-oxygen phase diagram; Investigating the solvus of fluorite's exsolution

Vauchy, R.; Hirooka, Shun; Horii, Yuta; Ogasawara, Masahiro*; Sunaoshi, Takeo*; Yamada, Tadahisa*; Tamura, Tetsuya*; Murakami, Tatsutoshi

Journal of Nuclear Materials, 599, p.155233_1 - 155233_11, 2024/10

https://doi.org/10.1016/j.jnucmat.2024.155233

Times Cited Count：2 Percentile：34.48(Materials Science, Multidisciplinary)

The fluorite exsolution/recombination in U $_{1-y}$ Pu $_{y}$ O $_{2-x}$ (y = 0.30 and 0.45) and PuO $_{2-x}$ was investigated using differential scanning calorimetry. The results are in relatively good agreement with the literature data, except for plutonia. Our values indicate that the critical temperature of the miscibility gap in Pu-O is 3050 K lower than previously reported. Finally, the systematic experimental procedure allowed us refining the locus of the solvus existing in hypostoichiometric U $_{0.70}$ 0Pu $_{0.30}$ O $_{2-x}$ , U $_{0.55}$ Pu $_{0.45}$ O $_{2-x}$ , and PuO $_{2-x}$ dioxides.

Journal Articles

Sintering behavior analysis of compacted dry recycled U $_{0.7}$ Pu $_{0.3}$ O $_{2}$ powder using master sintering curve theory

Nakamichi, Shinya; Sunaoshi, Takeo*; Hirooka, Shun; Vauchy, R.; Murakami, Tatsutoshi

Journal of Nuclear Materials, 595, p.155072_1 - 155072_11, 2024/07

https://doi.org/10.1016/j.jnucmat.2024.155072

Times Cited Count：2 Percentile：34.48(Materials Science, Multidisciplinary)

Journal Articles

Sintering and microstructural behaviors of mechanically blended Nd/Sm-doped MOX

Hirooka, Shun; Horii, Yuta; Sunaoshi, Takeo*; Uno, Hiroki*; Yamada, Tadahisa*; Vauchy, R.; Hayashizaki, Kohei; Nakamichi, Shinya; Murakami, Tatsutoshi; Kato, Masato

Journal of Nuclear Science and Technology, 60(11), p.1313 - 1323, 2023/11

https://doi.org/10.1080/00223131.2023.2192725

Times Cited Count：5 Percentile：56.11(Nuclear Science & Technology)

Additive MOX pellets are fabricated by a conventional dry powder metallurgy method. Nd $_{2}$ O $_{3}$ and Sm $_{2}$ O $_{3}$ are chosen as the additive materials to simulate the corresponding soluble fission products dispersed in MOX. Shrinkage curves of the MOX pellets are obtained by dilatometry, which reveal that the sintering temperature is shifted toward a value higher than that of the respective regular MOX. The additives, however, promote grain growth and densification, which can be explained by the effect of oxidized uranium cations covering to a pentavalent state. Ceramography reveals large agglomerates after sintering, and Electron Probe Micro-Analysis confirms that inhomogeneous elemental distribution, whereas XRD reveals a single face-centered cubic phase. Finally, by grinding and re-sintering the specimens, the cation distribution homogeneity is significantly improved, which can simulate spent nuclear fuels with soluble fission products.

Journal Articles

Oxygen potential of neodymium-doped U $_{0.817}$ Pu $_{0.180}$ Am $_{0.003}$ O $_{2 pm x}$ uranium-plutonium-americium mixed oxides at 1573, 1773, and 1873 K

Vauchy, R.; Sunaoshi, Takeo*; Hirooka, Shun; Nakamichi, Shinya; Murakami, Tatsutoshi; Kato, Masato

Journal of Nuclear Materials, 580, p.154416_1 - 154416_11, 2023/07

https://doi.org/10.1016/j.jnucmat.2023.154416

Times Cited Count：10 Percentile：82.28(Materials Science, Multidisciplinary)

Journal Articles

Liquid phase sintering of alumina-silica co-doped cerium dioxide CeO $_{2}$ ceramics

Vauchy, R.; Hirooka, Shun; Watanabe, Masashi; Yokoyama, Keisuke; Sunaoshi, Takeo*; Yamada, Tadahisa*; Nakamichi, Shinya; Murakami, Tatsutoshi

Ceramics International, 49(2), p.3058 - 3065, 2023/01

https://doi.org/10.1016/j.ceramint.2022.09.295

Times Cited Count：14 Percentile：57.03(Materials Science, Ceramics)

Journal Articles

Relative oxygen potential measurements of (U,Pu)O $_{2}$ with Pu = 0.45 and 0.68 and related defect formation energy

Hirooka, Shun; Matsumoto, Taku; Sunaoshi, Takeo*; Hino, Tetsushi*

Journal of Nuclear Materials, 558, p.153375_1 - 153375_8, 2022/01

https://doi.org/10.1016/j.jnucmat.2021.153375

Times Cited Count：8 Percentile：50.06(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Oxygen potential measurement of (U,Pu,Am)O $_{2 pm x}$ and (U,Pu,Am,Np)O $_{2 pm x}$

Hirooka, Shun; Matsumoto, Taku; Kato, Masato; Sunaoshi, Takeo*; Uno, Hiroki*; Yamada, Tadahisa*

Journal of Nuclear Materials, 542, p.152424_1 - 152424_9, 2020/12

https://doi.org/10.1016/j.jnucmat.2020.152424

Times Cited Count：14 Percentile：72.37(Materials Science, Multidisciplinary)

The measurement of oxygen potential was conducted at 1,673, 1,773, and 1,873 K for (U $_{0.623}$ Pu $_{0.350}$ Am $_{0.027}$ )O $_{2}$ and at 1,873 and 1,923 K for (U $_{0.553}$ Pu $_{0.285}$ Am $_{0.015}$ Np $_{0.147}$ )O $_{2}$ by using a thermo-gravimeter and an oxygen sensor. Am inclusion in terms of substituting the U significantly increased the oxygen potential. Similarly, the inclusion of Np as a substitute for U increased the oxygen potential; however, the effect was not as large as that with the Pu or Am addition at the same rate. The results were analyzed via defect chemistry and certain defect formations were suggested in the reducing region and the near-stoichiometric region by plotting the relationship between PO $_{2}$ and the deviation from the stoichiometry. The equilibrium constants of the defect reactions were arranged to reproduce the experiment such that Am/Np contents were included in the entropy with coefficients fitting the experimental data.

Journal Articles

Oxygen self-diffusion in near stoichiometric (U,Pu)O $_{2}$ at high temperatures of 1673-1873 K

Watanabe, Masashi; Kato, Masato; Sunaoshi, Takeo*

Journal of Nuclear Materials, 542, p.152472_1 - 152472_7, 2020/12

https://doi.org/10.1016/j.jnucmat.2020.152472

Times Cited Count：6 Percentile：41.38(Materials Science, Multidisciplinary)

The oxygen self-diffusion coefficients in near stoichiometric (U,Pu)O $_{2}$ at high temperatures were successfully measured by thermogravimetry combined with the oxygen isotope exchange method. The activation energy for oxygen diffusion in the stoichiometric composition of (U,Pu)O $_{2}$ was evaluated from experimental data, and the value was determined to be 248 kJ/mol. In addition, the defect migration energies of (U,Pu)O $_{2 pm x}$ were derived, and the oxygen self-diffusion coefficients were evaluated using these. As a result, good agreement was found between the experimental data and the oxygen self-diffusion coefficients calculated using the defect migration energies.

Journal Articles

Effect of O/M ratio on sintering behavior of (Pu $_{0.3}$ U $_{0.7}$ )O $_{2-x}$

Nakamichi, Shinya; Hirooka, Shun; Kato, Masato; Sunaoshi, Takeo*; Nelson, A. T.*; McClellan, K. J.*

Journal of Nuclear Materials, 535, p.152188_1 - 152188_8, 2020/07

https://doi.org/10.1016/j.jnucmat.2020.152188

Times Cited Count：19 Percentile：82.12(Materials Science, Multidisciplinary)

Oxygen-to-metal ratio (O/M) of uranium and plutonium mixed oxide depends on its oxygen partial pressure. To attain the desirable microstructure and O/M ratio of sintered pellets, it is important to investigate the relation between the sintering behavior and the atmosphere of sintering process. In this study, sintering behavior of (Pu $_{0.3}$ U $_{0.7}$ )O $_{2}$ and (Pu $_{0.3}$ U $_{0.7}$ )O $_{1.99}$ in controlled po $_{2}$ atmosphere were investigated. It was found activation energy of (Pu $_{0.3}$ U $_{0.7}$ )O $_{1.99}$ was higher than that of (Pu $_{0.3}$ U $_{0.7}$ )O $_{2}$ . On the other hand, it was observed grain growth during sintering was suppressed in hypo-stoichiometric composition.

Journal Articles

Thermal and mechanical properties of CeO $_{2}$

Suzuki, Kiichi; Kato, Masato; Sunaoshi, Takeo*; Uno, Hiroki*; Carvajal-Nunez, U.*; Nelson, A. T.*; McClellan, K. J.*

Journal of the American Ceramic Society, 102(4), p.1994 - 2008, 2019/04

https://doi.org/10.1111/jace.16055

Times Cited Count：65 Percentile：93.70(Materials Science, Ceramics)

The fundamental properties of CeO $_{2}$ were assessed using a range of experimental techniques. The oxygen potential of CeO $_{2}$ was measured by the thermogravimetric technique, and a numerical fit for the oxygen potential of CeO $_{2}$ is derived based on defect chemistry. Mechanical properties of CeO $_{2}$ were obtained using sound velocity measurement, resonant ultrasound spectroscopy and nanoindentation. The obtained mechanical properties of CeO $_{2}$ are then used to evaluate the Debye temperature and Gruneisen constant. The heat capacity and thermal conductivity of CeO $_{2}$ were also calculated using the Debye temperature and the Gruneisen constant. Finally, the thermal conductivity was calculated based upon laser flash analysis measurements. This result demonstrates that the thermal conductivity has strong dependence upon material purity.

Journal Articles

Oxygen chemical diffusion coefficients of (U, Pu)O $_{2-x}$

Watanabe, Masashi; Sunaoshi, Takeo*; Kato, Masato

Defect and Diffusion Forum, 375, p.84 - 90, 2017/05

https://doi.org/10.4028/www.scientific.net/DDF.375.84

The oxygen chemical diffusion coefficient in (U, Pu)O $_{2-x}$ was determined by thermo-gravimetry as functions of the Pu content, oxygen-to-metal ratio and temperature. The surface reaction was considered in the diffusion coefficient determination. The activation energy for the chemical diffusion coefficient was 60 kJ/mol and 65 kJ/mol, respectively, in (U $_{0.8}$ Pu $_{0.2}$ )O $_{2-x}$ and (U $_{0.7}$ Pu $_{0.3}$ )O $_{2-x}$ .

Journal Articles

Oxygen potential measurement and point defect chemistry of UO $_{2}$

Watanabe, Masashi; Kato, Masato; Sunaoshi, Takeo*

Transactions of the American Nuclear Society, 114, p.1081 - 1082, 2016/06

Many studies on the oxygen potential of UO $_{2}$ have been carried out so far. However, the oxygen potential data for UO $_{2}$ near the stoichiometric composition in the high temperature region (1673-1873 K) are limited. In this work, the oxygen potential data of UO $_{2+x}$ were extended to high temperature range of 1673-1873 K by gas equilibrium method. The measured data were analyzed based on a defect chemistry model.

Journal Articles

Thermal expansion measurement of (U,Pu)O $_{2-x}$ in oxygen partial pressure-controlled atmosphere

Kato, Masato; Ikusawa, Yoshihisa; Sunaoshi, Takeo*; Nelson, A. T.*; McClellan, K. J.*

Journal of Nuclear Materials, 469, p.223 - 227, 2016/02

https://doi.org/10.1016/j.jnucmat.2015.11.048

Times Cited Count：20 Percentile：80.48(Materials Science, Multidisciplinary)

Thermal expansion of (U $_{0.7}$ Pu $_{0.3}$ )O $_{2-x}$ (x = 0, 0.01, 0.02, 0.03) and (U $_{0.52}$ Pu $_{0.48}$ )O $_{2.00}$ was measured with a dilatometer in an oxygen partial pressure-controlled atmosphere. The oxygen partial pressure was controlled to hold a constant oxygen-to-metal ratio in the (U,Pu)O $_{2-x}$ during the measurement. Thermal expansion slightly increased with the decrease in oxygen-to-metal ratio. The relationship was derived to describe thermal expansion.

Journal Articles

Oxygen potential measurement of (Pu $_{0.928}$ Am $_{0.072}$ )O $_{2-x}$ at high temperatures

Matsumoto, Taku; Arima, Tatsumi*; Inagaki, Yaohiro*; Idemitsu, Kazuya*; Kato, Masato; Morimoto, Kyoichi; Sunaoshi, Takeo*

Journal of Nuclear Science and Technology, 52(10), p.1296 - 1302, 2015/10

https://doi.org/10.1080/00223131.2014.986243

Times Cited Count：12 Percentile：63.20(Nuclear Science & Technology)

The oxygen potentials of (Pu $_{0.928}$ Am $_{0.072}$ )O $_{2-x}$ were measured at 1873K, 1773K and 1473K by gas equilibrium method. It was shown that following the reduction of Am at the O/M ratio above 1.96, Pu was reduced at the O/M ratio below 1.96.

Journal Articles

Sintering behavior of (U,Ce)O $_{2}$ and (U,Pu)O $_{2}$

Nakamichi, Shinya; Hirooka, Shun; Sunaoshi, Takeo*; Kato, Masato; Nelson, A.*; McClellan, K.*

Transactions of the American Nuclear Society, 113(1), p.617 - 618, 2015/10

Cerium dioxide has been used as a surrogate material for plutonium dioxide. Dorr et al reported the use of hyper-stoichiometric conditions causes the start of shrinkage of (U,Ce)O $_{2}$ at low temperature compared with the sintering in reducing atmosphere. However, the precise stoichiometry of the samples investigated was not controlled or otherwise monitored, preventing any quantitative conclusions regarding the similarities or differences between (U,Ce)O $_{2}$ and (U,Pu)O $_{2}$ . The motivation for the present work is therefore to compare the sintering behavior of MOX and the (U,Ce)O $_{2}$ MOX surrogates under controlled atmospheres to assess the role of oxygen defects on densification in both systems.