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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：0 Percentile：0.00(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：1 Percentile：68.64(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：84.10(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：9 Percentile：94.05(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：11 Percentile：67.57(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：5 Percentile：51.35(Materials Science, Multidisciplinary)

no abstracts in English

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：2 Percentile：18.47(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

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：12 Percentile：73.70(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

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：15 Percentile：83.18(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：52 Percentile：92.70(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

http://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：14 Percentile：75.83(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：11 Percentile：61.22(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.

Journal Articles

Thermal expansion of PuO $_{2}$

Uchida, Teppei; Sunaoshi, Takeo*; Konashi, Kenji*; Kato, Masato

Journal of Nuclear Materials, 452(1-3), p.281 - 284, 2014/09

https://doi.org/10.1016/j.jnucmat.2014.05.039

Times Cited Count：11 Percentile：61.57(Materials Science, Multidisciplinary)

Experiment and simulation studies on physical properties of actinide oxides have been carried out. Thermal expansion is important data to evaluate the various properties from molecular dynamics (MD) simulation. In this study, thermal expansion of PuO $_{2}$ was evaluated by experiment and MD simulation. In the experimental study, thermal expansion of PuO $_{2.00}$ pellet was determined by a dilatometer in the temperature range of 300-1923 K. In the MD simulation, Born-Mayer-Huggins interatomic potential with a partially ionic model and Morse potential were employed. Lattice constants of PuO $_{2}$ were evaluated in the temperature range of 300-2800 K by MD simulation, and thermal expansion was evaluated. The experimental data was good agreement with the MD simulation result. Evaluation formula for thermal expansion of PuO $_{2}$ in the temperature range of 300-2800 K was derived from both data.

Journal Articles

Thermal expansion measurement and heat capacity evaluation of hypo-stoichiometric PuO $_{2.00}$

Kato, Masato; Uchida, Teppei; Matsumoto, Taku; Sunaoshi, Takeo*; Nakamura, Hiroki; Machida, Masahiko

Journal of Nuclear Materials, 451(1-3), p.78 - 81, 2014/08

https://doi.org/10.1016/j.jnucmat.2014.03.021

Times Cited Count：11 Percentile：61.57(Materials Science, Multidisciplinary)

Linear thermal expansion of PuO $_{2-x}$ was measured by dilatometry in an oxygen partial pressure-controlled atmosphere. The measured data of PuO $_{2-x}$ slightly increased with deviation . The linear thermal expansion of PuO $_{2-x}$ was determined as a function of temperature and O/M ratio, and the equation for the thermal expansion coefficient was derived. Heat capacity of PuO $_{2-x}$ was evaluated using this equation. The effect of O/M ratio on heat capacity was small. In addition to the vibration and dilatational terms, it is important to analyze the Schottky term in evaluating heat capacity of PuO $_{2}$ .

Journal Articles

Property measurements of (U $_{0.7}$ ,Pu $_{0.3}$ )O $_{2-x}$ in $_{O2}$ -controlled atmosphere

Kato, Masato; Murakami, Tatsutoshi; Sunaoshi, Takeo*; Nelson, A. T.*; McClellan, K.*

Proceedings of International Nuclear Fuel Cycle Conference; Nuclear Energy at a Crossroads (GLOBAL 2013) (CD-ROM), p.852 - 856, 2013/09

Uranium and plutonium mixed oxide (U $_{0.7}$ ,Pu $_{0.3}$ )O $_{2-x}$ which has been developed as a fast reactor fuel is nonstoichiometric compound. The stoichiometry significantly affects various properties. The relationship between oxygen potential and oxygen-to-metal (O/M) ratio has been investigated so far. The measurement results showed that it is essential to control oxygen partial pressure ( $_{O2}$ ) for holding constant O/M ratio in high temperature range. Therefore, properties of (U $_{0.7}$ ,Pu $_{0.3}$ )O $_{2-x}$ were measured in $_{O2}$ -controlled atmosphere in this work. In this work, properties of O/M change, sintering and thermal expansion were investigated in $_{O2}$ -controlled atmosphere.

Journal Articles

Effect of oxygen-to-metal ratio on densification behavior in the sintering process of PuO $_{2}$ pellet

Uchida, Teppei; Sunaoshi, Takeo*; Kato, Masato

Physica Status Solidi (C), 10(2), p.193 - 196, 2013/02

https://doi.org/10.1002/pssc.201200475

Times Cited Count：6 Percentile：91.09(Materials Science, Multidisciplinary)