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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

Journal Articles

Effect of a raw material powder on sintered CeO $_{2}$ pellets by 28 GHz microwave irradiation

Akashi, Masatoshi; Matsumoto, Taku; Kato, Masato

Transactions of the American Nuclear Society, 118, p.1391 - 1394, 2018/06

In this study, CeO $_{2}$ pellet sintering by irradiating microwave at a frequency of 28 GHz was carried out to investigate the effect of particle diameter of raw powder on the density of sintered pellet. The highest bulk density is 94.2 %T.D. under the condition of 30 min holding at 1473 K. The bulk density decreases with increasing the particle diameter of used raw powder. On the other hand, all of the apparent density of sintered pellet is more than 93.5 %T.D.. The difference between the bulk density and the apparent density is caused by the difference of open porosity for each sample pellet. It seems that the high density sintered pellets with porous structure are obtained because sample pellet is heated internally and uniformly in microwave sintering.

Journal Articles

Oxygen potentials, oxygen diffusion coefficients and defect equilibria of nonstoichiometric (U,Pu)O $_{2pm x}$

Kato, Masato; Watanabe, Masashi; Matsumoto, Taku; Hirooka, Shun; Akashi, Masatoshi

Journal of Nuclear Materials, 487, p.424 - 432, 2017/04

https://doi.org/10.1016/j.jnucmat.2017.01.056

Times Cited Count：21 Percentile：85.65(Materials Science, Multidisciplinary)

Oxygen potential of (U,Pu)O $_{2pm x}$ was evaluated based on defect chemistry using an updated experimental data set. The relationship between oxygen partial pressure and deviation in (U,Pu)O $_{2pm x}$ was analyzed, and equilibrium constants of defect formation were determined as functions of Pu content and temperature. Brouwer's diagrams were constructed using the determined equilibrium constants, and a relational equation to determine O/M ratio was derived as functions of O/M ratio, Pu content and temperature. In addition, relationship between oxygen potential and oxygen diffusion coefficients were described.

Journal Articles

The Influences of Pu and Zr on the melting temperatures of the UO $_{2}$ -PuO $_{2}$ -ZrO $_{2}$ pseudo-ternary system

Morimoto, Kyoichi; Hirooka, Shun; Akashi, Masatoshi; Watanabe, Masashi; Sugata, Hiromasa*

Journal of Nuclear Science and Technology, 52(10), p.1247 - 1252, 2015/10

https://doi.org/10.1080/00223131.2015.1047807

Times Cited Count：4 Percentile：29.61(Nuclear Science & Technology)

As a part of decommissioning plan of the damaged reactors at Fukushima Daiichi Nuclear Power Plant, some strategies for removing of debris from the reactors are discussed. In these considerations, it is necessary to predict a melt progression during the severe accident based on theoretical evidences. Melting temperature is one of the most important thermal characteristics to analyse a melt progression during the severe accident. In this study, the melting temperatures of specimens of U, Pu and Zr mixed oxide prepared as simulated debris were measured by the thermal arrest technique. From the results of this measurement, the influences of Pu $^{-}$ and Zr $^{-}$ contents on the melting temperature of the simulated debris were evaluated.

Journal Articles

Development of science-based fuel technologies for Japan's Sodium-Cooled Fast Reactors

Kato, Masato; Hirooka, Shun; Ikusawa, Yoshihisa; Takeuchi, Kentaro; Akashi, Masatoshi; Maeda, Koji; Watanabe, Masashi; Komeno, Akira; Morimoto, Kyoichi

Proceedings of 19th Pacific Basin Nuclear Conference (PBNC 2014) (USB Flash Drive), 12 Pages, 2014/08

Uranium and plutonium mixed oxide (MOX) fuel has been developed for Japan sodium-cooled fast reactors. Science based fuel technologies have been developed to analyse behaviours of MOX pellets in the sintering process and irradiation conditions. The technologies can provide appropriate sintering conditions, irradiation behaviour analysis results and so on using mechanistic models which are derived based on theoretical equations to represent various properties.

Journal Articles

Effect of oxygen-to-metal ratio on properties of corium prepared from UO $_{2}$ and zircaloy-2

Hirooka, Shun; Kato, Masato; Morimoto, Kyoichi; Komeno, Akira; Uchida, Teppei; Akashi, Masatoshi

Journal of Nuclear Materials, 437(1-3), p.130 - 134, 2013/06

https://doi.org/10.1016/j.jnucmat.2013.01.334

Times Cited Count：5 Percentile：35.86(Materials Science, Multidisciplinary)

Journal Articles

Thermal expansion of corium prepared from UO and zircalloy-2

Hirooka, Shun; Akashi, Masatoshi; Uchida, Teppei; Morimoto, Kyoichi; Kato, Masato

Materials Research Society Symposium Proceedings, Vol.1444, p.97 - 101, 2012/09

https://doi.org/10.1557/opl.2012.1004

Times Cited Count：0 Percentile：0.00(Materials Science, Multidisciplinary)

Journal Articles

Melting temperature and thermal conductivities of corium prepared from UO $_{2}$ and zircalloy-2

Kato, Masato; Uchida, Teppei; Hirooka, Shun; Akashi, Masatoshi; Komeno, Akira; Morimoto, Kyoichi

Materials Research Society Symposium Proceedings, Vol.1444, p.91 - 96, 2012/09

https://doi.org/10.1557/opl.2012.987

Times Cited Count：1 Percentile：54.57(Materials Science, Multidisciplinary)

Oral presentation

Characterization of melt-solidified (U, Gd, Zr)O $_{2-x}$ as simulated corium debris

Morimoto, Kyoichi; Hirooka, Shun; Akashi, Masatoshi; Watanabe, Masashi

no journal, ,

The influence of Gd on characteristics of debris is important for removing the debris from the reactors of Fukushima Daiichi Nuclear Power Plant because subassemblies of nuclear fuels containing Gd $_{2}$ O $_{3}$ were loaded in the some reactor cores. Additionally, it is important to assess the distribution state of Gd from the anxiety of re-criticality caused by the relocation of debris while removing them. In this study, sintered pellets of (U $_{0.95-y}$ Gd $_{0.05}$ Zr $_{y}$ )O $_{2-x}$ (y=0,0.5, 2-x=1.989-2.000) were melted and solidified to prepare specimens of simulated corium debris. Phase states and fundamental properties of them were evaluated.

Oral presentation

Phase state estimation of metallic inclusions in simulated corium debris

Akashi, Masatoshi; Morimoto, Kyoichi

no journal, ,

Oral presentation

Study on the condition of MOX granules suitable for constant die filling in MOX fuel pellet production

Akashi, Masatoshi; Nishina, Masahiro; Morimoto, Kyoichi; Kato, Yoshiyuki; Kato, Masato

no journal, ,

A die filling experiment was carried out using granulated MOX powder to investigate the condition of MOX granules suitable for constant die filling. It was found that the mean particle diameter and the Hausner ratio gave an indication of the die filling for the granulated MOX powder.

Oral presentation

The Outline of fuel debris characterization; Large scale MCCI tests and MOX simulated fuel, etc.

Kitagaki, Toru; Ikeuchi, Hirotomo; Yano, Kimihiko; Hoshino, Takanori; Okamura, Nobuo; Hirooka, Shun; Akashi, Masatoshi; Watanabe, Masashi; Morimoto, Kyoichi; Ogino, Hideki; et al.

no journal, ,

no abstracts in English

Oral presentation

Sintering of cerium dioxide by 28 GHz Microwave Radiation

Akashi, Masatoshi; Matsumoto, Taku; Kato, Masato

no journal, ,

no abstracts in English

Oral presentation

Science-based integrated models for MOX properties, 1; Relation among lattice constant, coefficient of thermal expansion, mechanical properties and heat capacity

Hirooka, Shun; Kato, Masato; Akashi, Masatoshi; Morimoto, Kyoichi; Matsumoto, Taku

no journal, ,

Sound velocity in MOX pellet at room temperature was measured by ultrasound pulse-echo method. With the newly obtained data of Pu dependence (0-1) and reported data of porosity (0.0543-0.1439) and O/M (1.963-2.000) dependences, equations to evaluate sound velocity with these parameters were derived. Mechanical properties such as Young's modulus can be evaluated from sound velocity. By using Debye model, Debye temperature is evaluated from sound velocity and lattice parameter. Debye temperature leads to heat capacity at constant volume. Sound velocity, lattice parameter, heat capacity at constant volume and coefficient of thermal expansion lead to Gruneisen constant and dilatational term of heat capacity. Suppose that Gruneisen constant is not temperature-dependent, temperature dependence of Young's modulus can be evaluated. Calculated result of Young's modulus of (U $_{0.8}$ Pu $_{0.2}$ )O $_{2}$ decreased with increasing temperature, which is the same trend as literature.

Oral presentation

Estimating how fuel debris stay by the physical properties

Hirooka, Shun; Akashi, Masatoshi; Komeno, Akira; Morimoto, Kyoichi; Kato, Masato

no journal, ,

To understand physical property on the fuel debris generated in core melt accident at Fukushima Daiichi Nuclear Power Plan, oxidation test and thermal conductivity measurement with simulated fuel debris were performed. By using the data, the status of the fuel debris in the damaged cores were estimated.

Oral presentation

The Heat capacity and thermal diffusivity of UO $_{2}$ at high temperature

Matsumoto, Taku; Hirooka, Shun; Akashi, Masatoshi; Kato, Masato; Ogasawara, Masahiro*

no journal, ,

The enthalpy and thermal diffusivity of UO up to 2000 $^{circ}$ C was measured with drop calorimetry and laser flash instrument, respectively. The heat capacity evaluated from the measurement results of enthalpy sharply increases from 1500 $^{circ}$ C that shows different results from the data obtained by some researchers. The thermal diffusivity decreases with increasing of temperature to 1500 $^{circ}$ C, and has almost stable value above 1500 $^{circ}$ C.