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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

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

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

 Times Cited Count:5 Percentile:33.43(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

 Times Cited Count:0 Percentile:100(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

 Times Cited Count:4 Percentile:57.09(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

 Times Cited Count:5 Percentile:49.3(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 $$x$$. 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 $$P$$$$_{O2}$$-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 ($$P$$$$_{O2}$$) 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 $$P$$$$_{O2}$$-controlled atmosphere in this work. In this work, properties of O/M change, sintering and thermal expansion were investigated in $$P$$$$_{O2}$$-controlled atmosphere.

Journal Articles

Measurement of oxygen chemical diffusion in PuO$$_{2-x}$$ and analysis of oxygen diffusion in PuO$$_{2-x}$$ and (Pu,U)O$$_{2-x}$$

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

Physica Status Solidi (C), 10(2), p.189 - 192, 2013/02

 Times Cited Count:3 Percentile:11.46

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

 Times Cited Count:3 Percentile:11.46

Journal Articles

Oxygen potentials of PuO$$_{2-x}$$

Komeno, Akira; Kato, Masato; Hirooka, Shun; Sunaoshi, Takeo*

Materials Research Society Symposium Proceedings, Vol.1444, p.85 - 89, 2012/09

 Times Cited Count:4 Percentile:6.66

Journal Articles

Measurement of oxygen potential of (U$$_{0.8}$$Pu$$_{0.2}$$)O$$_{2pm x}$$ at 1773 and 1873 K, and its analysis based on point defect chemistry

Kato, Masato; Nakamichi, Shinya; Takeuchi, Kentaro; Sunaoshi, Takeo*

CALPHAD; Computer Coupling of Phase Diagrams and Thermochemistry, 35(4), p.623 - 626, 2011/12

 Times Cited Count:6 Percentile:54.96(Thermodynamics)

Uranium and plutonium mixed oxide (MOX) has been used as fuels of fast reactors. The MOX having fluorite structure is an oxygen nonstoichiometric compound which is stable in hyper- and hypo-stoichiometric composition range. The stoichiometry of MOX significantly affects their properties. So, it is essential to know the relation between stoichiometry and oxygen potential to develop MOX fuels. In this work, the oxygen potentials of (U$$_{0.8}$$Pu$$_{0.2}$$)O$$_{2pm x}$$ were measured at high temperatures of 1773, and 1873K. The measurements were carried out by gas equilibrium method using thermo-gravimetry. Th The oxygen partial pressure was adjusted by controlling the ratio of H$$_{2}$$/H$$_{2}$$O in the flowing gas atmosphere, and the oxygen potential was determined. The oxygen potentials were determined as functions of O/M ratio, and temperature. The data at stoichiometric composition were estimated to be -311kJ/mol and -299kJ/mol at 1773K, and 1873K based on point defect model.

Journal Articles

Thermal properties of UO$$_{2}$$ by molecular dynamics simulation

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

Progress in Nuclear Science and Technology (Internet), 2, p.598 - 602, 2011/10

Interatomic potential function of UO$$_{2}$$ was determined and thermal expansion, specific heat and thermal conductivity were evaluated by molecular dynamics (MD) simulation. For thermal expansion, a dilatometry was conducted and a measurement result was compared with a result of MD simulation. Thermal expansion value was in good agreement with MD simulation. Specific heat and thermal conductivity was also in good agreement with literature data when the supercell contained Schottky defects. Thermal conductivity, especially, was affected at low temperature range and decreased with increasing Schottky defect concentration. It was considered that vacancies scattered phonon vibrations. The interatomic potential function of UO$$_{2}$$ was considered usable.

Journal Articles

Influence of O/M ratio on sintering behavior of (U$$_{0.8}$$,Pu$$_{0.2}$$)O$$_{2pm x}$$

Takeuchi, Kentaro; Kato, Masato; Sunaoshi, Takeo*

Journal of Nuclear Materials, 414(2), p.156 - 160, 2011/07

 Times Cited Count:5 Percentile:53.85(Materials Science, Multidisciplinary)

The sintering behavior of MOX pellets was investigated by thermal gravimetry and thermal dilatometry. The starting material was prepared by the microwave direct heating de-nitration method, in which the Pu/(Pu+U) ratio was controlled to 20% in the nitrate solution. The powder was pressed into sample pellets by the die wall lubrication method. The two kind of test, the constant heating rate test and the isochronal heating test, were carried out in various sintering atmospheres of $$P$$$$_{H_2}$$/$$P$$$$_{H_2O}$$ ratio. The results of the constant heating rate test showed that the shrinkage rate and O/M ratio increased with decreasing the $$P$$$$_{H_2}$$/$$P$$$$_{H_2O}$$ ratio. The isochronal heating test was carried out in the O/M range of 1.98 - 2.0005, and the densification behavior of the pellets was analyzed by use of the equation; y = ($$Delta$$$$L$$/$$L$$$$_{0}$$) = $$K$$($$T$$)$$^{n}$$ $$t$$$$^{n}$$. The result showed that the sintering mechanisms varied with the O/M ratio and temperature.

Journal Articles

Oxygen potential of (U$$_{0.88}$$Pu$$_{0.12}$$)O$$_{2pm x}$$ and (U$$_{0.7}$$Pu$$_{0.3}$$)O$$_{2pm x}$$ at high temperatures of 1673-1873 K

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

Journal of Nuclear Materials, 414(2), p.120 - 125, 2011/07

Many studies on oxygen potentials have been reported, but their data were scattered and the data at high temperatures are limited. In this work, the oxygen potential of (U$$_{0.88}$$Pu$$_{0.12}$$)O$$_{2pm x}$$ and (U$$_{0.7}$$Pu$$_{0.3}$$)O$$_{2pm x}$$ was measured at high temperatures of 1673-1873 K using gas equilibrium method using thermo-gravimetry. The influence of Pu addition on the oxygen potential of MOX was discussed. The oxygen potential and the O/M ratio were decided by in-situ analysis. The oxygen partial pressure was adjusted by controlling the ratio of $$P$$$$_{H_{2}}$$/$$P$$$$_{H_{2}}$$O in the flowing gas atmosphere, and the oxygen potential was determined. The oxygen potentials measured by the point defect model. The deviation x varied with the relation of in the near stoichiometric composition region. The oxygen potential increased with increasing Pu content. The values of stoichiometric MOX containing 12% and 30%Pu were determined to be -334 kJ/mol and -296 kJ/mol, respectively, at 1773 K.

Journal Articles

Effects of H$$_{2}$$/H$$_{2}$$O ratio in the sintering atmosphere on the sintering behavior of MOX pellets

Takeuchi, Kentaro; Kato, Masato; Sunaoshi, Takeo*

Proceedings of 2010 International Congress on Advances in Nuclear Power Plants (ICAPP '10) (CD-ROM), p.1866 - 1873, 2010/06

The effect of the O/M ratio on the sintering behavior of MOX pellets was evaluated from the measurement results of O/M ratio and shrinkage ratio of pellets during the sintering process. Change of O/M ratio was caused by approaching the equilibrium condition which was decided from oxygen potential depending on the H$$_{2}$$/H$$_{2}$$O ratio in the sintering atmosphere. The lower the H$$_{2}$$/H$$_{2}$$O ratio, the more shrinkage of pellets proceeded at lower temperatures and higher O/M values. The curves of shrinkage rate were observed to have two peaks at 1000 to 1300$$^{circ}$$C and 1400 to 1600$$^{circ}$$C. The shrinkage in the lower temperature range increased with decreasing the H$$_{2}$$/H$$_{2}$$O ratio. These results suggested that different mechanisms dominate the sintering behavior at each temperature range. It was confirmed that the sintering behavior of MOX pellet depends on H$$_{2}$$/H$$_{2}$$O ratio and changes significantly with O/M ratio.

Journal Articles

Oxygen chemical diffusion in hypo-stoichiometric MOX

Kato, Masato; Morimoto, Kyoichi; Tamura, Tetsuya*; Sunaoshi, Takeo*; Konashi, Kenji*; Aono, Shigenori; Kashimura, Motoaki

Journal of Nuclear Materials, 389(3), p.416 - 419, 2009/06

 Times Cited Count:9 Percentile:40.42(Materials Science, Multidisciplinary)

Plutonium and uranium mixed oxide (MOX) has been developed to use as a core fuel of the fast reactor. The oxygen to metal ratio (O/M) of the MOX fuel is an important parameter to control the FCCI. The oxygen potential and the oxygen diffusion coefficient of the MOX are essential data to understand the oxygen behaviour in MOX. The oxygen potentials of the MOX were measured with accuracy as a function of O/M and temperatures in the previous work. In this work the oxygen chemical diffusion coefficient in (Pu$$_{0.2}$$U$$_{0.8}$$)O$$_{2-x}$$ and (Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{2-x}$$ were investigated using thermo gravimetric technique. The kinetics of the reduction processes of (Pu$$_{0.2}$$U$$_{0.8}$$)O$$_{2-x}$$ and (Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{2-x}$$ were measured by TG-DTA method. The oxygen chemical diffusion coefficients have been estimated from the reduction curves. It was concluded that the oxygen chemical diffusion coefficient in (Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{2-x}$$ is a smaller than that of (Pu$$_{0.2}$$U$$_{0.8}$$)O$$_{2-x}$$.

Journal Articles

The Phase state at high temperatures in the MOX-SiO$$_{2}$$ system

Nakamichi, Shinya; Kato, Masato; Sunaoshi, Takeo*; Uchida, Teppei; Morimoto, Kyoichi; Kashimura, Motoaki; Kihara, Yoshiyuki

Journal of Nuclear Materials, 389(1), p.191 - 196, 2009/05

 Times Cited Count:2 Percentile:79.72(Materials Science, Multidisciplinary)

Japan Atomic Energy Agency researchers have developed mixed oxide (MOX) fuels containing minor actinides (MA). These fuels were irradiated for ten minutes in the FBR Joyo in some short-term irradiation tests. The Si-condensed phases were observed at the center of the pellets in the post irradiation examination. Si impurities came to be mixed into the raw materials in the ball milling process, because Si rubber was used as the lining of the milling pot. Content of Si in the pellets was within the specification of the fuel. It is important to investigate the Si state in MOX at high temperatures like the reactor operating temperature of the fuel to evaluate irradiation behavior. In the present work, MOX specimens with mixed SiO$$_{2}$$ impurity were prepared. The ratio of MOX to SiO$$_{2}$$ was controlled at a mol fraction of 3 to 1. The specimens were first heated at 1973K in atmospheres of three different oxygen partial pressures to adjust the O/M ratio. Then these specimens were sealed in a tungsten capsule, and heated at 2273K or 2673K. Compounds consisting of Pu and Si were observed at grain boundaries of the MOX matrix in specimens after heat treatment. These compounds were not observed in grain interior and MOX matrix was not affected significantly by Si impurity. These compounds tended to form in specimens with low O/M ratio and in specimens heated at higher temperatures.

Journal Articles

Effective thermal conductivity of MOX raw powder

Takeuchi, Kentaro; Kato, Masato; Sunaoshi, Takeo*; Aono, Shigenori; Kashimura, Motoaki

Journal of Nuclear Materials, 385(1), p.103 - 107, 2009/03

 Times Cited Count:2 Percentile:79.72(Materials Science, Multidisciplinary)

The effective thermal conductivities were evaluated by measuring radial temperature distribution in MH-MOX powder. The effective thermal conductivities were very small and they changed significantly with O/M, bulk density and atmosphere gas. The results in this work were analyzed by the model of Hamilton and Crosser and the new model for the effective thermal conductivity for MH-MOX powder was derived as a function of powder properties, thermal conductivity of atmosphere gas and temperature.

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