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Journal Articles

A Science-based mixed oxide property model for developing advanced oxide nuclear fuels

Kato, Masato; Oki, Takumi; Watanabe, Masashi; Hirooka, Shun; Vauchy, R.; Ozawa, Takayuki; Uwaba, Tomoyuki; Ikusawa, Yoshihisa; Nakamura, Hiroki; Machida, Masahiko

Journal of the American Ceramic Society, 107(5), p.2998 - 3011, 2024/05

Journal Articles

Uranium-plutonium-americium cation interdiffusion in polycrystalline (U,Pu,Am)O$$_{2 pm x}$$ mixed oxides

Vauchy, R.; Matsumoto, Taku; Hirooka, Shun; Uno, Hiroki*; Tamura, Tetsuya*; Arima, Tatsumi*; Inagaki, Yaohiro*; Idemitsu, Kazuya*; Nakamura, Hiroki; Machida, Masahiko; et al.

Journal of Nuclear Materials, 588, p.154786_1 - 154786_13, 2024/01

Journal Articles

Machine learning molecular dynamics reveals the structural origin of the first sharp diffraction peak in high-density silica glasses

Kobayashi, Keita; Okumura, Masahiko; Nakamura, Hiroki; Itakura, Mitsuhiro; Machida, Masahiko; Urata, Shingo*; Suzuya, Kentaro

Scientific Reports (Internet), 13, p.18721_1 - 18721_12, 2023/11

The first sharp peak diffraction peak (FSDP) in the structure factor of amorphous materials is thought to reflect the medium-range order structure in amorphous materials, and the structural origin of the FSDP has been a subject of ongoing debate. In this study, we employed machine learning molecular dynamics (MLMD) with nearly first-principles calculation accuracy to investigate the structural origin of the FSDP in high-density silica glass. First, we successfully reproduced various experimental data of high-density silica glass using MLMD. Furthermore, we revealed that the development (or reduction) of the FSDP in high-density silica glass is characterized by the deformation behavior of ring structures in Si-O covalent bond networks under compression.

Journal Articles

Machine learning molecular dynamics simulations for evaluation of high-temperature properties of nuclear fuel materials

Kobayashi, Keita; Nakamura, Hiroki; Itakura, Mitsuhiro; Machida, Masahiko; Okumura, Masahiko

Materia, 62(3), p.175 - 181, 2023/03

no abstracts in English

Journal Articles

Correlations for the specific heat capacity of (U$$_x$$Pu$$_{1-x}$$)$$_{1-y}$$Gd$$_y$$O$$_{2-z}$$ derived from molecular dynamics

Galvin, C. O. T.*; Machida, Masahiko; Nakamura, Hiroki; Andersson, D. A.*; Cooper, M. W. D.*

Journal of Nuclear Materials, 572, p.154028_1 - 154028_8, 2022/12

 Times Cited Count:0 Percentile:0.01(Materials Science, Multidisciplinary)

UO$$_2$$ is the primary conventional fuel used in most nuclear reactors with Gd$$_2$$O$$_3$$ commonly added as a burnable absorber to produce a more level power distribution in the reactor core at the beginning of operation. It can also be mixed with other actinide oxides to produce mixed oxide (MOx) fuel. In this study, molecular dynamics simulations were used to predict the specific heat capacity of Gd-doped PuO$$_2$$, UO$$_2$$ and (U,Pu)O$$_2$$ MOx accommodating Gd$$^{3+}$$ substituted at cation sites via two charge compensation mechanisms - oxygen vacancy formation and the oxidation of U$$^{4+}$$ to U$$^{5+}$$. The specific heat capacity values for PuO$$_2$$ and UO$$_2$$ are in good agreement with other studies showing a distinct peak at high temperatures - above 1800 K. As Gd$$^{3+}$$ is added, the peak height reduces for each composition considered. An analytical fit was applied to the data where Gd$$^{3+}$$ was fully charge compensated by either oxygen vacancies or U$$^{5+}$$. The expression was then validated by predicting the specific heat capacity for three compositions of (U$$_x$$Pu$$_{1-x}$$)$$_{1-y}$$Gd$$_y$$O$$_{2-z}$$ containing both oxygen vacancies and U$$^{5+}$$, and compared to molecular dynamics data.

Journal Articles

Materials science and fuel technologies of uranium and plutonium mixed oxide

Kato, Masato; Machida, Masahiko; Hirooka, Shun; Nakamichi, Shinya; Ikusawa, Yoshihisa; Nakamura, Hiroki; Kobayashi, Keita; Ozawa, Takayuki; Maeda, Koji; Sasaki, Shinji; et al.

Materials Science and Fuel Technologies of Uranium and Plutonium mixed Oxide, 171 Pages, 2022/10

Innovative and advanced nuclear reactors using plutonium fuel has been developed in each country. In order to develop a new nuclear fuel, irradiation tests are indispensable, and it is necessary to demonstrate the performance and safety of nuclear fuels. If we can develop a technology that accurately simulates irradiation behavior as a technology that complements the irradiation test, the cost, time, and labor involved in nuclear fuel research and development will be greatly reduced. And safety and reliability can be significantly improved through simulation of nuclear fuel irradiation behavior. In order to evaluate the performance of nuclear fuel, it is necessary to know the physical and chemical properties of the fuel at high temperatures. And it is indispensable to develop a behavior model that describes various phenomena that occur during irradiation. In previous research and development, empirical methods with fitting parameters have been used in many parts of model development. However, empirical techniques can give very different results in areas where there is no data. Therefore, the purpose of this study is to construct a scientific descriptive model that can extrapolate the basic characteristics of fuel to the composition and temperature, and to develop an irradiation behavior analysis code to which the model is applied.

Journal Articles

Machine learning molecular dynamics simulations toward exploration of high-temperature properties of nuclear fuel materials; Case study of thorium dioxide

Kobayashi, Keita; Okumura, Masahiko; Nakamura, Hiroki; Itakura, Mitsuhiro; Machida, Masahiko; Cooper, M. W. D.*

Scientific Reports (Internet), 12(1), p.9808_1 - 9808_11, 2022/06

 Times Cited Count:1 Percentile:73.76(Multidisciplinary Sciences)

no abstracts in English

Journal Articles

Defect equilibria and thermophysical properties of CeO$$_{2-x}$$ based on experimental data and density functional theory calculation result

Watanabe, Masashi; Nakamura, Hiroki; Suzuki, Kiichi; Machida, Masahiko; Kato, Masato

Journal of the American Ceramic Society, 105(3), p.2248 - 2257, 2022/03

 Times Cited Count:1 Percentile:7.37(Materials Science, Ceramics)

Properties of CeO$$_{2}$$ were evaluated by DFT simulation to determine band gap, Frenkel defect formation energy and defect migration energy. Band gap and Frenkel defect formation energy were used to analyze defect equilibria. Oxygen partial pressure dependence of defect equilibria was evaluated based on oxygen potential experimental data and DFT calculation, and a Brouwer diagram was derived. The defect formation energies, including Frenkel defect, electron-hole pair and so on, were determined and used to evaluate the properties, including oxygen diffusion coefficients, electrical conduction, heat capacity and thermal conductivity. Mechanisms of various properties were discussed for a deeper understanding based on defect chemistry, and the relationship among properties were systematically described.

Journal Articles

Machine learning potentials for tobermorite minerals

Kobayashi, Keita; Nakamura, Hiroki; Yamaguchi, Akiko; Itakura, Mitsuhiro; Machida, Masahiko; Okumura, Masahiko

Computational Materials Science, 188, p.110173_1 - 110173_14, 2021/02

 Times Cited Count:12 Percentile:74.37(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Machine-learning molecular dynamics study of thermal properties of CaF$$_2$$

Nakamura, Hiroki; Machida, Masahiko

Proceedings of Joint International Conference on Supercomputing in Nuclear Applications + Monte Carlo 2020 (SNA + MC 2020), p.104 - 108, 2020/10

Journal Articles

First-principles calculation of mechanical properties of simulated debris Zr$$_x$$U$$_{1-x}$$O$$_2$$

Itakura, Mitsuhiro; Nakamura, Hiroki; Kitagaki, Toru; Hoshino, Takanori; Machida, Masahiko

Journal of Nuclear Science and Technology, 56(9-10), p.915 - 921, 2019/09

 Times Cited Count:2 Percentile:21.95(Nuclear Science & Technology)

To elucidate the mechanical properties of fuel debris inside the Fukushima Daiichi Nuclear Power Plant, we use first-principles calculations to evaluate mechanical properties of cubic Zr$$_{x}$$U$$_{1-x}$$O$$_{2}$$, which is a main component of the fuel debris. We focus on the dependence of mechanical properties on the fraction x of zirconium, compare our results with recent experiment of simulated debris, in which dependences of elastic moduli and fracture toughness on the ZrO$$_{2}$$ content showed deviation from a simple linear relation. We show that elastic moduli drop at around x=0.25 and increase again for larger values of x, as has been observed in experiments. The reason of the drop is a softening owing to disordered atomistic structures induced by the solute zirconium atoms. We also find that stress-strain curves for the x=0.125 case show marked hysteresis owing to the existence of many meta-stable states. We show that this hysteresis leads to slightly increased fracture toughness, but it is not enough to account for the significant increase of fracture toughness observed in experiments.

Journal Articles

Coexisting spin resonance and long-range magnetic order of Eu in EuRbFe$$_{4}$$As$$_{4}$$

Iida, Kazuki*; Nagai, Yuki; Ishida, Shigeyuki*; Ishikado, Motoyuki*; Murai, Naoki; Christianson, A. D.*; Yoshida, Hiroyuki*; Inamura, Yasuhiro; Nakamura, Hiroki; Nakao, Akiko*; et al.

Physical Review B, 100(1), p.014506_1 - 014506_8, 2019/07

 Times Cited Count:29 Percentile:85.61(Materials Science, Multidisciplinary)

Magnetic excitations and magnetic structure of EuRbFe$$_{4}$$As$$_{4}$$ were investigated by neutron scattering measurements.

Journal Articles

First-principles calculation study on phonon thermal conductivity of thorium and plutonium dioxides; Intrinsic anharmonic phonon-phonon and extrinsic grain-boundary-phonon scattering effects

Nakamura, Hiroki; Machida, Masahiko

Journal of Nuclear Materials, 519, p.45 - 51, 2019/06

 Times Cited Count:10 Percentile:75.06(Materials Science, Multidisciplinary)

Thermal conductivity of nuclear fuel materials is one of the most essential thermophysical data in analyzing nuclear reactor safety and fuel performance. To establish a reliable technique of computation of their thermal conductivity, we apply a scheme based on first-principles electronic calculations and calculate phonon thermal conductivity of both plutonium and thorium dioxides. In the employed scheme, all phonon properties are calculated from harmonic and third-order anharmonic force constants obtained through first-principles calculations. Using these phonon properties, phonon thermal conductivity are computed with a standard phonon transport theory. The calculation results show a good agreement with experiments. Moreover, we evaluate grain-boundary effects on phonon thermal conduction. These results prove that the present first-principles scheme is reliably applicable to accurate computation of phonon thermal conductivity of fuel materials in the considered temperature range.

Journal Articles

Development of the ReaxFF methodology for electrolyte-water systems

Fedkin, M. V.*; Shin, Y. K.*; Dasgupta, N.*; Yeon, J.*; Zhang, W.*; van Duin, D.*; Van Duin, A. C. T.*; Mori, Kento*; Fujiwara, Atsushi*; Machida, Masahiko; et al.

Journal of Physical Chemistry A, 123(10), p.2125 - 2141, 2019/03

 Times Cited Count:38 Percentile:94.63(Chemistry, Physical)

no abstracts in English

Journal Articles

New antiferromagnetic order with pressure-induced superconductivity in EuFe$$_2$$As$$_2$$

Ikeda, Shugo*; Tsuchiya, Yu*; Zhang, X.-W.*; Kishimoto, Shunji*; Kikegawa, Takumi*; Yoda, Yoshitaka*; Nakamura, Hiroki; Machida, Masahiko; Glasbrenner, J.*; Kobayashi, Hisao*

Physical Review B, 98(10), p.100502_1 - 100502_6, 2018/09

 Times Cited Count:4 Percentile:26.15(Materials Science, Multidisciplinary)

The interplay between magnetism and superconductivity is one of important subjects to investigate the pairing mechanism in novel superconductors. We have found new coexistence between an antiferromagnetic order in the Fe sublattice and superconductivity of the FeAs-based EuFe$$_2$$As$$_2$$ superconductor in the pressure range from 2.4 to 3.0 GPa by $$^{57}$$Fe nuclear forward scattering (NFS) using a single crystal sample. The magnetic state in the Fe sublattice changes to a new antiferromagnetic one with superconductivity from a stripe-type antiferromagnetic one observed in normal conducting state at 2.7 GPa. Below the superconducting transition temperature, the temperature dependence of $$^{57}$$Fe NFS spectra reveals that the new antiferromagnetic order develops with the superconductivity. This non-trivial coupling of two ordered states in EuFe$$_2$$As$$_2$$ under pressure demonstrates a new and intriguing relationship between magnetism and superconductivity in Fe-based superconductors.

Journal Articles

Defect chemistry and basic properties of non-stoichiometric PuO$$_{2}$$

Kato, Masato; Nakamura, Hiroki; Watanabe, Masashi; Matsumoto, Taku; Machida, Masahiko

Defect and Diffusion Forum, 375, p.57 - 70, 2017/05

The basic properties of PuO$$_{2-x}$$ were reviewed, and the equilibrium defects in PuO$$_{2-x}$$ were evaluated from the experimental data of the oxygen potential and electrical conductivity as well as the Ab-initio calculation results. Consistency among various properties was confirmed, and the mechanistic models for thermal property representations were derived.

Journal Articles

High-temperature properties of thorium dioxide; A First-principles molecular dynamics study

Nakamura, Hiroki; Machida, Masahiko

Journal of Nuclear Materials, 478, p.56 - 60, 2016/09

 Times Cited Count:8 Percentile:60.71(Materials Science, Multidisciplinary)

Thorium has been considered a potential nuclear fuel for decades. To develop evaluation method for high-temperature properties of thorium dioxide as a candidate nuclear fuel, we perform first-principles molecular dynamics. The calculated enthalpy and thermal expansion agree well with the observed data. The Bredig transition temperature also coincides with experiments. Our results indicate that this method can provide reliable data of thermal properties of nuclear fuels.

Journal Articles

Multi-band Eilenberger theory of superconductivity; Systematic low-energy projection

Nagai, Yuki; Nakamura, Hiroki

Journal of the Physical Society of Japan, 85(7), p.074707_1 - 074707_18, 2016/07

 Times Cited Count:3 Percentile:27.93(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Effect of magnetism on lattice dynamics in SrFe$$_2$$As$$_2$$ using high-resolution inelastic X-ray scattering

Murai, Naoki*; Fukuda, Tatsuo; Kobayashi, Tatsuya*; Nakajima, Masamichi*; Uchiyama, Hiroshi*; Ishikawa, Daisuke*; Tsutsui, Satoshi*; Nakamura, Hiroki; Machida, Masahiko; Miyasaka, Shigeki*; et al.

Physical Review B, 93(2), p.020301_1 - 020301_5, 2016/01

 Times Cited Count:7 Percentile:34.33(Materials Science, Multidisciplinary)

Journal Articles

Phonons of Fe-based superconductor Ca$$_{10}$$Pt$$_4$$As$$_8$$(Fe$$_{1-x}$$Pt$$_x$$As)$$_{10}$$

Ikeuchi, Kazuhiko*; Kobayashi, Yoshiaki*; Suzukia, Kazunori*; Ito, Masayuki*; Kajimoto, Ryoichi; Bourges, P.*; Christianson, A. D.*; Nakamura, Hiroki; Machida, Masahiko; Sato, Masatoshi*

Journal of Physics; Condensed Matter, 27(46), p.465701_1 - 465701_7, 2015/11

 Times Cited Count:1 Percentile:4.84(Physics, Condensed Matter)

We report the results of inelastic neutron scattering measurements on particular phonons of a superconducting (SC) Ca$$_{10}$$Pt$$_4$$As$$_8$$(Fe$$_{1-x}$$Pt$$_x$$As)$$_{10}$$ with the onset transition temperature $$T_c sim 33$$ K to investigate mainly what roles orbital fluctuation plays in Cooper pairing, where we observed a slight softening of the in-plane transverse acoustic mode corresponding to the elastic constant $$C_{66}$$. This softening starts at temperature $$T$$ well above the SC $$T_c$$, as $$T$$ decreases. An anomalously strong change of the scattering intensity of in-plane optical modes was observed at the M point of the pseudo tetragonal reciprocal space in the range of 35 $$< omega <$$ 40 meV with decreasing $$T$$ from far above $$T_c$$. Because this $$omega$$ region mainly corresponds to the motion of Fe and As atoms in the FeAs planes, the finding presents information on the coupling between the orbital fluctuation of Fe $$3d$$ electrons and the lattice system, useful for studying the possible roles of orbital fluctuation in the pairing mechanism and/or the appearance of the so-called nematic phase.

235 (Records 1-20 displayed on this page)