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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:0 Percentile:100(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

New insights into the Cs adsorption on montmorillonite clay from $$^{133}$$Cs solid-state NMR and density functional theory calculations

Okubo, Takahiro*; Okamoto, Takuya*; Kawamura, Katsuyuki*; Gu$'e$gan, R.*; Deguchi, Kenzo*; Oki, Shinobu*; Shimizu, Tadashi*; Tachi, Yukio; Iwadate, Yasuhiko*

Journal of Physical Chemistry A, 122(48), p.9326 - 9337, 2018/12

 Times Cited Count:2 Percentile:73.63(Chemistry, Physical)

The structures of Cs adsorption on montmorillonite were investigated by the nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra of Cs adsorbed on montmorillonite samples were measured under different Cs contents and relative humidity levels. NMR parameters were evaluated by the first principle calculations in order to identify the relationship between adsorbed Cs structures and NMR parameters. The comparisons between experimental and theoretical NMR spectra revealed that Cs is preferentially adsorbed at sites near Al for low Cs substituted montmorillonites, and that non-hydrated Cs present in partially Cs substituted samples, even after being hydrated under high relative humidity.

Journal Articles

Interpretation of thermal desorption spectra of hydrogen from aluminum using numerical simulation

Ebihara, Kenichi; Yamaguchi, Masatake; Tsuru, Tomohito; Itakura, Mitsuhiro

Keikinzoku, 68(11), p.596 - 602, 2018/11

Hydrogen embrittlement (HE) is considered as one cause of stress corrosion cracking. HE is a serious problem in the development of high strength aluminum alloy as with steels. For understanding HE, it is inevitable to know hydrogen trapping states in the alloys and it can be identified using thermal desorption spectrometry of H. In this study, we numerically simulated thermal desorption spectra of hydrogen in aluminum for a cylindrical and a plate specimens and interpreted the desorption peaks included in them on the basis of the trap site concentration and the trap energy. As a result, we found that the peak at the lowest-temperature side can result from grain boundaries and confirmed that the reported interpretation for other peaks is reasonable. We also obtained the result showing the possibility that the trap site concentration of defects changes during heating the specimens. This result may give a suggestion for the interpretation of temperature desorption spectra of steels.

Journal Articles

Radiocesium interaction with clay minerals; Theory and simulation advances Post-Fukushima

Okumura, Masahiko; Kerisit, S.*; Bourg, I. C.*; Lammers, L. N.*; Ikeda, Takashi*; Sassi, M.*; Rosso, K. M.*; Machida, Masahiko

Journal of Environmental Radioactivity, 189, p.135 - 145, 2018/09

 Times Cited Count:16 Percentile:11.34(Environmental Sciences)

no abstracts in English

Journal Articles

Atomistic simulation of phosphorus segregation to $$Sigma$$3(111) symmetrical tilt grain boundary in $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki

Modelling and Simulation in Materials Science and Engineering, 26(6), p.065005_1 - 065005_10, 2018/09

 Times Cited Count:1 Percentile:85.18(Materials Science, Multidisciplinary)

Irradiation-induced grain boundary phosphorus segregation is an important factor for estimating the embrittlement of nuclear reactor pressure vessel steels, but the physical process of phosphorus migration to grain boundaries is still unclear. We numerically studied phosphorus migration toward $$Sigma$$3(111) symmetrical tilt grain boundary in $$alpha$$-iron using molecular dynamics. We found that, in the vicinity of the grain boundary within $$sim$$1 nm distance, an iron-phosphorus mixed dumbbell and an octahedral interstitial phosphorus atom push a self-interstitial atom into the grain boundary, and the phosphorus atom becomes a substitutional atom. A phosphorus vacancy complex in the region also becomes dissociated, and the vacancy is absorbed in the grain boundary without dragging phosphorus. The results claim that a novel view of the segregation process is required.

Journal Articles

First-principles study of solvent-solute mixed dumbbells in body-centered-cubic tungsten crystals

Suzudo, Tomoaki; Tsuru, Tomohito; Hasegawa, Akira*

Journal of Nuclear Materials, 505, p.15 - 21, 2018/07

 Times Cited Count:2 Percentile:48.87(Materials Science, Multidisciplinary)

Tungsten (W) is considered as a promising candidate for plasma-facing materials for future nuclear fusion devices, and selecting optimal alloying constituents is a critical issue to improve radiation resistance of the W alloys as well as to improve their mechanical properties. We conducted in the current study a series of first-principles calculations for investigating solvent-solute mixed dumbbells in W crystals. The results suggested that titanium (Ti), vanadium (V), and chromium (Cr) are favorable as solutes for W alloys from irradiation-effect perspectives because these elements are expected to promote vacancy-interstitial recombination without causing radiation-induced precipitation that reduces ductility of irradiated materials.

Journal Articles

Localization of cesium on montmorillonite surface investigated by frequency modulation atomic force microscopy

Araki, Yuki*; Sato, Hisao*; Okumura, Masahiko; Onishi, Hiroshi*

Surface Science, 665, p.32 - 36, 2017/11

 Times Cited Count:5 Percentile:53.19(Chemistry, Physical)

no abstracts in English

Journal Articles

Transmutation effects on long-term Cs retention in phyllosilicate minerals from first principles

Sassi, M.*; Okumura, Masahiko; Machida, Masahiko; Rosso, K. M.*

Physical Chemistry Chemical Physics, 19(39), p.27007 - 27014, 2017/10

 Times Cited Count:4 Percentile:64.2(Chemistry, Physical)

no abstracts in English

Journal Articles

Origin of 6-fold coordinated aluminum at (010)-type pyrophyllite edges

Okumura, Masahiko; Sassi, M.*; Rosso, K. M.*; Machida, Masahiko

AIP Advances (Internet), 7(5), p.055211_1 - 055211_9, 2017/05

 Times Cited Count:5 Percentile:53.38(Nanoscience & Nanotechnology)

no abstracts in English

Journal Articles

Modeling of Phosphorus Transport by Interstitial Dumbbell in $$alpha$$-Iron Using First-Principles-Based Kinetic Monte Carlo

Ebihara, Kenichi; Suzudo, Tomoaki; Yamaguchi, Masatake

Materials Transactions, 58(1), p.26 - 32, 2017/01

 Times Cited Count:3 Percentile:64.77(Materials Science, Multidisciplinary)

In order to evaluate grain boundary (GB) phosphorous (P) segregation in nuclear reactor pressure vessel steels under irradiation, the rate-theory model based on first-principles calculations is developed. In this study, we evaluated the diffusion coefficient of the mixed interstitial dumbbell of a P atom and an iron(Fe) atom using a kinetic Monte Carlo (kMC) simulation based on first-principles calculations. The evaluated diffusion coefficient was almost the same with the diffusion coefficient of P atoms which migrate via octahedral interstitial sites, and was much faster than that for P transport by vacancies. Furthermore, from the simulation of the irradiation induced GB P segregation using the model which was modified to include P atoms of octahedral interstitial sites, it was found that the boundary condition at GB is not valid for P atoms of octahedral interstitial sites

Journal Articles

Simulation of He embrittlement at grain boundaries of bcc transition metals

Suzudo, Tomoaki; Yamaguchi, Masatake

Journal of Nuclear Materials, 465, p.695 - 701, 2015/10

 Times Cited Count:9 Percentile:22.79(Materials Science, Multidisciplinary)

To investigate what atomic properties largely determine vulnerability to He embrittlement at grain boundaries (GB) of bcc transition metals, we introduced a computational model composed of first principles density functional theory and a He segregation rate theory model. Predictive calculations of He embrittlement at the first wall of the future DEMO fusion concept reactor indicated that variation in the He embrittlement is originated not only from He production rate related to neutron irradiation, but also from the He segregation energy at the GB that has a systematic trend in the periodic table.

Journal Articles

First-principles calculation studies on cesium in environmental situations; Hydration structures and adsorption on clay minerals

Machida, Masahiko; Okumura, Masahiko; Nakamura, Hiroki; Sakuramoto, Kazuhiro*

Proceedings of Joint International Conference on Mathematics and Computation, Supercomputing in Nuclear Applications and the Monte Carlo Method (M&C + SNA + MC 2015) (CD-ROM), 12 Pages, 2015/04

In order to clarify physicochemical behaviors of radioactive Cs released into environment from the Fukushima Daiichi Nuclear Power Plants, we study on two issues, i.e., hydration structures of Cs$$^{+}$$ and its adsorption on a specific edge in a clay particle (mica) by employing first principles calculations. In this paper, firstly, we report on hydration structures of Cs$$^{+}$$ by using Born-Oppenheimer first-principles molecular dynamics. Our striking finding in the hydration structures is that Cs$$^{+}$$ has no clear second hydration shell in contrast to any other alkali cations. Secondly, we construct a model of the Frayed Edge Site and confirm that the model actually becomes selective for Cs when expanding the interlayer distance from that of the original crystal structure through the calculation of the ion-exchange energy.

Journal Articles

Cs adsorption mechanism on clay minerals based on material sciences using synchrotron radiation and first principle calculation

Yaita, Tsuyoshi; Kobayashi, Toru; Ikeda, Takashi; Matsumura, Daiju; Machida, Masahiko; Okumura, Masahiko; Nakamura, Hiroki

Hoshako, 27(6), p.315 - 322, 2014/11

no abstracts in English

Journal Articles

First-principles studies on cesium adsorption of clay minerals; Mechanism and chemical bonding

Okumura, Masahiko; Nakamura, Hiroki; Machida, Masahiko

Nippon Genshiryoku Gakkai-Shi, 56(6), p.372 - 377, 2014/06

no abstracts in English

Journal Articles

The Effect of hydrogen atoms on the screw dislocation mobility in bcc iron; A First-principles study

Itakura, Mitsuhiro; Kaburaki, Hideo; Yamaguchi, Masatake; Okita, Taira*

Acta Materialia, 61(18), p.6857 - 6867, 2013/10

 Times Cited Count:49 Percentile:4.94(Materials Science, Multidisciplinary)

The interaction between dislocations and impurity atoms in metals determines various properties of plastic deformation, such as the dependence of the yield stress on the impurity contents. Since the direct observation of atomistic structure of screw dislocation is almost impossible, several hypothetical assumptions have been employed to explain conveniently experimental observations. Recent advancement of computational hardware, as well as the development of elaborated techniques to reduce the size-effect in the first-principles calculation, enabled direct calculations of dislocation-impurity interaction. We have succeeded to evaluate the effect of hydrogen atoms on the dislocation mobility in iron.

Journal Articles

Defect layer in SiO$$_2$$-SiC interface proved by a slow positron beam

Maekawa, Masaki; Kawasuso, Atsuo; Yoshikawa, Masahito; Miyashita, Atsumi; Suzuki, Ryoichi*; Odaira, Toshiyuki*

Physica B; Condensed Matter, 376-377, p.354 - 357, 2006/04

 Times Cited Count:2 Percentile:86.21(Physics, Condensed Matter)

no abstracts in English

Journal Articles

Positron study of electron irradiation-induced vacancy defects in SiC

Kawasuso, Atsuo; Yoshikawa, Masahito; Ito, Hisayoshi; Krause-Rehberg, R.*; Redmann, F.*; Higuchi, Takatoshi*; Betsuyaku, Kiyoshi*

Physica B; Condensed Matter, 376-377, p.350 - 353, 2006/04

 Times Cited Count:10 Percentile:52.7(Physics, Condensed Matter)

In this presentation, we report identification of vacancy defects in cubic and hexagonal SiC irradiated with fast electrons through electron-pisitron momentum distribution measurements and theoretical analyses. In cubic SiC isolated silicon vacancies are responsible for positron trapping. The lifetime of positrons trapped at silicon vacancies is prolonged due to the outward lattice relaxation. Because of the local tetrahedral symmetry of silicon vacancies, the observed momentum distributions are consistently explained. In the case of hexagonal SiC, one particular vacancy defects appearing after annealing of isolated silicon vacancies have dangling bonds along the c-axis. From the enhancement of positron annihilation probability with carbon 1s electrons, the above defects are attributed to carbon-vacancy-antisite-carbon complexes.

Journal Articles

First-principles molecular dynamics simulation of SiC devices; Generation of amorphous SiO$$_{2}$$/SiC interface

Miyashita, Atsumi; Yoshikawa, Masahito; Kano, Takuma; Onuma, Toshiharu*; Sakai, Takayuki*; Iwasawa, Misako*; Soneda, Naoki*

Annual Report of the Earth Simulator Center April 2004 - March 2005, p.287 - 291, 2005/12

Silicon carbide semiconductor device is expected to be used under a severe environment like the nuclear reactor and the space environment. On the semiconductor device interface, the electric charge state of the defect decides an electric characteristic. To emulate interfacial structure the $$rm SiO_{2}/SiC$$ interface structure is generated and the electronic geometry is decided by the first-principle molecular dynamics simulation with the earth simulator. The amorphous $$rm SiO_{2}/SiC$$ interface structure is made by medium-scale model of about 400 atoms. The heating temperature is 4000K, the heating time is 3.0ps, the speed of rapid cooling is -1000K/ps, and SiC movable layers in the interface are assumed to be 4 layers. In temperature 2200K the $$rm SiO_{2}$$ terminal was opened to make the $$rm SiO_{2}$$ layer more amorphous. The model has almost abrupt interface, however, some defects energy levels were still observed in the band gap. The energy levels are originated from interfacial oxygen. The localized electronic distribution of the dangling bond causes defect energy levels.

Journal Articles

Response to comment on "Grain Boundary Decohesion by Impurity Segregation in a Nickel-Sulfur System"

Yamaguchi, Masatake; Shiga, Motoyuki; Kaburaki, Hideo

Science, 309(5741), P. 1677d, 2005/09

Geng et al. claim in their comment that the segregation energy of sulfur atoms in a nickel grain boundary should be calculated not on average as we did but incrementally. From their own calculations using the incremental binding energies, they concluded that the two-monolayer (GB0 4/4, GB2 4/4) segregation that bring about the reduction of the tensile strength by one order of magnitude has only 1% occupation possibility. However, they miss the point that there are many paths to the (GB0 4/4, GB2 4/4) configuration. In our re-examined calculations, one path has the 50% possibility of the occupation even if we use the incremental binding energies. Furthermore, there may be other paths that have larger possibility. To begin with, Geng et al.'s claims are not meaningful because the prediction of the occupation possibility using the simple McLean's model is not quantitative but qualitative. On the other hand, they claim that another two-monolayer (GB1 4/4, GB2 4/4) configuration brings about the reduction of the tensile strength by one order of magnitude. Their results may be miscalculations, because that is not the case in our re-examined calculations. Furthremore, they suggest a new mechanism of the strong decohesion due to the directional change of the Ni-S bonds. However, they do not show any clear evidence.

Journal Articles

Grain boundary decohesion by impurity segregation in a nickel-sulfur system

Yamaguchi, Masatake; Shiga, Motoyuki; Kaburaki, Hideo

Science, 307(5708), p.393 - 397, 2005/01

 Times Cited Count:216 Percentile:1.65(Multidisciplinary Sciences)

Sulfur-induced embrittlement of nickel has long been wrapped in mysteries why and how sulfur weakens the grain boundaries of nickel and why a critical intergranular sulfur concentration is required. From first-principles calculations, we found that a large grain boundary expansion is caused by a short-range overlap repulsion among densely segregated and neighboring sulfur atoms. This expansion results in a drastic grain boundary decohesion that reduces the grain boundary tensile strength by one order of magnitude. This decohesion may directly causes the embrittlement since the critical sulfur concentration of this decohesion agrees well with experimental data of the embrittlement.

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