Long-timescale transformations of self-interstitial atom clusters of Cu using the SEAKMC method; The Effect of setting an activation energy threshold for saddle point searches

早川 頌*; 山本 耀二郎*; 沖田 泰良*; 板倉 充洋; 鈴木 克幸*

Computational Materials Science, 218, p.111987_1 - 111987_10, 2023/02

https://doi.org/10.1016/j.commatsci.2022.111987

On-the-fly kinetic Monte Carlo (kMC), a computational technique for atomistic simulations, has attracted attention because it increases the simulation timescale beyond that of molecular dynamics (MD) simulations while maintaining atomistic fidelity. However, for most kMC methods, when events with high and low activation energies coexist in the event list, trivial events with extremely low activation energies that do not essentially affect the phenomena of interest, so-called flicker events, are frequently selected, making it challenging to observe the key dynamics. In this study, we use Self-Evolving Atomistic kMC (SEAKMC), one of the on-the-fly kMC methods, to model the unstable-to-stable transformations of irregular three-dimensional self-interstitial-atom (SIA) clusters in Cu generated through collision cascade. By setting an activation energy threshold once every five steps, transformations into stable configurations are enhanced. The algorithm renders the simulation timescales one or two orders of magnitude longer than those possible with MD simulations. Further, the probability of transformations into stable configurations is increased by 40 times compared to that of the original SEAKMC method. In addition, we find that the stable configurations obtained by the transformation of the SIA clusters are mostly Frank loops. In summary, this new algorithm for the SEAKMC method helps to resolve the inefficiency of kMC methods resulting from the selection of flicker events and will aid the study of meso-timescale atomistic dynamics.

Molecular dynamics simulations to quantify the interaction of a rigid and impenetrable precipitate with an edge dislocation in Cu

津川 聖人*; 早川 頌*; 岩瀬 祐樹*; 沖田 泰良*; 鈴木 克幸*; 板倉 充洋; 愛知 正温*

Computational Materials Science, 210, p.111450_1 - 111450_9, 2022/07

https://doi.org/10.1016/j.commatsci.2022.111450

Precipitation strengthening has been utilized to improve the properties of metallic materials so far. Since interactions between precipitates and dislocations are micro-mechanisms responsible for this phenomenon, a molecular dynamics (MD) simulation is a powerful tool for quantifying this phenomenon. In this study, we introduced a method to simulate a rigid and impenetrable precipitate against a direct contact with a dislocation using a single interatomic potential representing the bulk material. The total force exerted on all atoms in the precipitate region was divided by the number of atoms in the region. This average force was then applied to each atom in the region to simulate one super particle that moved depending on the total force exerted by the matrix atoms on the precipitate atoms. We used MD simulations to quantify the interaction of a precipitate with an edge dislocation. After the dislocation overcame the precipitate, an Orowan loop was formed along the outer circumference of the precipitate. The energy of the loop was 2.1 0.1 eV/b, which was higher than that obtained using the elasticity. The hardening caused by the precipitate was larger than that caused by voids of the same size. The proposed method can be applied to simulate interactions of precipitates with dislocations in any type of metallic material, especially when a dislocation bypasses a precipitate without changing its structure, except when a strong repulsive force acts between them.

Construction of machine-learning Zr interatomic potentials for identifying the formation process of c-type dislocation loops

沖田 泰良*; 寺山 怜志*; 津川 聖人*; 小林 恵太; 奥村 雅彦; 板倉 充洋; 鈴木 克幸*

Computational Materials Science, 202, p.110865_1 - 110865_9, 2022/02

https://doi.org/10.1016/j.commatsci.2021.110865

In this study, a Neural Network Potential (NNP) using an Artificial Neural Network (ANN) was developed for Zr, which is used as fuel cladding material in light water reactors. The reference data were obtained through first-principles calculations of various quantities, such as strained hexagonal-closed-packed (hcp) cells, strained face-centered cubic cells, cells containing a vacancy, several vacancies, and surfaces and -surface energy on all five slip planes in the hcp structures. These data were converted to training data for the ANN, which were invariant to the rotation and translation of the atoms and independent of the number of atoms in the cells. The ANN was defined as a three-layer structure and the number of the nodes was set to 26-12-18-1. The NNP reproduced the first-principles calculations, particularly for the shear deformation, vacancy formation energy, surface energy, and -surface energy, with much higher accuracy than any of the existing potentials that have been developed for classical molecular dynamics simulations. The NNP was applied to identify the formation process of c-type dislocation loops in Zr, which is a key microstructure responsible for abrupt increases in hydrogen absorption. The formation process was determined by the balance of the vacancy formation energy, surface energy and the -surface energy on the basal plane, both of which were precisely reproduced only by the NNP developed in this study. The formation process was identified based on the atomistic behavior of the NNP.

Devil's staircase transition of the electronic structures in CeSb

黒田 健太*; 新井 陽介*; Rezaei, N.*; 國定 聡*; 櫻木 俊輔*; Alaei, M.*; 木下 雄斗*; Bareille, C.*; 野口 亮*; 中山 充大*; et al.

Nature Communications (Internet), 11, p.2888_1 - 2888_9, 2020/06

https://doi.org/10.1038/s41467-020-16707-6

Solids with competing interactions often undergo complex phase transitions. Among them, CeSb is the most famous material where a number of the distinct magnetic phases called devil's staircase appear. We observed the electronic structure evolution across the devil's staircase transitions using bulk-sensitive angle-resolved photoemission spectroscopy.

Atomistic simulations for the effects of stacking fault energy on defect formations by displacement cascades in FCC metals under Poisson's deformation

早川 頌*; 沖田 泰良*; 板倉 充洋; 川畑 友弥*; 鈴木 克幸*

Journal of Materials Science, 54(16), p.11096 - 11110, 2019/08

https://doi.org/10.1007/s10853-019-03688-1

We performed molecular dynamics simulations of displacement cascades in FCC metals under Poisson's deformation using interatomic potentials differing in stacking fault energy (SFE), in order to investigate the effect of tensile strain on the SFE dependence of defect formation processes. There was no clear SFE dependence of the number of residual defects and the size distribution of defect clusters under both no strain and the applied strain, while the strain enhanced the defect formation to a certain extent. We also observed that the strain affected the formations of self-interstitial atom (SIA) clusters depending on their size and the Burgers vector. These results were consistent with the analysis based on the defect formation energies. Meanwhile, the number of SIA perfect loops was higher at lower SFE under both no strain and the applied strain, leading to an increase in the ratio of glissile SIA clusters with a decrease in SFE. Further, the absolute number of SIA perfect loops was increased by the applied strain, while the SFE dependence of the number of SIA perfect loops was not affected. These findings were associated with the difference in formation energy between an SIA perfect loop and an SIA Frank loop. The insights extracted from this study significantly contribute to the modeling of microstructural evolution in nuclear materials under irradiation, especially for low SFE metals such as austenitic stainless steels.

Interactions between clusters of self-interstitial atoms via a conservative climb in BCC-Fe

早川 頌*; 沖田 泰良*; 板倉 充洋; 愛知 正温*; 鈴木 克幸*

Philosophical Magazine, 98(25), p.2311 - 2325, 2018/06

https://doi.org/10.1080/14786435.2018.1486047

BCC鉄における格子間原子クラスタは容易方向へのすべり運動とは別に垂直方向への保存的上昇運動を行い、これにより三次元的な運動を行う。この拡散の三次元性は転位への吸収確率に大きく影響し、材料のスエリングなどの照射劣化挙動を左右する。この保存的上昇運動を分子動力学計算とモンテカルロ計算を組み合わせた手法で解析し、移動障壁エネルギーを評価したところ、従来の弾性理論により予想されていた値よりはるかに大きい値が得られた。

Experimental determination of the topological phase diagram in Cerium monopnictides

黒田 健太*; 越智 正之*; 鈴木 博之*; 平山 元昭*; 中山 充大*; 野口 亮*; Bareille, C.*; 明比 俊太朗*; 國定 聡*; 室 隆桂之*; et al.

Physical Review Letters, 120(8), p.086402_1 - 086402_6, 2018/02

https://doi.org/10.1103/PhysRevLett.120.086402

We use bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy and investigate bulk electronic structures of Ce monopnictides (CeX; X=P, As, Sb and Bi). By exploiting a paradigmatic study of the band structures as a function of their spin-orbit coupling (SOC), we draw the topological phase diagram of CeX and unambiguously reveal the topological phase transition from a trivial to a nontrivial regime in going from CeP to CeBi induced by the band inversion.

Measurements of two-phase pressure drop in a simulated debris bed

Nava, M.*; 伊藤 大介*; 齊藤 泰司*; 青柳 光裕; 神山 健司; 鈴木 徹*

Proceedings of 25th International Conference on Nuclear Engineering (ICONE-25) (CD-ROM), 5 Pages, 2017/07

Two-phase flow through porous media should be well understood to develop a severe accident analysis code not only for light water reactor but also sodium cooled fast reactor (SFR). When a core disruptive accident occurs in SFR, the fuel inside the core become melted and interacts with the coolant. As a result, gas-liquid two-phase flow will be formed in the debris bed, which may have porous nature depending on the cooling process. Thus, as first step, the present work focuses on the characteristics of pressure drop in single and two-phase flows in different porous media conditions (porous size, liquid and gas flow velocity). In addition, in order to construct an experimental database, the measured pressure drop under different conditions was compared with existing correlations.

Validation and improvement of a numerical model for freezing and blockage formation of solid-liquid flow of molten fuel in the core disruptive accident of FBR

青柳 光裕; 神山 健司; 飛田 吉春; 鈴木 徹

Proceedings of 10th International Topical Meeting on Nuclear Thermal Hydraulics, Operation and Safety (NUTHOS-10) (USB Flash Drive), 14 Pages, 2014/12

ナトリウム冷却高速炉の炉心損傷時における溶融燃料流出挙動の評価精度向上のため、高速炉安全解析コードSIMMERにおける溶融燃料の固化・閉塞挙動に関する固液混相流の解析モデルを検証し、モデルの改良を行った。本研究では溶融燃料の流出挙動の分離効果試験であるTHEFIS試験を参照データとした。当該試験の体系に準じた解析体系を設定して、SIMMERコードによる解析を行った。既存の解析モデルによって、概ね試験結果を再現したが、条件によっては試験と比べて融体の流動が過大に制限された。試験での物理現象と既存解析モデルを対比すると、解析モデルは固体粒子による流動抵抗を過大評価しており、このことが流動を制限する解析結果をもたらしたと考えられた。そこで、実現象に応じたモデルに改良を行い、改良モデルを適用した結果、試験結果をより適切に再現することが確認できた。

ROSA-V/LSTF vessel top head LOCA tests SB-PV-07 and SB-PV-08 with break sizes of 1.0 and 0.1% and operator recovery actions for core cooling

鈴木 光弘; 竹田 武司; 中村 秀夫

JAEA-Research 2009-057, 188 Pages, 2010/02

JAEA-Research-2009-057.pdf:16.46MB

加圧水型原子炉(PWR)頂部の小破断冷却材喪失事故を模擬し、高圧注入系(HPI)不作動時のアクシデントマネジメント策の効果を調べるため、ROSA-V計画の大型非定常試験装置を用いて一連の破断サイズパラメータ実験(SB-PV-07, SB-PV-08)を実施した。本報では、破断サイズ1.00.1%(コールドレグ破断相当)における頂部破断LOCA事象の特徴的現象、すなわち破断口蒸気流出と頂部水位の関係,1次系保有水量と炉心露出の関係,炉心過熱を検出する炉心出口温度計(CET)の特性及び炉心と出口部の3次元蒸気流れ等を明らかにした。炉心ボイルオフ過程で623KへのCET温度上昇により開始した1.0%破断実験のHPI回復操作と、0.1%破断実験の蒸気発生器減圧操作とは、ともに炉心冷却を直ちに回復する効果を示した。