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論文

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

 被引用回数:0 パーセンタイル:0(Materials Science, Multidisciplinary)

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 $$pm$$ 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.

論文

Modeling changes in the second harmonic generation of ultrasonic waves having wavelengths beyond the length scale of conventional molecular dynamics

森 承宇*; 松田 那由多*; 沖田 泰良*; 愛知 正温*; 板倉 充洋; 鈴木 克幸*

Materialia, 21, p.101371_1 - 101371_6, 2022/03

The nonlinear ultrasonic (NLU) technique is a nondestructive method for detecting nanostructure in crystalline materials. In this study, a method was developed to quantify the changes in NLU signals associated with nanostructure using molecular dynamics (MD). A nonreflective boundary, which reduces the computational cost to the first power of the wavelength, was used to achieve this. This method is distinct from previous studies using a conventional MD, for which the computational cost is proportional to the square of the wavelength. The nonreflective boundary eliminates the influence of reflected waves at the detection position by setting a buffer region at the end of the simulation cell opposite from the wave source, and periodically resetting the displacements and velocities of all atoms in this region. This method allows the introduction of elastic waves with wavelengths longer than the cell size, and only an extension of time is required, according to the extension of the wavelength, without increasing the cell size. Hence, it is possible to extend the NLU wavelength by approximately four orders of magnitude, which approaches the wavelengths used for inspections and, thus, to use MD to simulate the changes in the NLU signals induced by nanostructure. The NLU signal values obtained by the two methods were in good agreement for a perfect Fe crystal and a Fe crystal containing 1% monovacancies. No significant frequency dependence of the acoustic nonlinearity parameter was found at 0 K. This method will contribute to the development of an inspection technique based on scientific principles.

論文

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

 被引用回数:0 パーセンタイル:0(Materials Science, Multidisciplinary)

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 $$gamma$$-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 $$gamma$$-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 $$gamma$$-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.

論文

Molecular dynamic simulations evaluating the effect of the stacking fault energy on defect formations in face-centered cubic metals subjected to high-energy particle irradiation

寺山 怜志*; 岩瀬 祐樹*; 早川 頌*; 沖田 泰良*; 板倉 充洋; 鈴木 克幸*

Computational Materials Science, 195, p.110479_1 - 110479_12, 2021/07

 被引用回数:2 パーセンタイル:50.94(Materials Science, Multidisciplinary)

Austenitic stainless steels, which are used as incore structural materials in light water reactors, are characterized by an extremely low stacking fault energy (SFE) among face-centered cubic (FCC) metals. To evaluate the effects of SFE on defect formation under high-energy particle irradiation, molecular dynamics simulations were performed using the interatomic potential sets for FCC metals with different SFEs and a primary knock-on atom energy (E$$_{rm PKA}$$) of 100 keV at 600 K. The results show that the number of residual defects is independent of the SFE. However, the characteristics of self-interstitial atom (SIA) clusters do depend on the SFE. For clusters smaller than a certain size, the ratio of glissile SIA clusters decreases as the SFE increases, which is similar to the trend observed at the low E$$_{rm PKA}$$. However, for larger clusters, which can be detected only at a high E$$_{rm PKA}$$, the ratio of glissile clusters increases. These results correspond to static energy calculations, in which the difference in the formation energy between a Frank loop and perfect loop ($$Delta$$E$$_{rm F-P}$$) for the small clusters decreases as the SFE increases. In contrast, for the larger clusters, the SFE dependence of $$Delta$$E$$_{rm F-P}$$ changes due to the shape restrictions of stable perfect loops. At a high temperature of 600 K, large vacancy clusters with stacking faults can be detected at E$$_{rm PKA}$$ = 100 keV, resulting in the enhanced formation of these clusters at lower SFEs. Furthermore, several of these clusters were similar to perfect loops, with the edges split into two partial dislocations with stacking faults, although the largest clusters detected at low E$$_{rm PKA}$$s were similar to stacking fault tetrahedrons.

論文

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

 被引用回数:9 パーセンタイル:68.19(Multidisciplinary Sciences)

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.

論文

Screw dislocation-spherical void interactions in fcc metals and their dependence on stacking fault energy

早川 頌*; 土井原 康平*; 沖田 泰良*; 板倉 充洋; 愛知 正温*; 鈴木 克幸*

Journal of Materials Science, 54(17), p.11509 - 11525, 2019/09

 被引用回数:8 パーセンタイル:49.62(Materials Science, Multidisciplinary)

We performed molecular dynamics simulations to evaluate the effects of stacking fault energy (SFE) on interactions between a screw dislocation and spherical voids in face-centered cubic (fcc) metals. It was observed that the frequency of the cross-slips is a critical factor affecting the interaction, with primarily three different interaction morphologies being observed: (1) the two partial dislocations detach from the void independently with a time lag, (2) the two partial dislocations detach from the void almost simultaneously on a single slip plane, and (3) the two partial dislocations detach from the void almost simultaneously while involving more than one cross-slip and a jog formation. The magnitude of the critical resolved shear stress (CRSS) increases in the order mentioned above. The CRSS values for interaction morphology (2), which was observed most frequently in this study, were in good agreement with those predicted analytically by adjusting the parameters dependent on the SFE. Based on the obtained results, we discussed the applicability of the analytical model for void hardening in fcc metals. The results of this work contribute significantly to the modeling of mechanical property degradation in irradiated metals.

論文

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

 被引用回数:5 パーセンタイル:33.9(Materials Science, Multidisciplinary)

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.

論文

Effects of stacking fault energies on formation of irradiation-induced defects at various temperatures in face-centred cubic metals

中西 大貴*; 川畑 友弥*; 土井原 康平*; 沖田 泰良*; 板倉 充洋; 鈴木 克幸*

Philosophical Magazine, 98(33), p.3034 - 3047, 2018/09

 被引用回数:5 パーセンタイル:36.12(Materials Science, Multidisciplinary)

原子炉内部で使用されるオーステナイト鋼は積層欠陥エネルギーが低いことで知られ、そのため積層欠陥を伴う照射欠陥クラスタが大きくなることが予想される。この傾向を定量評価するため、FCC金属のモデルポテンシャルにおいて積層欠陥エネルギーだけを変化させたものを用い照射欠陥生成の分子動力学シミュレーションを様々な温度において実行し、生成した照射欠陥クラスタの種類と数を調べた。その結果移動可能な格子間原子クラスタの生成数が積層欠陥エネルギーとともに大きく変化することが見出された。

論文

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

 被引用回数:4 パーセンタイル:29.9(Materials Science, Multidisciplinary)

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。

論文

Atomic simulations to evaluate effects of stacking fault energy on interactions between edge dislocation and spherical void in face-centred cubic metals

土井原 康平*; 沖田 泰良*; 板倉 充洋; 愛知 正温*; 鈴木 克幸*

Philosophical Magazine, 98(22), p.2061 - 2076, 2018/05

 被引用回数:13 パーセンタイル:68.46(Materials Science, Multidisciplinary)

原子炉材料、特に炉内構造材のオーステナイトの照射脆化を評価する際に、オーステナイトの特徴である低い積層欠陥エネルギーの影響を評価する必要がある。本研究では積層欠陥エネルギーだけを変化させたFCC金属のモデルポテンシャルを用いて転位とボイドの相互作用形態を評価し、ボイドの大きさと積層欠陥エネルギーの値によって様々な相互作用形態があることを見出した。

論文

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

 被引用回数:33 パーセンタイル:90.28(Physics, Multidisciplinary)

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.

論文

Two-phase flow measurements in a simulated debris bed

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

Proceedings of 17th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-17) (USB Flash Drive), 10 Pages, 2017/09

Two-phase flow through porous media must be well understood to develop a severe accident analysis code not only for light water reactor (LWR) 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 two-phase flows in different porous media conditions (porous size, liquid and gas flow velocity). To construct an experimental database, the measured pressure drop under different conditions was compared with existing correlations. In addition, X-ray radiography, which is very helpful to understand the two-phase structure inside the porous media, was applied to measure porosity and void fraction distribution in the packed bed of spheres.

論文

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.

論文

Characteristics of pressure drop and void fraction in a simulated debris bed

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

Proceedings of 2017 Japan-US Seminar on Two-Phase Flow Dynamics (JUS 2017), 4 Pages, 2017/06

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 two-phase flows in different porous media conditions (porous size, liquid and gas flow velocity). In addition, to construct an experimental database, the measured pressure drop under different conditions was compared with existing correlations.

論文

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。

論文

Reliability of core exit thermocouple for accident management action during SBLOCA and abnormal transient tests at ROSA/LSTF

鈴木 光弘; 中村 秀夫

Journal of Nuclear Science and Technology, 47(12), p.1193 - 1205, 2010/12

 被引用回数:1 パーセンタイル:10.69(Nuclear Science & Technology)

機構が、PWRを模擬するROSA計画大型非定常試験装置(LSTF)を用いて行った小破断冷却材喪失事故(SBLOCA)と異常過渡事象に関する13回の模擬実験を対象に、炉心の過熱を検出してアクシデントマネジメント策開始の判断に用いられる炉心出口温度計(CET)の特性を系統的に調べ、あらためて次の諸点をまとめた。LSTFは、高さ実寸,実機圧力で容積比1/48を有する世界最大のPWR模擬装置である。(1)一般にCETで炉心過熱の検出が可能であるが、炉心過熱部と比べて検出時間と温度上昇に遅れを伴う。(2)この遅れの共通原因の1つは、炉心出口と外周部の金属構造材による冷却効果である。(3)著しく高圧あるいは低圧の条件下では、CET温度の代替指標として、その過熱度が必要である。(4)ホットレグから著しい量の流下水がある場合には、CETは過熱温度を検出しなかった。さらに、これらの実験結果のPWRへの適用性を検討した。

報告書

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1.0$$sim$$0.1%(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。

論文

Reliability of core exit thermocouple (CET) for accident management action during SBLOCA and abnormal transient tests at ROSA/LSTF

鈴木 光弘; 中村 秀夫

Proceedings of 13th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-13) (CD-ROM), 17 Pages, 2009/09

日本原子力研究開発機構が大型非定常試験装置(LSTF)を用いて行った小破断冷却材喪失事故(SBLOCA)と異常過渡事象に関する12回の模擬実験を対象として、炉心出口温度計(CET)の特性とそのPWRアクシデントマネジメント措置への適用性をまとめた。LSTFは、高さ実寸で容積比1/48を有する世界最大のPWR模擬装置である。これらの実験より、次の一般的CET特性を明らかにした。(1)CETは大部分のケースで炉心過熱の検出が可能であるが、炉心過熱部と比べて検出時間と温度上昇に遅れを伴う。(2)この遅れを生じる原因の1つは、炉心出口と外周部の構造材による冷却効果である。(3)蒸気発生器による減圧操作を伴う微少破断の場合には、10%低温側配管破断と同様、ホットレグからの著しい流下水の影響でCETは過熱温度を検出しなかった。(4)著しく高圧あるいは低圧条件下では、CET温度の代替指標として、その過熱度が必要である。

報告書

CET performance at ROSA/LSTF tests; Twelve tests with core heat-up

鈴木 光弘; 中村 秀夫

JAEA-Research 2009-011, 155 Pages, 2009/07

JAEA-Research-2009-011.pdf:31.62MB

本報は、ROSA/LSTFで実施した12実験の炉心出口温度計(CET)特性をまとめたものであるが、これには10件の小破断冷却材喪失事故(SBLOCA)模擬実験と2件の異常過渡模擬実験を含み、既に検討されたOECD/NEA ROSAプロジェクト実験6-1の結果に対する追加資料として作成したものである。すなわちこの内容は、PWRアクシデントマネジメント策にCETを利用するうえで必要な知識基盤を再検討し統合するため、2008年4月に新たに設置されたOECD/NEA WGAMAのタスクグループに対して準備され、提出された。これらの実験は、破断サイズや位置,1次系圧力,炉心出力,リフラックス水流下の有無及び運転員操作等の広範囲の実験条件を含む。各々の実験について炉心温度履歴に対応するCET温度特性を調べ、時間遅れや温度上昇の緩慢さに焦点を当てたCETの一般特性をまとめた。

論文

Performance of core exit thermocouple for PWR accident management action in vessel top break LOCA simulation experiment at OECD/NEA ROSA Project

鈴木 光弘; 竹田 武司; 中村 秀夫

Journal of Power and Energy Systems (Internet), 3(1), p.146 - 157, 2009/00

本報は、LSTFで実施したOECD/NEA ROSAプロジェクトの最初の実験であるTest 6-1に関して、PWR頂部小破断LOCA模擬条件における炉心過熱検出用炉心出口温度計(CET)の特性をまとめたものである。破断サイズは1.9%コールドレグ破断相当であり、CET温度623K以上を検出した場合にアクシデントマネジメント(AM)として蒸気発生器2次系急減圧操作を開始することとした。しかし、炉心過熱状態の検出には約230sの時間遅れが生じ、またCETsと炉心最高温度部との温度乖離が大きかった。炉心と炉心出口部の3次元蒸気流れを含めてこれらの原因を解明するとともに、PWR条件でのCET特性への適用可能性と、原子炉水位計のような兆候ベースプラント計装による早期AM開始のための代替指標可能性とについて検討結果を述べた。

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