Fukushima Daiichi Nuclear Power Plant Unit 2 Accident analysis considering the thermal stratification and containment leakage

中村 勇気*; 小島 良洋*; 山下 拓哉; 下村 健太; 溝上 伸也

Journal of Nuclear Science and Technology, 62(12), p.1226 - 1230, 2025/11

https://doi.org/10.1080/00223131.2025.2570371

In 2011, at the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, it has been reported that several Units of containment vessel had failed, and large quantity of radionuclides had been released into the environment, however, the detail of accident progression with core melt, reactor and containment vessel failure, has still large uncertainties. Especially for the Unit 2 and Unit 3, even they had succeeded in the initial core cooling, at last lost cooling system and fell into severe accident into large release of the fission product into the environment. To clarify these uncertainties in accident scenario, considering the latest information and several insights, the latest accident scenario for Unit 2 and Unit 3 are studied using the severe accident analysis code in this study. It is shown that both Units would result in the thermal stratification in the containment water which encouraged the containment pressure increase at the early phase of the accident. On the other hand, it would be also possible that containment leakage happened to decrease the containment pressure at the later phase of the accident.

Potassium abundances in extremely metal poor stars; Implications for nucleosynthesis in the final stages of massive star evolution

石垣 美歩*; 冨永 望*; 青木 和光*; 和南城 伸也*; 滝脇 知也*; 中村 航*; 岩本 信之; 野本 憲一*; 小林 千晶*

Astrophysical Journal, 992(2), p.215_1 - 215_17, 2025/10

https://doi.org/10.3847/1538-4357/ae00b8

We present the abundance analysis of Potassium (K) in extremely metal-poor stars based on high-resolution spectra obtained by High-Dispersion Spectrograph on Subaru Telescope. One-dimensional LTE abundances of K as well as other elemental abundances including Na, Mg, Ca, Ti, Cr, and Ni are obtained by a spectral synthesis method. We detect K lines in seven stars with [Fe/H] . We find that [K/Fe] and [K/Ca] ratios of seven stars are enhanced relative to the solar value with scatter smaller than the typical uncertainty. While a significant variation in [Na/Mg] ratios can be seen, the observed small scatter in [K/Fe] and [K/Ca] is in line with a hypothesis that the K in those stars has been produced in the innermost ejecta of core-collapse supernovae whose yields are independent of progenitor masses or metallicities.

クリープ破断時間および高温引張強度予測モデルの連合学習

櫻井 惇也*; 鳥形 啓輔*; 松永 学*; 高梨 直人*; 日比野 真也*; 木津 健一*; 森田 聡*; 井元 雅弘*; 下畠 伸朗*; 豊田 晃大; et al.

鉄と鋼, 111(5), p.246 - 262, 2025/04

https://doi.org/10.2355/tetsutohagane.TETSU-2024-124

Creep testing is time-consuming and costly, leading institutions to limit the number of tests conducted to the minimum necessary for their specific objectives. By pooling data from each institution, it is anticipated that predictive models can be developed for a wide range of materials, including welded joints and degraded materials exposed to service conditions. However, the data obtained by each institution is often highly confidential, making it challenging to share with others. Federated learning, a type of privacy-preserving computation technology, allows for learning while keeping data confidential. Utilizing this approach, it is possible to develop creep life prediction models by leveraging data from various institutions. In this paper, we constructed global deep neural network models for predicting creep rupture life of heat-resistant ferritic steels in collaboration with eight institutions using the federated learning system we developed for this purpose. Each institution built a local model using only its own data for comparison. While these local models demonstrated good predictive accuracy for their respective datasets, their predictive performance declined when applied to data from other institutions. In contrast, the global model constructed using federated learning showed reasonably good predictive performance across all institutions. The distance between each institution's data was defined in the space of explanatory variables, with the NIMS data, which had the largest dataset, serving as the reference point. The global model maintained high predictive accuracy regardless of the distance from the NIMS data, whereas the predictive accuracy of the NIMS local model significantly decreased as the distance increased.

Sm synchrotron-radiation-based Mssbauer and SR studies of SmRuGe

筒井 智嗣; 伊藤 孝; 中村 仁*; 吉田 実生*; 小林 義男*; 依田 芳卓*; 中村 惇平*; 幸田 章宏*; 東中 隆二*; 青木 大*; et al.

Interactions (Internet), 245(1), p.55_1 - 55_9, 2024/12

https://doi.org/10.1007/s10751-024-01900-6

Sm SR-based Mssbauer and muon spin relaxation (SR) spectroscopies have been applied to SmRuGe. The temperature dependence of the Sm SR-based Mssbauer spectra in the paramagnetic state implies the presence of dynamical nuclear Zeeman and/or quadrupole interactions. The time differential SR spectra also exhibit a marked temperature dependence in the paramagnetic state, indicating the presence of magnetic fluctuation in SmRuGe at least. These results in the present work infer that the dynamical hyperfine interactions observed using the mutually complementary spectroscopic methods are connected with the mechanism of the heavy fermion behavior in SmRuGe.

Imaging Josephson vortices on curved junctions

藤澤 唯太*; Krishnadas, A*; 中村 友謙*; Hsu, C.-H.*; Smith, B.*; Pardo-Almanza, M*; 比屋根 芳周*; Chang, G.*; 永井 佑紀; 町田 理*; et al.

Physical Review B, 110(22), p.224511_1 - 224511_5, 2024/12

https://doi.org/10.1103/PhysRevB.110.224511

エキゾチックな超伝導体の超伝導磁束の振る舞いを明らかにすることは、今後のデバイス開発にとって重要である。超伝導体LiTiO(111)の薄膜を作製し、磁場中での電子状態を走査型トンネル顕微鏡によって観測した。その際、接合部分の曲率に応じて超伝導磁束の状態が大きく変化することがわかった。そして、その変化は理論計算によっても確かめられた。この結果は、Josephson磁束を実験的な構造変化によってコントロールできる可能性を示唆しており、今後の新たなデバイス開発において重要な結果となるだろう。

原子力機構-東海タンデム加速器の現状

株本 裕史; 中村 暢彦; 沓掛 健一; 乙川 義憲; 遊津 拓洋; 松井 泰; 中川 創平; 池亀 拓麻; 加藤 佑太; 石崎 暢洋; et al.

Proceedings of 21st Annual Meeting of Particle Accelerator Society of Japan (インターネット), p.1165 - 1169, 2024/10

https://www.pasj.jp/web_publish/pasj2024/proceedings/PDF/TFSP/TFSP01.pdf

原子力機構-東海タンデム加速器施設は最高運転電圧が約18MVの大型静電加速器で、重イオンビーム等を用いた核物理、核化学、原子物理、材料照射などの各分野で利用されている。本発表では、2023年度における加速器の運転・整備状況及びビーム利用開発等について報告する。2023年度の運転状況としては、例年よりも運転日数が減少傾向となった。当施設は1982年より運転開始しているが、建屋のインフラの一つである水配管が経年劣化し、一部を更新するための工事に日数を要したためである。また、静電加速器の絶縁ガスとして六フッ化硫黄ガス(SF)を使用しているが、経年劣化した一部のバルブの更新に日数を要したことも運転日数が減少した理由の一つである。整備関係では、実験室へビーム輸送するための振分電磁石の電源の故障が発生し、パワーモジュール素子の交換を行って復旧した。その他には、イオン源周辺の初段加速用の高圧電源(ターミナルECRイオン源の20kV電源、負イオン源の240kV電源)の故障が発生し、それぞれ素子交換等の修理を行って運転を継続した。また、ビーム利用開発としては、近年、宇宙用機器に使用する半導体等へイオンビームを照射するソフトエラー試験の需要が高まっている。当施設としても、それらの需要に応えるために、試験に必要となる照射用チャンバー等の機器類やビームライン配置の検討を開始した。

Polarized and unpolarized neutron scattering for magnetic materials at the triple-axis spectrometer PONTA in JRR-3

中島 多朗*; 齋藤 開*; 小林 尚暉*; 川崎 卓郎; 中村 龍也; 古川 はづき*; 浅井 晋一郎*; 益田 隆嗣*

Journal of the Physical Society of Japan, 93(9), p.091002_1 - 091002_5, 2024/09

https://doi.org/10.7566/JPSJ.93.091002

Neutron scattering is a powerful tool to study magnetic structures and cross-correlated phenomena originated from symmetry of the magnetic structures in matter. Among a number of neutron scattering techniques, polarized neutron scattering is quite sensitive to the orientations of the magnetic moments, which are essential to understand microscopic mechanisms of the spin-driven emergent phenomena. Here, we present POlarized Neutron Triple-Axis spectrometer PONTA in the Japan Research Reactor 3 (JRR-3), and show the present capabilities of polarized and unpolarized neutron scattering by introducing recent results from the instrument.

Fukushima Daiichi Nuclear Power Plant accident analysis considering the thermal stratification and containment leakage

中村 勇気*; 小島 良洋*; 山下 拓哉; 下村 健太; 溝上 伸也

Proceedings of 14th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation, and Safety (NUTHOS-14) (Internet), 12 Pages, 2024/08

At the Fukushima Daiichi Nuclear Power Plant accident, it has been reported that several units of containment vessel had failed, and large quantity of radionuclides had been released into the environment. However, the detailed accident progression of such a containment failure, which includes core melt, reactor vessel failure and following containment vessel behavior, has still large uncertainties. Especially for the unit 2 and 3, they had succeeded in the initial core cooling, but at last lost their cooling system and fell into severe accident to release the fission product into the environment. Nowadays, several information has been obtained by the internal inspection into the containment of the Fukushima Daiichi Nuclear Power Plants. To clarify the uncertainties in the accident scenario, considering the information and several insights already accustomed by previous research, the latest accident scenario in unit 2 and unit 3 of the Fukushima Daiichi Nuclear Power Plants accident are suggested and tested by the severe accident analysis code, MAAP in this study. It is shown that unit 2 and 3 both accident scenario would have resulted in the thermal stratification in suppression pool which encouraged the containment pressure response in the early phase of the accident. In addition, containment vessel leakage would have occurred and affected the containment depressurization.

2023年度夏期休暇実習報告

石塚 悦男; 長住 達; 長谷川 俊成; 川井 大海*; 脇坂 真司*; 長瀬 颯太*; 中村 建斗*; 矢口 陽樹*; 石井 俊晃; 中野 優美*; et al.

JAEA-Technology 2024-008, 23 Pages, 2024/07

JAEA-Technology-2024-008.pdf:1.69MB

「HTTRに関する技術開発」をテーマとした2023年度夏期休暇実習において、3つの大学から5名が参加した。参加者は、HTTR炉心の解析、強制冷却機能喪失時の挙動解析、一次冷却系統のヨウ素沈着挙動解析、高温ガス炉用エネルギー貯蔵システムの概念検討について実習した。実習後のアンケートでは、就業体験として有益であったこと、一部の学生においては自身の研究に役立ったこと等の感想があり、本実習は概ね良好な評価を得た。

原子力機構-東海タンデム加速器の現状

松井 泰; 中村 暢彦; 沓掛 健一; 株本 裕史; 遊津 拓洋; 乙川 義憲; 池亀 拓麻; 中川 創平; 加藤 佑太; 石崎 暢洋; et al.

第36回タンデム加速器及びその周辺技術の研究会報告集, p.17 - 21, 2024/06

当施設には20UR型ペレトロンタンデム加速器(最大電圧約18.5MV)が設置されており、核物理、核化学、材料照射などの研究に利用されている。2023年度の加速器の運転日数は110日で、利用運転における最大電圧は16.8MVであった。加速したイオン種は16元素(分子を含め22種)であり、うち高電圧端子内のECRイオン源からのビームが45.3%を占めた。本研究会では2023年度の加速器の運転および整備状況、トラブル対応等について報告する。