Small-scale experiments on melt spreading and deposition via melt-jet impingement on a dry substrate; Evaluation of empirical correlations for deposition area of continuous layered debris

岩澤 譲; 柴本 泰照; 丸山 結

Nuclear Engineering and Design, 446(Part B), p.114599_1 - 114599_16, 2026/01

https://doi.org/10.1016/j.nucengdes.2025.114599

Continuous layered debris deposited due to the molten core (melt) spreading across the floor of a containment vessel can pose a serious threat to containment integrity during severe accidents in light water reactors. The present study conducted small-scale experiments to investigate melt spreading process and subsequent deposition of continuous layered debris via melt-jet impingement onto a floor. The small-scale experiments were conducted using a low-melting-point metal under dry conditions without coolant water. High-speed imaging and image processing techniques were employed to elucidate the influence of melt injection conditions on melt spreading and subsequent deposition of continuous layered debris. The use of larger nozzle sizes and more highly superheated melts enabled the expansion of the experimental database. Based on the experimental results, we identified appropriate correlations from those proposed in previous studies to estimate the debris deposition area and evaluated their predictive accuracies. These correlations were then applied to estimate the potential spreading area of the relocated melt under anticipated reactor-scale conditions. The analysis revealed that thermal effects, such as heat transfer to the floor, influence the potential spreading area, could be incorporated into the correlations for applications under the anticipated reactor-scale conditions.

Analysis of Mo isotopes originating from the reactor core of the Fukushima Daiichi Nuclear Power Station using extraction chromatographic separation and ICP-MS/MS measurement with an oxygen dynamic reaction

島田 亜佐子; 飯田 芳久; 丸山 結

ACS Omega (Internet), 10(46), p.56533 - 56538, 2025/11

https://dx.doi.org/10.1021/acsomega.5c08680

Oxygen dynamic reactions with Zr, Mo, and Ru were elucidated to analyze Mo isotopes originating from the reactor core of the Fukushima Daiichi Nuclear Power Station. The main products were ZrO, MoO, and Ru, respectively. The measured Mo isotope ratio in a mixed solution containing 1 ng/mL each of Zr, Mo, and Ru, using ICP-MS/MS with 0.30 mL/min flow rate of oxygen reaction cell, agreed within 4 % with that of the Mo solution, indicating that MoO was successfully separated from ZrO and Ru. The developed method was applied to analysis of Mo isotopes in smear samples collected in reactor buildings at the Fukushima Daiichi Nuclear Power Station, following dissolution and extraction chromatographic separation. A clearly elevated high Mo/Cs ratio was observed in the smear sample collected on the 2nd floor of the fuel handling machine room on the operating floor in Unit 2. In contrast, the ratios for smear samples collected from other floors of the Unit 2 reactor building were comparable to the composition in the reactor core.

New insight on the thermal impact on cementitious materials due to high-temperature with water supply; Continuous expansive spalling in water

三浦 泰人*; 宮本 慎太郎*; 丸山 一平*; Aili, A.*; 佐藤 拓未; 永江 勇二; 五十嵐 豪*

Case Studies in Construction Materials, 21, p.e03571_1 - e03571_14, 2024/12

https://doi.org/10.1016/j.cscm.2024.e03571

In this study, the expansion behavior of cement materials after high-temperature heating and water immersion was observed experimentally. Two experiments were conducted using mortar specimens with different sand-to-cement ratios subjected to different high-temperature histories up to 1000C. In Case 1, the specimens were immersed in water after high-temperature heating and then cooled naturally; in Case 2, the specimens were immersed in water at high temperatures without the cooling process. Based on the results, it was confirmed that lime expansion due to the rehydration of CaO by heating occurred in Case 1. In contrast, dynamic continuous explosive spalling occurred in Case 2 because of water penetration into the specimen at a high temperature. The explosive spalling in water observed in Case 2 is a phenomenon that has not been reported to date. Possible failure mechanisms for lime expansion and continuous expansive spalling in water are suggested.

多層膜スーパーミラーを用いたWolter I型中性子磁気顕微鏡の光学設計

曽山 和彦; 林田 洋寿*; 丸山 龍治; 山崎 大; 後藤 惟樹*; 小林 勇輝*; 荒川 翔平*; 山本 有悟*; 須場 健太*; 山村 和也*

JAEA-Research 2024-006, 15 Pages, 2024/10

JAEA-Research-2024-006.pdf:1.51MB

磁性体中の磁場イメージングを目的として、Wolter I型光学系に多層膜スーパーミラーを適用した中性子磁気顕微鏡の光学設計を行った。拡大率12.5倍の拡大結像光学系について、軌跡シミュレーションコードを用いて、偏極中性子の磁性体試料によるdepolarizationの解析、レプリカ法で作製されるWolterミラーの形状精度が顕微鏡の空間分解能へ与える影響の検討を行った。その結果、マイクロメートルオーダーの空間分解能を得るために必要な形状精度に関する知見を得た。

Development of risk importance measures for dynamic PRA based on risk triplet, 2; Trial measurement of risk importance through dynamic level 2 PRA with RAPID

Zheng, X.; 玉置 等史; 柴本 泰照; 丸山 結; 高田 毅士; 成川 隆文*; 高田 孝*

Proceedings of Probabilistic Safety Assessment and Management & Asian Symposium on Risk Assessment and Management (PSAM17 & ASRAM2024) (Internet), 10 Pages, 2024/10

Traditional frequency-based risk importance measures (RIMs) have demonstrated its practicability in the nuclear regulation. The authors investigate the definitions of existing RIMs and associated applications in risk-informed nuclear regulations, for instance, the risk-informed categorization of structures, systems, and components (SSCs), risk-informed changes to technical specifications, etc. However, when evaluating mitigation effects of accident countermeasures, importance assessments involving consequence and timing has the potential of providing valuable information for decision making. By widely using numerical simulations of possible accident progressions, dynamic PRA enables a straightforward assessment of risk triplets. Recent advancements in the development of dynamic PRA tend to explicitly incorporate the dynamics of accident progression and failure events into risk assessment, and it allows a provision of more detailed risk information. The approach to appropriate estimation of risk importance within this framework has not been established, exposing a significant research challenge in the use of risk information for decision making in the nuclear industry. Possible accident sequences are sampled using RAPID by randomly branching, and risk triplets are quantified, including key quantities such as source term release amount and release timing to the environment, and the associated frequencies. Risk triplets are used to calculate the new RIMs to rank the importance of pivotal headings in the event tree model. As the exemplary results of the analysis, source term release amount and timing are largely influenced by the mode of containment failure and the termination timing of reactor coolant injection. As the conclusion, when issues such as timing or seriousness of consequence are important for judgement, dynamic PRA and the new RIMs is capable of supporting decision making by providing more detailed risk information.

A Focusing supermirror for time-of-flight grazing-incidence small-angle neutron scattering measurement

山崎 大; 丸山 龍治; 青木 裕之; 花島 隆泰*; 阿久津 和宏*; 宮田 登*; 曽山 和彦

Quantum Beam Science (Internet), 9(2), p.20_1 - 20_12, 2024/06

https://doi.org/10.3390/qubs9020020

This study developed a neutron-beam focusing supermirror for grazing-incidence small-angle neutron scattering (GISANS) measurements. It was fabricated by depositing NiC/Ti supermirror film with ion-beam sputtering on a precise elliptic surface of fused quartz figured using the elastic emission machining technique. Neutron measurements at a pulsed neutron reflectometer BL17 of MLF, J-PARC successfully demonstrated that the focusing supermirror enhances a beam intensity twentyfold compared with an optimally collimated beam, achieving the signal-to-background ratio of the focal spot as high as 500. The mirror can be readily installed and used at BL17.

Microbiome analysis of the restricted bacteria in radioactive element-containing water at the Fukushima Daiichi Nuclear Power Station

藁科 友朗*; 佐藤 朝子*; 比内 浩; Shaikhutdinov, N.*; Shagimardanova, E.*; 森 宙史*; 玉木 聡志*; 斎藤 元文*; 眞田 幸尚; 佐々木 祥人; et al.

Applied and Environmental Microbiology, 90(4), p.e02113-23_1 - e02113-23_23, 2024/04

https://doi.org/10.1128/aem.02113-23

A major incident occurred at the Fukushima Daiichi Nuclear Power Station following the tsunami triggered by the Tohoku-Pacific Ocean Earthquake in March 2011, whereby seawater entered the torus room in the basement of the reactor building. Here, we identify and analyze the bacterial communities in the torus room water and several environmental samples. Samples of the torus room water (1 10 BqCs/L) were collected by the Tokyo Electric Power Company Holdings from two sampling points between 30 cm and 1 m from the bottom of the room (TW1) and the bottom layer (TW2). A structural analysis of the bacterial communities based on 16S rRNA amplicon sequencing revealed that the predominant bacterial genera in TW1 and TW2 were similar. TW1 primarily contained the genus Limnobacter, a thiosulfate-oxidizing bacterium. -Irradiation tests on Limnobacter thiooxidans, the most closely related phylogenetically found in TW1, indicated that its radiation resistance was similar to ordinary bacteria. TW2 predominantly contained the genus Brevirhabdus, a manganese- oxidizing bacterium. Although bacterial diversity in the torus room water was lower than seawater near Fukushima, 70% of identified genera were associated with metal corrosion. Latent environment allocation - an analytical technique that estimates habitat distributions and co-detection analyses - revealed that the microbial communities in the torus room water originated from a distinct blend of natural marine microbial and artificial bacterial communities typical of biofilms, sludge, and wastewater. Understanding the specific bacteria linked to metal corrosion in damaged plants is important for advancing decommissioning efforts.

超高真空材料プロセス研究のためのガス精密制御の自動化; 表面反応の放射光リアルタイム観察への応用

中村 孝史*; 山本 幸男*; 荒川 正和*; 丸山 晃生*; 吉越 章隆

産業応用工学会論文誌, 11(2), p.109 - 114, 2023/09

https://doi.org/10.12792/jjiiae.11.2.109

SPring-8のBL23SUに設置された表面化学実験ステーションは、放射光軟X線を使って様々な機能性材料の表面および界面の研究に利用されている。固体表面とガスとの化学反応の理解を進めるためには、ガスの精密流量制御が必須である。本論文では、超高真空(分子流領域)におけるガスと表面の反応の実験精度および再現性を改善するための自動ガス流量コンピュター制御システムを報告する。ガス圧力制御のために、スローリークバルブのフィードバック制御システムを開発した。開発したシステムによって、装置エキスパートの実験者と同等以上の反応実験が可能となった。

Main outputs from the OECD/NEA ARC-F Project

丸山 結; 杉山 智之*; 島田 亜佐子; Lind, T.*; Bentaib, A.*; Sogalla, M.*; Pellegrini, M.*; Albright, L.*; Clayton, D.*

Proceedings of 20th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-20) (Internet), p.4782 - 4795, 2023/08

http://dx.doi.org/10.13182/NURETH20-40302

The Analysis of Information from Reactor Buildings and Containment Vessels of Fukushima Daiichi Nuclear Power Station (FDNPS) (ARC-F) project was initiated in January 2019 for three years with 22 signatories from 12 countries. Three main tasks were implemented in the ARC-F project, which were relevant to 1) refinement of analysis for accident scenarios and associated fission product (FP) transport and dispersion, 2) compilation and management of data and information, and 3) discussion for the next-phase project. Various activities were performed in Task 1, covering improvement of analysis for accident scenarios, and in-depth analyses for specific phenomena such as in-vessel melt progression, molten core/concrete interaction, FP transport and source term, hydrogen combustion and atmospheric dispersion of FPs. Through these studies, analyses for accident scenarios with severe accident codes were refined and important phenomena with large uncertainties were clarified. In order to share well selected and organized information from the FDNPS with the project partners, two databases, information source database and sample database, were built under Task 2. The analysis techniques including the separation of iodine species were developed also in Task 2 and applied to the analysis of FPs in several samples taken from the FDNPS. The next-phase project was discussed in Task 3, resulting in launching the Fukushima Daiichi Nuclear Power Station Information Collection and Evaluation (FACE) project. The FACE project officially started in July 2022 with the participation of 23 organizations from 12 countries and the European Commission.

商用高温ガス炉使用済燃料の再処理廃棄物処分に関する研究

深谷 裕司; 丸山 貴大; 後藤 実; 大橋 弘史; 樋口 英明

JAEA-Research 2023-002, 19 Pages, 2023/06

JAEA-Research-2023-002.pdf:1.48MB

商用高温ガス炉使用済燃料の再処理に由来する廃棄物の処分に関する研究を行った。軽水炉の再処理と高温ガス炉の再処理では燃料の構造の違いによる大きな違いがあるため、軽水炉に対して制定された再処理の廃棄物処理に関する法律の高温ガス炉廃棄物への適用性を確認すべきである。そこで、技術の違いを比較するとともに、全炉心燃焼計算を用いて、黒鉛廃棄物の放射化量及び表面汚染による放射能濃度を評価することにより、再処理廃棄物について比較を行った。その結果、SiC残渣廃棄物は、特定放射性廃棄物の最終処分に関する法律(2000年法律第117号)の第二種特定放射性廃棄物として軽水炉のハル・エンドピースと同様に地層処分されるべきことが分かった。黒鉛廃棄物については、軽水炉のチャンネルボックスと同様に、核原料物質、核燃料物質及び原子炉の規制に関する法律(1957年法律第166号)の第二種廃棄物としてピット処分による浅地中処分されるべきことが分かった。