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under high temperatures and various water vapor concentrationsMohamad, A. B.; Nakajima, Kunihisa; Imoto, Jumpei*; Takano, Masahide
Journal of Nuclear Materials, 631, p.156826_1 - 156826_12, 2026/09
Times Cited Count：0Shi, W.*; Machida, Masahiko; Okamoto, Koji*; Luo, X.*; Feng, W.*; Liu, X.*
Reliability Engineering & System Safety, 272, Part1, p.112538_1 - 112538_18, 2026/08
The reliability of emergency response in severe nuclear accidents critically depends on robust real-time monitoring of radioactive source distributions. However, this safety function is challenged by physical constraints that create monitoring blind spots and by the inadequacy of static methods in tracking dynamic releases. To enhance the reliability and robustness of source term estimation, this study proposes a dynamic reconstruction framework based on LASSO regression with temporal regularization. A sliding-window time-penalty mechanism is introduced, imposing 
-norm constraints on inter-step source variations to ensure physical continuity. The contribution matrix and measurement vector are normalized to counteract biases from radiation shielding and time-varying intensities. Validation using a two-room model with internal shielding, with PHITS Monte Carlo simulation, demonstrates accurate reconstruction of dynamic sources from remote measurements. Temporal regularization enhances situational awareness by suppressing spatial aliasing: at sliding-window width 
(no regularization), hotspot locations fluctuate significantly, with quantitative mean absolute error fluctuations at around 
, whereas 
yields improved spatial consistency and the fluctuation quantities decrease to the 
range. Comparative analysis identifies 
as optimal in balancing accuracy and computational cost. This work establishes a more reliable pathway for dynamic hazard assessment, enabling accurate localization and intensity tracking under challenging conditions. The proposed framework provides a decision-support tool enhancing the resilience and safety of emergency management in nuclear facilities.
Yamashita, Takuma*; Hayashi, Teppei*; Mitsuyasu, Yusuke*; Ono, Kenta*; Iwami, Satone*; Kino, Yasushi*; Sekine, Tsutomu*; Oka, Toshitaka; Takahashi, Atsushi*; Shimizu, Yoshinaka*; et al.
International Journal of Radiation Biology, 102(7), p.880 - 887, 2026/07
Times Cited Count：0 Percentile：50.28(Biology)no abstracts in English
Hayashi, Teppei*; Yamashita, Takuma*; Mitsuyasu, Yusuke*; Ono, Kenta*; Iwami, Satone*; Kino, Yasushi*; Sekine, Tsutomu*; Oka, Toshitaka; Takahashi, Atsushi*; Shimizu, Yoshinaka*; et al.
International Journal of Radiation Biology, 102(7), p.888 - 895, 2026/07
Times Cited Count：0 Percentile：0.00(Biology)no abstracts in English
Collaborative Laboratories for Advanced Decommissioning Science; Tohoku University*
JAEA-Review 2026-016, 64 Pages, 2026/06
JAEA-Review-2026-016.pdf:3.06MB
The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), has been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") from FY2019. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2022, this report summarizes the research results of the "Development of a high-resolution imaging camera for alpha dust and high-dose rate monitor" conducted from FY2022 to FY2024. The present study aims to develop a high-resolution imaging camera for alpha dust and a high-dose rate monitor. To realize the high-resolution imaging camera for alpha dust, we have developed novel scintillation materials with emission bands of 500-800 nm. Moreover, we have prepared several materials for the camera and software. We have also developed novel scintillation materials with emission bands of 650-1,000 nm, and simulation studies have been conducted for the high-dose-rate monitor system consisting of optical fiber. In addition, we demonstrated this monitoring system, and the dose-rate dynamic range was found to be 10 mSv/h to 1 kSv/h.
Collaborative Laboratories for Advanced Decommissioning Science; Institute of Science Tokyo*
JAEA-Review 2026-007, 65 Pages, 2026/06
JAEA-Review-2026-007.pdf:4.67MB
The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), has been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") from FY2019. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2024, this report summarizes the research results of the "Development of a laser deflection-type ultrasonic wideband 3D imaging system for in-vessel visualization in high-radiation and non-visible environments" conducted in FY2024. The present study aims to maximize the safety of debris-cutting operations by enabling visualization of in-vessel structures, fuel debris shapes, and scattered particles at distances on the order of several meters, even under dusty and turbid water conditions during work. To achieve this, a compact and portable ultrasonic device suitable for mounting on robots and manipulator arms is employed to develop a laser deflection-type ultrasonic wideband 3D imaging system. In FY2024, the project carried out imaging performance evaluation and studies for the advancement and acceleration of the ultrasonic imaging system, numerical simulations, prototyping and full-scale verification of the system, radiation resistance tests, construction of a submillimeter ultrasonic ranging system, application of the ultrasonic sub-millimeter ranging system to LIBS, as well as battery-powered remote operation of the measurement system. These outcomes are summarized in this report.
Collaborative Laboratories for Advanced Decommissioning Science; Tohoku University*
JAEA-Review 2026-003, 173 Pages, 2026/06
JAEA-Review-2026-003.pdf:12.5MB
The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), has been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") from FY2019. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station (1F), Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2024, this report summarizes the research results of the "Human resource development for research based on fuel debris study and SEEM-ology buildup" conducted in FY2024. The present study aims to buildup of "Severe Engineering Management (SEEM)-ology" that enables more rational decision-making in an uncertain and severe environment, and at the same time, to develop research personnel supporting the academic foundation of the extraction, long-term storage, treatment and disposal of fuel debris, which is a central issue in the decommissioning of 1F. Experts in different fields promote multiple research and human resource development tasks in organic collaboration. In the first fiscal year of this study, it was necessary to clarify specific research methods and set the direction in the [SEEM Fields] [Specialized Fields 1
2] and [Common Engineering Fields 13], as well as to prepare various conditions for necessary tests and analyses. It was also necessary to promote the operation of educational programs related to nuclear decommissioning and the planning of SEEM-related courses. For this reason, the purpose of this fiscal year was to obtain the prescribed results by examining specific research methods and making various preparations for conducting necessary tests, preparing various conditions, building and analyzing some measurement systems, and examining human resource development. Based on this objective, we set the performance targets and implementation methods for this fiscal year as an operational plan, implemented them as planned, and were able to achieve the targeted results in each technical field and education and human resource development activities.
Collaborative Laboratories for Advanced Decommissioning Science; The University of Tokyo*
JAEA-Review 2026-001, 140 Pages, 2026/06
JAEA-Review-2026-001.pdf:9.25MB
The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), has been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") from FY2019. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station (1F), Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2024, this report summarizes the research results of the "Research and education for human resource development in integrated remote robot and measurement technologies for fuel debris removal" conducted in FY2024. This research aims to develop robotic technologies, sensors, and radiation measurement techniques to remotely characterize and assess the properties of fuel debris at 1F reactor, and to cultivate personnel capable of integrating these technologies into systems. Furthermore, it seeks to establish SEEM science and deploy it in actual educational settings. Achievements in FY2024 include: optimizing neutron detectors with high radiation resistance ; constructing a simulator capable of appropriately generating radiation incident events; designing and developing a rover for generating 3D volume models; building physical environments for remote operation support; examined sensor configurations for radiation distribution estimation; proposal of a multi-arm orbital structure as a transport-capable modular orbital structure; development of lightweight arms and examined interfaces for multi-view remote control systems and orbital planners; image processing methods for full-scale environmental structure modeling; investigation on image data transmission methods; development of an integrated DX platform; studies on sensor and robot modularization; development of rigid-body and elastic-body analysis methods; characterization and waste management strategies and investigation of the applicability of geopolymers as backfill materials; establishing SEEM education.
Sakuma, Kazuyuki; Yoshimura, Kazuya
Journal of Environmental Radioactivity, 297, p.108055_1 - 108055_4, 2026/06
Times Cited Count：0Tritium (
H) is the principal radionuclide remaining in ALPS-treated water discharged from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), and its potential environmental impact has drawn considerable attention. To evaluate possible terrestrial effects, H concentrations were monitored in precipitation, groundwater, and river water in the upper Ota River catchment in Fukushima Prefecture. Monthly samples were collected before and after the initiation of oceanic discharge in August 2023. Time-series comparison showed no discernible increase in H concentrations after the start of discharge for any water type. Non-parametric statistical analysis further confirmed the absence of significant differences for groundwater and river water. Observed variations were consistent with natural background levels and known seasonal patterns in precipitation in Japan. These results indicate that ALPS-treated water discharge has had no detectable impact on terrestrial waters in the study area.
Gubarevich, A.*; Kubo, Ryotaro*; Arisaka, Makoto; Osugi, Takeshi; Yoshida, Katsumi*; Takeshita, Kenji*
Journal of Nuclear Science and Technology, 63(6), p.667 - 676, 2026/06
Times Cited Count：1 Percentile：50.28(Nuclear Science & Technology)To immobilize and solidify carbonate-based Advanced Liquid Processing System (ALPS) sediment wastes, this study introduces a chemical transformation process that converts these wastes into calcium and magnesium phosphate phases, followed by densification using a novel cold sintering press (CSP) technique. Simulated calcium-magnesium carbonate slurries were treated with phosphoric acid to synthesize calcium and magnesium phosphates, which were then sintered at 300-500
C using CSP. The effects of the calcium-to-magnesium ratio, strontium incorporation, and sodium chloride addition on phase composition and CSP densification were investigated. Dense bulk samples were successfully fabricated and characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and the Archimedes method. The results showed that the chemical transformation process led to the formation of whitlockite and newberyite, with the calcium-to-magnesium ratio determining the relative proportions of these phases. Strontium was effectively incorporated into the whitlockite crystal structure, while newberyite enhanced densification through a dehydration-driven process. Sodium chloride had no effect on chemical transformation and was not found in the final solid product. These results show that direct conversion of calcium-magnesium carbonate slurries to whitlockite-based phosphate ceramics, followed by CSP, enables stable solidification, making this method promising for ALPS sediment waste management.
Nakajima, Kunihisa; Imoto, Jumpei*; Nishioka, Shunichiro*; Osaka, Masahiko; Miwa, Shuhei
Journal of Nuclear Science and Technology, 63(6), p.727 - 736, 2026/06
Times Cited Count：0 Percentile：0.00(Nuclear Science & Technology)Water-leaching tests at 303 K were performed on cesium (Cs) chemisorbed on stainless steels to investigate its long-term dissolution behavior. The findings showed that Cs continued to dissolve into water even after 1200 hours and that Cs was found to coexist with silicon as ring-shaped particles. This indicates that even water-insoluble Cs, which is defined in the Cs-chemisorption models incorporated into existing SA analysis codes, can dissolve in water over extended periods, with the dissolution attributed to the ring-shaped Cs silicate particles. Additionally, these water-leaching behaviors were accurately described by the Noyes-Whitney equation, suggesting the potential development of a water-dissolution model for the water-insoluble Cs in the chemisorption models. These insights imply that long-term redistributions of chemisorbed Cs could occur within the reactor pressure vessels in Fukushima Daiichi Nuclear Power Station, necessitating a water-dissolution model to predict redistributions through the aqueous phase.
Hanari, Toshihide; Nakamura, Keita*; Imabuchi, Takashi; Kawabata, Kuniaki
Keisoku To Seigyo (Internet) , 65(6), p.438 - 443, 2026/06
no abstracts in English
Takahashi, Hiroki*; Kato, Toru*; Yamashita, Meguru*; Doi, Akio*; Imabuchi, Takashi
Artificial Life and Robotics, 31(2), p.598 - 610, 2026/05
Kawabe, Tomosaburo*; Sano, Yoshihiko*; Kuwahara, Fujio*; Uesawa, Shinichiro; Yoshida, Hiroyuki
Dai-63-Kai Nihon Dennetsu Shimpojiumu Koen Rombunshu (Internet), 1 Pages, 2026/05
To estimate the thermal behavior of fuel debris inside the Primary Containment Vessels (PCVs) of TEPCO's Fukushima Daiichi Nuclear Power Station, a simulation method has been developed using the JUPITER code with porous medium models. However, it has been found that the selection of models and parameters according to the internal structure of the porous media significantly affects the accuracy of simulation results, highlighting the need for appropriate model selection strategies. In this study, we focus on packed beds with particles as one form of porous media and investigate a methodology for determining model parameters that characterize their flow behavior. Systematic knowledge of packed beds with heterogeneous particle size ratios and packing structures remains limited, and comprehensive organization accounting for structural differences has not yet been fully established. Therefore, numerical simulations were employed to evaluate the permeability and Forchheimer coefficient of packed beds with different particle size ratios and packing structures. Based on the obtained results, a method for appropriately determining these parameters according to the structural characteristics of packed beds is proposed, and insights are provided that contribute to improving the accuracy of flow analysis models in porous media.
Sato, Ikken*; Yamashita, Takuya
Nihon Genshiryoku Gakkai-Shi ATOMO
, 68(5), p.306 - 310, 2026/05
Understanding fuel debris behavior during the accident and its final distribution and properties is crucial for fuel debris retrieval at the Fukushima Daiichi Nuclear Power Station (1F). By comprehensively synthesizing existing knowledge, including internal plant investigations, accident-stage plant data, and consistent analytical evaluations, this study estimates the most probable fuel debris behavior for 1F Units 1 to 3. The estimation focuses on the thermal conditions of the debris during each relocation stage from the reactor pressure vessel (RPV) to the pedestal. Finally, we identify key issues for future investigation.
Sato, Yuhi*; Tani, Takashi*; Ikenoue, Tsubasa; Kawamura, Hideyuki; Omori, Yuko*
Environmental Science and Pollution Research, 33(13), p.5818 - 5826, 2026/04
Tokyo Electric Power Company Holdings began the oceanic release of treated water from the Fukushima Daiichi Nuclear Power Station (FDNPS) in August 2023, in which radioactive materials were effectively removed using the Advanced Liquid Processing System (ALPS). The environmental behavior of tritium, accounting for almost all radioactivity in ALPS-treated water, is of critical scientific and social concern. The accumulation possibility of tritium in marine organisms under the release conditions of ALPS-treated water was reviewed to ensure the safety of fishery products collected off the Fukushima coast and prevent unfounded reputation damages to the products. First, previous findings from actual measurements and numerical model estimations of the distribution of tritium derived from ALPS-treated water in seawater off the Fukushima coast are summarized to discuss the impact of oceanic release on tritium levels in seawater. As a result, the impact is suggested to be highly limited, which is indistinguishable from a natural level except for within 200 km from FDNPS. Second, the accumulation possibility of organically bound tritium (OBT) in marine organisms, such as phytoplankton, seaweed, and fish, was assessed using previous findings obtained from experimental and numerical studies, resulting in far smaller OBT accumulations in those organisms compared to the food-chain guideline proposed by FAO/WHO. Finally, the risks of internal exposure through the ingestion of fishery products collected off the Fukushima coast are discussed and quantitatively explained to be minimal especially in comparison with the food-chain guideline. However, continuous environmental monitoring of the oceanic release of ALPS-treated water is considered essential.
Sato, Yuki; Kakuto, Takeshi*; Tanaka, Takayuki*; Shimano, Hiroyuki*
European Physical Journal; Special Topics, 235(4), p.949 - 958, 2026/04
Times Cited Count：1 Percentile：41.22(Physics, Multidisciplinary)Batsaikhan, M.; Oba, Hironori*; Karino, Takahiro; Akaoka, Katsuaki; Wakaida, Ikuo*; Iwata, Yoshihiro; Sakamoto, Kan*
Journal of Analytical Atomic Spectrometry, 41(4), p.1324 - 1335, 2026/04
Times Cited Count：0 Percentile：0.00(Chemistry, Analytical)Pshenichnikov, A.
Proceedings of the Pacific Basin Nuclear Conference (PBNC2026) (Internet), 8 Pages, 2026/04
