Energy release characteristics of prompt criticality induced by molten fuel sloshing in fast reactors

Zou, Z.*; Morita, Koji*; Ishida, Shinya; Yamano, Hidemasa

International Communications in Heat and Mass Transfer, 180, Part 2, p.112532_1 - 112532_22, 2026/11

https://doi.org/10.1016/j.icheatmasstransfer.2026.112532

Research of omnidirectional neutron imaging system for ensuring safety in debris removal (Contract research); FY2024 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; Kyoto University*

JAEA-Review 2026-014, 61 Pages, 2026/08

JAEA-Review-2026-014.pdf:5.86MB

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 "Research of omnidirectional neutron imaging system for ensuring safety in debris removal" conducted in FY2024. The present study aims to develop a compact and light omnidirectional neutron imager to identify the neutron sources for ensuring safety during debris removal operations. There are two operational environments: the reactor building where workers can go, and the high dose rate environment near primary containment vessel. For the former environment, we are developing the neutron imaging system based on the omnidirectional detectors for /-rays. It can be used for worker evacuation instructions by indicating the neutron direction. In order to manufacture the detector, we investigated the response characteristics analysis of the system via Monte Carlo simulation and conducted the performance tests of the detector elements. We also constructed a system combining information from the detector with a 360 degree camera to visually determine the neutron direction. For the latter environment, we should develop the detector with high discrimination performance against -rays. Therefore, detection elements with extremely short decay times were selected and the structure of the omnidirectional neutron detector will be constructed. To evaluate the characteristics of these detectors, the irradiation fields of research reactor were evaluated before the irradiation tests.

Development of a high-resolution imaging camera for alpha dust and high-dose rate monitor (Contract research); FY2024 Nuclear Energy Science & Technology and Human Resource Development Project

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.

Investigations on distribution of radioactive substances owing to the Fukushima Daiichi Nuclear Power Station Accident in the fiscal year 2024 (Contract research)

Group for Fukushima Mapping Project

JAEA-Technology 2025-013, 206 Pages, 2026/03

JAEA-Technology-2025-013.pdf:34.64MB

This report presents results of the investigations on the distribution-mapping project of radioactive substances conducted in FY2024. Car-borne and walk surveys, a measurement using survey meters, and an unmanned helicopter survey were carried out to obtain air dose rate data on land to create their distribution maps, and temporal changes of those air dose rates were analyzed. In order to confirm the applicability of unmanned aircraft to monitoring in mountainous areas, a basic performance of unmanned aircraft was investigated in mountainous areas. Surveys on depth profile of radiocesium and in-situ measurements as for radiocesium deposition were performed. These measurement results were published on the WEB site. Based on these measurement results, effective half-lives of the temporal changes in the air dose rates and the deposition were evaluated. Using the Bayesian hierarchical modeling approach, we obtained maps that integrated air dose rate distribution data acquired through surveys such as car-borne and walk surveys. Radiation monitoring and analysis of environmental samples owing to the comprehensive radiation monitoring plan were carried out. Representative life patterns that can be expected after the return to the evacuation-designated restricted area were set, and the cumulative exposure doses were evaluated for the local governments and residents in the area. Score maps to classify the importance of the measurement points were created, and the temporal changes in the score were analyzed. A system to report the tritium concentration level in seawater to the Nuclear Regulation Authority was operated, and the variation of tritium concentration before and after the discharge of ALPS treated water to the ocean was analyzed. Monitoring data in coastal area performed owing to the comprehensive radiation monitoring plan until FY2024 was analyzed.

Ambient dose rate variation in the Fukushima region visualized using explainable AI techniques

Yoshida, Ryu*; Kurikami, Hiroshi; Nagao, Fumiya; Takahashi, Shigeo*; Sanada, Yukihisa

Journal of Environmental Radioactivity, 293, p.107900_1 - 10790_13, 2026/02

https://doi.org/10.1016/j.jenvrad.2026.107900

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

Iwasawa, Yuzuru; Shibamoto, Yasuteru; Maruyama, Yu

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

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

An Estimation method for the total amount of Cs contamination on the bottom of top shield plug in the Fukushima Daiichi Nuclear Power Plant Unit 2

Kanno, Ikuo; Matsumura, Taichi; Okumura, Keisuke

Journal of Nuclear Science and Technology, 7 Pages, 2026/00

https://doi.org/10.1080/00223131.2026.2725018

Activity report of the emergency monitoring committee, 2; Transition and long-term monitoring

Mikami, Satoshi; Saito, Kimiaki; Hokama, Tomonori; 9 of others*

Hoken Butsuri (Internet), 60(4), p.275 - 303, 2025/12

https://doi.org/10.5453/jhps.60.275

no abstracts in English

Development of a prototype shielding-free radiation-resistant diamond neutron measurement system (Contract research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; Hokkaido University*

JAEA-Review 2025-028, 66 Pages, 2025/11

JAEA-Review-2025-028.pdf:3.59MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2023. 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 FY2023, this report summarizes the research results of the "Development of a prototype shielding-free radiation-resistant diamond neutron measurement system" conducted in FY2023. The present study aims to develop a prototype of a shielding-free neutron measurement system for 1F. The system consists of diamond neutron detectors and radiation-resistant silicon integrated circuits, and has radiation resistance of more than 10 MGy and 4 MGy, respectively, at the component level in terms of integrated dose, and has a track record of stable operation under -ray dose rate environment of 1.5 kGy/h. Future applications are expected to include neutron detectors for debris investigation, criticality proximity monitoring monitors, and neutron detectors for dry tube investigation in pressure vessels. In this development, a prototype consisting of 100 diamond detector elements of 5 mm square will be developed to obtain system construction technology and to evaluate system performance. In addition, a subcriticality evaluation method will be developed. This development will lead to the completion of system development, development of the actual system in collaboration with the manufacturer, and introduction of the system into 1F decommissioning project.

Heat transfer coefficient modeling for downward saturated boiling flows in vertical pipes

Wada, Yuki; Shibamoto, Yasuteru; Hibiki, Takashi*

International Journal of Heat and Mass Transfer, 249, p.127219_1 - 127219_16, 2025/10

https://doi.org/10.1016/j.ijheatmasstransfer.2025.127219