Real-time inversion of radioactive source distribution using air dose rate measurements via least absolute shrinkage and selection operator method

Shi, W.*; Machida, Masahiko; Okamoto, Koji*; Luo, X.*; Feng, W.*; Liu, X.*

Reliability Engineering & System Safety, 272, Part1, p.112538_1 - 112538_18, 2026/08

http://doi.org/10.1016/j.ress.2026.112538

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.

Research and education for human resource development in integrated remote robot and measurement technologies for fuel debris removal (Contract research); FY2024 Nuclear Energy Science & Technology and Human Resource Development Project

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.

Cold sintering of whitlockite-based phosphate ceramics for ALPS carbonate slurry waste immobilization

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

https://doi.org/10.1080/00223131.2025.2592963

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 300-500C 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.

Floating object removal in underwater ROV video images using segment anything model and generative image in-painting

Takahashi, Hiroki*; Kato, Toru*; Yamashita, Meguru*; Doi, Akio*; Imabuchi, Takashi

Artificial Life and Robotics, 31(2), p.598 - 610, 2026/05

https://doi.org/10.1007/s10015-025-01100-7

Accumulation possibility of tritium released from the Fukushima Daiichi Nuclear Power Station in marine organisms

Sato, Yuhi*; Tani, Takashi*; Ikenoue, Tsubasa; Kawamura, Hideyuki; Omori, Yuko*

Environmental Science and Pollution Research, 33(13), p.5818 - 5826, 2026/04

https://doi.org/10.1007/s11356-026-37606-1

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.

Measurement of uranium and fission products in a spent fuel sample using fiber-optic laser-induced breakdown spectroscopy

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

https://doi.org/10.1039/D5JA00451A

Aerial monitoring around TEPCO's Fukushima Daiichi Nuclear Power Station and development of radiation monitoring technology for unmanned vehicle in fiscal year 2024 (Contract research)

Futemma, Akira; Ochi, Kotaro; Sasaki, Miyuki; Nakama, Shigeo; Kawasaki, Yoshiharu*; Iwai, Takeyuki*; Hiraga, Shogo*; Haginoya, Masashi*; Matsunaga, Yuki*; Yamada, Tsutomu*; et al.

JAEA-Technology 2025-016, 253 Pages, 2026/03

JAEA-Technology-2025-016.pdf:20.16MB

Aerial Radiation Monitoring (ARM) has been used to quickly and widely measure radiation distribution caused by the TEPCO's Fukushima Daiichi Nuclear Power Station (FDNPS) accident resulted from the tsunami accompanying the Pacific coast of Tohoku Earthquake on March 11, 2011. Since the accident, As a commissioned project of the Nuclear Regulation Authority, the Japan Atomic Energy Agency (JAEA) has continuously conducted ARM around FDNPS. This report summarizes the results of the 2024 monitoring activities, evaluates temporal changes in ambient dose rates, and identifies factors contributing to these changes. A terrain-corrected analysis was applied to improve dose rates conversion accuracy, and results with and without this correction were compared. A radon-progeny discrimination method was also used to assess its impact on manned-helicopter measurements. Furthermore, development of unmanned airplane monitoring technologies was advanced to enhance the efficiency of wide-area surveys.

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.

Survey on the radioactive substance in the coastal areas near Fukushima Prefecture in FY2022

Misono, Toshiharu; Shiribiki, Takehiko*; Urabe, Yoshimi*; Sanada, Yukihisa

JAEA-Research 2025-014, 88 Pages, 2026/03

JAEA-Research-2025-014.pdf:7.52MB

An accident occurred at the TEPCO's Fukushima Daiichi Nuclear Power Station (1F) in 2011 and a large amount of radioactive materials were deposited around the 1F. Japan Atomic Energy Agency has continued to conduct research on the dynamics of radioactive materials after the accident. This report summarizes the results of the survey conducted in FY 2022 on the status of marine monitoring survey on radioactive substances. Furthermore, in order to evaluate the inflow of radioactive Cs from the river, the horizontal distribution of the radioactive Cs concentration on the surface sediment in front of the rivers was measured. As basic information on the effects of radioactive materials on marine products, the distribution status of fish was investigated. From these results, we estimated the distribution and its dynamics of radioactive Cs in the sediments in the front area on the 1F.

Effects of non-decontamination human activities on the reduction of ambient dose equivalent rates in residential areas near the Fukushima Daiichi Nuclear Power Plant

Kim, M.; Yoshimura, Kazuya; Sakuma, Kazuyuki; Malins, A.*; Abe, Tomohisa; Nakama, Shigeo; Machida, Masahiko; Saito, Kimiaki

Journal of Environmental Radioactivity, 294, p.107931_1 - 107931_8, 2026/03

https://doi.org/10.1016/j.jenvrad.2026.107931

This study quantitatively evaluated the effects of non-decontamination human activities, such as traffic, on ambient dose rates in residential areas near the Fukushima Daiichi Nuclear Power Plant through field measurements and simulations. Field surveys showed that areas with higher traffic had greater reductions in Cs deposition, suggesting that vehicular movement may enhance cesium weathering. Monte Carlo simulations using 3D-ADRES confirmed that human activities accelerate the decrease in ambient dose rates on paved surfaces, with reductions of up to approximately 50%. These results indicate that non-decontamination human activities significantly contribute to lowering ambient dose rates.