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.

Annual report of Engineering Services Department on JFY2024

Engineering Services Department, Nuclear Science Research Institute

JAEA-Review 2026-011, 97 Pages, 2026/06

JAEA-Review-2026-011.pdf:4.39MB

The Engineering Services Department is in charge of operation and maintenance of utility facilities (water distribution systems, electricity supply systems, steam generation systems and drain water systems etc.) in whole of the institute. Furthermore, it also oversees the operation and maintenance of specific systems (power receiving and transforming facilities, an emergency electric power supply system, an air/liquid waste treatment system, a compressed air supply system) in nuclear reactor facilities, nuclear fuel material usage facilities and usual facilities or buildings. In addition, the department is in charge of maintenance of buildings, design and repair of electrical/mechanical equipment. This annual report describes summary of activities, operation and maintenance data and technical developments of the department carried out in JFY 2024. We hope that this report may help to future work.

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.

No detectable impact of ALPS-treated water discharge on tritium levels in terrestrial waters of the upper Ota River catchment, Fukushima, Japan

Sakuma, Kazuyuki; Yoshimura, Kazuya

Journal of Environmental Radioactivity, 297, p.108055_1 - 108055_4, 2026/06

https://doi.org/10.1016/j.jenvrad.2026.108055

Tritium (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.

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.

Implementation of a new function for handling nuclear data of outgoing particles and residual excitation states in PHITS

Furuta, Takuya; Hashimoto, Shintaro; Ogawa, Tatsuhiko; Tanimura, Yoshihiko

Nuclear Instruments and Methods in Physics Research A, 1086, p.171320_1 - 171320_8, 2026/06

https://doi.org/10.1016/j.nima.2026.171320

A new function to incorporate nuclear data libraries with outgoing particles plus residual nuclei in specific excitation states for neutron-induced reactions has been implemented in a Monte Carlo simulation code, Particle and Heavy Ion Transport code System (PHITS). With this function, accurate predictions of outgoing particle spectra and angular distributions according to the nuclear data libraries become possible, while accounting for production of residual nuclei and de-excitation gammas, conserving total energy and momentum in each event. This feature allows users to perform high-precision simulations of detector responses and radiation damage in materials.

Assessment report of research and development activities in FY 2025 activity; "Nuclear Science and Engineering Research" (Interim evaluation)

Nuclear Science and Engineering Center; NXR Development Center

JAEA-Evaluation 2026-001, 44 Pages, 2026/05

JAEA-Evaluation-2026-001.pdf:0.93MB
JAEA-Evaluation-2026-001-appendix(CD-ROM).zip:18.67MB

Japan Atomic Energy Agency (hereinafter referred to as "JAEA") consults an assessment committee, "Evaluation Committee of Research Activities for Nuclear Science and Engineering" (hereinafter referred to as "Committee") for interim evaluation of "Nuclear Science and Engineering", in accordance with "General Guideline for the Evaluation of Government Research and Development (R&D) Activities" by Cabinet Office, Government of Japan, "Guideline for Evaluation of R&D in Ministry of Education, Culture, Sports, Science and Technology" and "Regulation on Conduct for Evaluation of R&D Activities" by the JAEA. In response to the JAEA's request, the Committee evaluated research activities based on the materials prepared by JAEA. This report summarizes the results of the assessment by the Committee with the Committee report attached.

None

Ito, Rintaro

Enerugi Rebyu, 46(6), p.9 - 12, 2026/05

no abstracts in English

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