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.

Analysis of fracture conditions of Cr-coated Zr alloy claddings under LOCA conditions calculated using FEMAXI fuel performance code

Luu, V. N.; Taniguchi, Yoshinori; Udagawa, Yutaka; Tasaki, Yudai; Katsuyama, Jinya

Annals of Nuclear Energy, 230, p.112114_1 - 112114_14, 2026/06

https://doi.org/10.1016/j.anucene.2026.112114

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.

Hydrogen in the Earth core inferred from neutron imaging and diffraction

Takahashi, Naoki*; Sakamaki, Tatsuya*; Hattori, Takanori; Funakoshi, Kenichi*; Arima-Osonoi, Hiroshi*; Sano, Asami; Abe, Jun*; Suzuki, Akio*

Scientific Reports (Internet), 16, p.14162_1 - 14162_13, 2026/05

https://doi.org/10.1038/s41598-026-49969-z

We performed high-pressure and high-temperature neutron diffraction and imaging experiments in situ to determine the hydrogen content in liquid iron. We observed that liquid iron contains 0.17(3) wt.% H at 3.4 GPa and 1400 K, indicating that liquid iron is hydrogenated in the magma ocean during core formation. For the hydrogen content in the liquid iron at the base of the magma ocean, we estimated that the outer and inner cores contain 0.60-0.72 and 0.30-0.44 wt.% H, corresponding to 70-85 and 1.9-2.7 times the mass of hydrogen in the ocean, respectively. This suggests that hydrogen can contribute more than half of the density deficit in the outer core. For the magma ocean equilibrating with the hydrogen-rich primary atmosphere, the study findings show that liquid iron plays a crucial role in transporting a large amount of hydrogen into the core.

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

Numerical analysis of A and A value ratios for off-site transportation of small-amount of fuel debris retrieved from the Fukushima Daiichi Nuclear Power Station

Sakamoto, Masahiro; Okumura, Keisuke; Kanno, Ikuo; Matsumura, Taichi; Riyana, E. S.; Terashima, Kenichi; Kaneko, Junichi*; Mizokami, Masato*; Mizokami, Shinya*

Radioisotopes, 75(S-01), p.S-001_1 - S-001_5, 2026/04

https://doi.org/10.3769/radioisotopes.S-001

Suppression of electrolytic corrosion at accelerator cooling-water facility by changing of rust inhibitors; Reduction of rust and cost

Dei, Tatsumi; Suganuma, Kazuaki; Fujirai, Kosuke; Suzuki, Katsuo; Suzuki, Hiroshi*; Nakata, Morihiro*; Hosokawa, Hideaki*; Onose, Yuichiro*; Watanabe, Yasuhiro; Shinozaki, Shinichi; et al.

JAEA-Technology 2026-003, 27 Pages, 2026/03

JAEA-Technology-2026-003.pdf:1.3MB

Accelerator cooling-water facility cannot prevent electrolytic corrosion because the systems consist of several different kinds of metals. We have confirmed that the electrolytic corrosion certainly occurs between oxygen-free copper and carbon steel, and cannot be suppressed by the rust inhibitor used so far. We have chosen a new rust inhibitor and confirmed that the new rust inhibitor can prevent the corrosion progression of carbon steel as well as oxygen-free copper. Moreover, using the new rust inhibitor brings about saving water and the cost reduction of the inhibitors.

Background aerial monitoring and UAV radiation monitoring technology development for emergency response and preparedness in the fiscal year 2024 (Contract research)

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

JAEA-Technology 2025-015, 171 Pages, 2026/03

JAEA-Technology-2025-015.pdf:11.43MB

On March 11, 2011, the 2011 off the Pacific coast of Tohoku Earthquake and tsunami caused the Fukushima Daiichi Nuclear Power Station accident, releasing radioactive material. Since then, Aerial Radiation Monitoring (ARM) with manned helicopters has been used to assess radiation distribution quickly. In FY2024, the Japan Atomic Energy Agency (JAEA), under commission from the Nuclear Regulation Authority, conducted ARM around the Shimane Nuclear Power Station, producing background dose rate maps validated against ground and other data. During a nuclear emergency drill, UAV training flights complemented manned monitoring, confirming the effectiveness of real-time communication and rapid mapping. The UAV data system was developed and demonstrated for real-time analysis and multi-platform use. Skill training for multicopters was also conducted to strengthen operational capability. Additionally, joint monitoring with the U.S., France, South Korea, and Canada provided insights into international technologies and practices, emphasizing the value of information sharing. This report summarizes the results and technical challenges from these FY2024 activities, contributing to the advancement of emergency radiation monitoring.

Annual report of Nuclear Science Research Institute, JFY 2024

Nuclear Science Research Institute

JAEA-Review 2025-061, 183 Pages, 2026/03

JAEA-Review-2025-061.pdf:4.01MB

Nuclear Science Research Institute (NSRI) was composed of Planning and Management Department and six departments, namely Department of Operational Safety Administration, Department of Radiation Protection, Engineering Services Department, Department of Research Reactor and Tandem Accelerator, Department of Criticality and Hot Examination Technology, and Department of Decommissioning and Waste Management, and each department manages facilities and develops related technologies to achieve the "Medium- to Long-term Plan" successfully and effectively. On November 1, NSRI unified Department of Research Reactor and Tandem Accelerator, and Department of Criticality and Hot Examination Technology, newly organized Department of Research Infrastructure Technology Development. And, Planning and Management Department was reorganized to Promotion Office. Continuously, four research centers which are Advanced Science Research Center, Nuclear Science and Engineering Center, Nuclear Engineering Research Collaboration Center and Materials Sciences Research Center, belong to NSRI. In order to contribute to future research and development, and to promote management business, this annual report summarizes information on the activities of NSRI of JFY 2024 as well as the activity on research and development carried out by Collaborative Laboratories for Advanced Decommissioning Science, Nuclear Safety Research Center and activities of Nuclear Human Resource Development Center, using facilities of NSRI.

Development of a bioassay method for internal radiation dose assessment FY 2021 (Joint research)

Nagaoka, Mika; Maehara, Yushi; Ono, Masako*; Nihei, Hidekazu*; Hirao, Moe; Fujita, Hiroki

JAEA-Research 2026-001, 115 Pages, 2026/03

JAEA-Research-2026-001.pdf:6.32MB

In fiscal year 2021, Japan Atomic Energy Agency conducted a joint research with Tokyo Electric Power Company Holdings, Inc. to develop a bioassay method. This report presents the results of studies on a systematic analysis method for -ray and pure -ray nuclides in urine samples. Specifically, experiments with tracer-containing samples were conducted to verify the nuclide separation performance of the systematic analysis method utilizing multiple solid-phase extraction resins and the sample preparation method for radioactivity measurement. Furthermore, calculation methods for uncertainty and detection limit of radioactivity in -ray nuclide analysis were summarized.