Unveiling the effects of Mn, Cr, Al, and Si on the low-temperature tempering behaviors of high-carbon martensite

Zhang, Y.*; Marusawa, Kenji*; Kudo, Kohei*; Morooka, Satoshi; Gong, W.; Harjo, S.; Miyamoto, Goro*; Furuhara, Tadashi*

Journal of Materials Science & Technology, 275, p.250 - 259, 2026/12

https://doi.org/10.1016/j.jmst.2026.04.015

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.

Experimental evaluation of high-temperature fuel-coolant thermal interaction behavior using unirradiated fuel powder

Mihara, Takeshi; Urano, Kenta; Udagawa, Yutaka; Kakiuchi, Kazuo

JAEA-Technology 2026-009, 15 Pages, 2026/06

JAEA-Technology-2026-009.pdf:1.15MB

Mechanical energy generated during fuel failure under reactivity-initiated accident (RIA) conditions, such as pressure pulse and water hammer, strongly depends on the fragmentation state and temperature of the fuel. When failure caused by pellet/cladding mechanical interaction (PCMI) occurs rapidly at low temperature, fission gas release drives pellet fragments to move at high velocity in water, leading to extremely efficient heat transfer between the fuel and coolant. This results in rapid vapor generation and the production of impulsive mechanical energy. These observations indicate that both the particle surface area and the highly efficient heat transfer associated with high-velocity fragment motion are key influencing factors. In the 264-2 and 264-24 experiments, test conditions were designed to simulate fuel-coolant interaction under conditions where the driving force for pellet fragment motion, which is characteristic of RIA events, is absent. In the 264-24 test, the specific surface area of the pellet particles (surface area per unit mass) was designed to exceed that of previously tested high-burnup fuels. In addition, the fuel enthalpy (thermal energy per unit mass) was set based on prior observations to conditions where significant mechanical energy generation is expected. As a result, both the pressure pulse and water hammer energies were significantly lower than those observed in high-burnup fuel failure cases, where the driving force for pellet fragment motion is considered to be present. This clearly demonstrates the critical importance of the driving force for pellet fragment motion in the generation of mechanical energy.

Annual report of Engineering Services Department on JFY2024

Engineering Services Department, Nuclear Science Research Institute

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

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.

Neutronic characteristics of a partially damaged reactor model with varying numbers of damaged fuel assemblies

Nguyen, H. H.

Annals of Nuclear Energy, 230, p.112171_1 - 112171_13, 2026/06

https://doi.org/10.1016/j.anucene.2026.112171

This study examined the effects of the moderator-to-fuel volume ratio, fuel debris shape, and the number of damaged fuel assemblies on the neutronic characteristics of a partially damaged reactor model, where the fuel assemblies at the core center melt to fuel debris while the fuel assemblies at the outer region remain intact. The investigations were conducted using the Serpent code and the JENDL-5 library. The results show that when fuel debris is surrounded by intact fuel assemblies, the k can be classified into two groups based on the shape of the fuel debris. Conversely, in scenarios where the fuel debris is not fully encircled by intact fuel assemblies, the shape of the fuel debris has a negligible impact on the k. Additionally, the relationship between the number of neutrons entering and leaving the fuel debris determines how the shape of the fuel debris affects the k.

Development of phenomenological degradation models for Cr-Coated Zr alloy cladding under high-temperature oxidation conditions

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

Annals of Nuclear Energy, 231, p.112177_1 - 112177_16, 2026/06

https://doi.org/10.1016/j.anucene.2026.112177

Parameter estimation of river incision models of soft sedimentary rocks; A Case study on the Kamikita Coastal Plain, northeast Japan

Takai, Shizuka; Sanga, Tomoji*; Shimada, Taro; Takeda, Seiji

Earth Surface Dynamics, 14(3), p.417 - 432, 2026/06

https://doi.org/10.5194/esurf-14-417-2026

Understanding river incision model is crucial for predicting long-term landscape evolution. For the bedrock channel incision model (detachment-limited (DL) model: erosion rate where is drainage area, is channel gradient), parameters (, , and ), can be estimated via slope-area analysis if is known. Using Be denudation rate, previous studies globally compiled the parameter values for variable lithology. However, limited data availability for soft sedimentary rock restricts the applicability of global compilation. In addition, measuring the Be concentration in sedimentary rock is challenging in humid and tectonically active regions. To address this, slope-area analysis was conducted in the Kamikita Coastal Plain, Japan, where lithology (Miocene to Pleistocene sedimentary rocks) and uplift rate ( 0.2 mm y for the past 300 ka) are assumed to be uniform. River incision rates were derived approximately from widely distributed marine terraces (MIS 5e-11). For six target rivers, DL-like behaviour was confirmed in the limited areas located upstream of the alluvium distribution. The reference concavity was 0.44 0.10, typical for steady-state channels. Across the range of 0.4-0.6, the exponent consistently exhibited nonlinearity ranging between 1.14 to 1.34, which is consistent with the previous global compilations. This observed nonlinearity likely reflects transient landscape responses to past sea-level changes, which generated slope-break knickpoints at similar elevations. Finally, the estimated erosion coefficient (10-10) agreed with the global relationship with unconfined compressive strength (), supporting the significant influences of bedrock lithology on .

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.

Verification of countermeasures to prevent spalling of shaft walls during deep shaft excavation; A Case study at the Horonobe Underground Research Center, Japan

Nago, Makito*; Tsusaka, Kimikazu*; Aoyagi, Kazuhei; Sugawara, Kentaro*; Kodama, Junichi*

Journal of MMIJ, 142, p.46 - 57, 2026/06

https://doi.org/10.2473/journalofmmij.MMIJ-2025-034

This study examines the effects of rock spalling on support elements in order to prevent cracking and to ensure the integrity of the support system and safety during shaft sinking. When the ventilation shaft at the Horonobe Underground Research Center reached a depth of more than 250 m, severe rock spalling occurred, and cracks developed in the concrete lining immediately above the spalling zone. Therefore, a three-dimensional numerical analysis of the shaft was conducted to estimate changes in the stress distribution within the concrete lining caused by the spalling. The simulation results indicated that the vertical tensile stress in the concrete lining increased as the spalling progressed. By integrating the analytical results with field observations and considering various support patterns to prevent further rock spalling, a flowchart for selecting the optimal support pattern was developed. Shaft sinking was subsequently completed to a depth of 500 m without significant damage to the concrete lining or excessive spalling. The flowchart developed in this study will contribute to the selection of optimal support patterns for future shaft sinking projects.