Sampling of radioactive materials remaining in JMTR Reactor Facility

Ouchi, Takuya; Nagata, Hiroshi; Shinoda, Yuya; Yoshida, Hayato; Inoue, Shuichi; Chinone, Marina; Abe, Kazuyuki; Ide, Hiroshi; Watahiki, Shunsuke

JAEA-Technology 2025-006, 25 Pages, 2025/10

JAEA-Technology-2025-006.pdf:1.59MB

In the future, radioactive waste which generated from nuclear research facilities in Japan Atomic Energy Agency are planning to be buried for the near surface disposal. It is necessary to establish the method to evaluate the radioactivity concentrations of the radioactive wastes. Therefore, at the Oarai Nuclear Engineering Institute, in order to contribute to the study of methods for evaluating radioactivity concentrations of the radioactive wastes from nuclear research facilities, samples were taken from radioactive waste that are expected to be buried in the future and radiochemical analysis is used to obtain data on the radioactivity concentration of each nuclide contained in the radioactive waste. This report presents the concept of selecting sample collection targets and summarizes the sampling of radioactive materials conducted at the JMTR reactor facility in fiscal years 2023 and 2024 to obtain data on radioactivity concentrations.

Speciation of cesium in a radiocesium-bearing microparticle emitted from Unit 1 during the Fukushima nuclear accident by XANES spectroscopy using transition edge sensor

Takahashi, Yoshio*; Miura, Hikaru*; Yamada, Shinya*; Sekizawa, Oki*; Nitta, Kiyofumi*; Hashimoto, Tadashi*; Yomogida, Takumi; Yamaguchi, Akiko; Okada, Shinji*; Itai, Takaaki*; et al.

Journal of Hazardous Materials, 495, p.139031_1 - 139031_19, 2025/09

https://doi.org/10.1016/j.jhazmat.2025.139031

In this presentation, we analyzed the chemical state of cesium in radiocesium-bearing microparticles (CsMPs) released during the 2011 Fukushima Daiichi Nuclear Power Plant accident using high-resolution X-ray absorption spectroscopy (XANES) and micro X-ray fluorescence (-XRF). The results identified two forms of cesium: one dissolved in glass and the other enriched on the surfaces of internal voids. The latter is considered to have originally existed as a gas and became concentrated during the cooling and solidification of the molten glass. These findings are crucial for understanding the formation process of CsMPs during the accident, as well as for future decommissioning and safety assessments.

3D visualization in complicated flow channel using deep learning-based bubble detection

Uesawa, Shinichiro; Ono, Ayako; Yoshida, Hiroyuki

Gazo Rabo, p.1 - 5, 2025/08

This paper introduces a new measurement technique for visualizing the three-dimensional distribution of bubbles in a complex channel such as a nuclear reactor fuel assembly. Bubbly flow is important in many engineering fields, and especially in nuclear engineering, where bubble behavior significantly affects the performance and safety of nuclear reactors, and thus requires detailed understanding. Conventional rule-based image recognition has difficulty identifying bubbles overlapping in the line-of-sight direction, but in this study, deep learning (Mask R-CNN and Swin Transformer) is used to achieve highly accurate bubble detection with a small amount of training data. Furthermore, the tracking technique using ByteTrack made it possible to track many bubbles with complex motions, and by combining images taken from different viewpoints using two high-speed cameras and reconstructing the 3D shape of the bubbles using the ellipsoid approximation, 3D instantaneous local information such as bubble position, diameter, and velocity was obtained. To eliminate the effects of refraction and obstruction of vision by structures in the channel, a simulated fuel rod was made of a transparent material (PFA tube) with a refractive index similar to that of water, enabling distortion-free imaging and measurement even in channels with complex structures. This enabled 3D visualization of bubble behavior in complex channels, which had been difficult to achieve in the past. Since this technology enables highly accurate 3D visualization with a small number of cameras and a small amount of learning, it is expected to be applied to objects other than bubbles.

Characteristics of droplet evaporation on high-temperature porous surfaces for estimating cooling time of fuel debris

Yuki, Kohei*; Horiguchi, Naoki; Yoshida, Hiroyuki; Yuki, Kazuhisa*

Mechanical Engineering Journal (Internet), 12(4), p.24-00451_1 - 24-00451_8, 2025/08

https://doi.org/10.1299/mej.24-00451

Fuel debris at the Fukushima Daiichi nuclear power station is typically cooled under immersion. However, an unexpected significant drop in water level results in coolant contact with high-temperature fuel debris having porous structure. In such scenarios, rapid cooling is essential, yet the thermal behavior at the liquid-solid interface, including capillary phenomena, is not well understood. This paper presents basic research evaluating the evaporation characteristics of droplets upon contact with metallic porous media featuring small pores under 1 mm. We conducted experiments using bronze or stainless steel porous media with pore diameters of 1, 40, or 100 m to derive lifetime curves for droplets. Our findings indicate that Leidenfrost effect is mitigated on porous surfaces as the vapor can escape through the pores. Moreover, in bronze porous media, as the temperature increases, oxide film with a fine structure facilitates capillary action. In contrast, the low wettability of stainless steel porous media prevents capillary action, inhibiting droplet absorption and dispersion into the pores. Consequently, rapid cooling via the capillary action is unlikely if the fuel debris shares similar characteristics with steel porous media. Therefore, for risk management, the cooling system should be established assuming that capillary force does not act in the fuel debris.

Numerical analysis of natural convective heat transfer with porous medium using JUPITER

Uesawa, Shinichiro; Yamashita, Susumu; Sano, Yoshihiko*; Yoshida, Hiroyuki

Journal of Nuclear Science and Technology, 62(6), p.523 - 541, 2025/06

https://doi.org/10.1080/00223131.2025.2451023

Japan Atomic Energy Agency (JAEA) has developed a numerical method with the JUPITER code with a porous medium model to calculate the thermal behavior in PCVs of 1F. In this study, we performed an experiment and numerical simulation of the natural convective heat transfer with the porous medium to validate JUPITER with the porous medium model. In comparison of the temperature and velocity distributions between the experiment and simulation, the temperature distribution in the simulation was in good agreement with the distribution in the experiment except the temperature near the top surface of the porous medium. The velocity distribution also agreed qualitatively with the experimental result. In addition, we also performed the numerical simulations with various effective thermal conductivity models to discuss the effect of the conductivity based on the internal structure of porous media on the natural convective heat transfer. The result indicated that the temperature distribution in the porous medium and the velocity distribution of the natural convection were significantly different for each model, and thus the conductivity of the fuel debris was one of the key parameters of in the thermal behavior analysis in 1F.

Neutron diffraction study of the crystal and magnetic structures of antiferromagnetic manganese deuteride at high temperatures and high pressures

Machida, Akihiko*; Saito, Hiroyuki*; Aoki, Katsutoshi*; Komatsu, Kazuki*; Hattori, Takanori; Sano, Asami; Funakoshi, Kenichi*; Machida, Shinichi*; Sato, Toyoto*; Orimo, Shinichi*

Physical Review B, 111(22), p.224413_1 - 224413_6, 2025/06

https://doi.org/10.1103/yf8k-cy61

The crystal and magnetic structures of antiferromagnetic Mn deuterides formed by hydrogenating Mn metal at high temperature and high pressure, fcc -MnDx and hcp -MnDx, were investigated by in-situ neutron powder diffraction. Deuterium atoms partially occupied the octahedral interstitial positions of the fcc and hcp metal lattices. The site occupancies increased rapidly with decreasing temperature from 700 to 450 K and remained down to 300 K. Nel temperature of 543(10) K was determined for -MnD. For -MnD, saturation magnetic moment and Nel temperature were determined to be 0.82(1) and 347(3) K, respectively. The Nel temperatures determined for -MnD and -MnD are consistent with those predicted by the respective Slater-Pauling curves proposed in previous studies. The updated Nel temperatures provide insights into the development of more accurate Slater-Pauling curves based on electronic band structure calculations.

Achievements and status of the STRAD project for radioactive liquid waste management

Arai, Yoichi; Watanabe, So; Nakahara, Masaumi; Funakoshi, Tomomasa; Hoshino, Takanori; Takahatake, Yoko; Sakamoto, Atsushi; Aihara, Haruka; Hasegawa, Kenta; Yoshida, Toshiki; et al.

Progress in Nuclear Science and Technology (Internet), 7, p.168 - 174, 2025/05

https://doi.org/10.15669/pnst.7.168

The Japan Atomic Energy Agency (JAEA) has been conducting a project named "Systematic Treatment of RAdioactive liquid waste for Decommissioning (STRAD)" project since 2018 for fundamental and practical studies for treating radioactive liquid wastes with complicated compositions. Fundamental studies have been conducted using genuine liquid wastes accumulated in a hot laboratory of the JAEA called the Chemical Processing Facility (CPF), and treatment procedures for all liquid wastes in CPF were successfully designed on the results obtained. As the next phase of the project, new fundamental and practical studies on primarily organic liquid wastes accumulated in different facilities of JAEA are in progress. This paper reviews the representative achievements of the STRAD project and introduces an overview of ongoing studies.

Background aerial monitoring and UAV radiation monitoring technology development for emergency response and preparedness in fiscal year 2023 (Contract research)

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

JAEA-Technology 2024-022, 170 Pages, 2025/03

JAEA-Technology-2024-022.pdf:15.09MB

On March 11, 2011, the 2011 off the Pacific coast of Tohoku Earthquake caused a tsunami that led to the Fukushima Daiichi Nuclear Power Station accident, releasing radioactive material into the environment. Since then, Aerial Radiation Monitoring (ARM) using manned helicopters has been employed to measure radiation distribution. As a commissioned project from the Nuclear Regulation Authority, the Japan Atomic Energy Agency (JAEA) utilizes this technology for emergency monitoring during nuclear facility accidents, aiming to provide prompt results by pre-arranging information on background radiation, topography, and control airspaces around nuclear power plants nationwide. In fiscal year 2023, the commissioned project included conducting ARM around the Sendai Nuclear Power Station and preparing related information. To enhance effectiveness during emergencies, ARM and the first domestic training flight of Unmanned Aerial Vehicles (UAVs) were conducted during the FY2023 Nuclear Energy Disaster Prevention Drill. Furthermore, UAVs radiation monitoring technology was advanced by selecting UAVs and investigating their performance. This report summarizes the results and technical issues identified providing insights to improve emergency preparedness.

Anisotropic magnetic field-temperature phase diagrams in CeCoSi

Hidaka, Hiroyuki*; Yanagiya, Shun*; Onodera, Shintaro*; Yanagisawa, Tatsuya*; Amitsuka, Hiroshi*; Tanida, Hiroshi*; Ishitobi, Takayuki

Journal of the Physical Society of Japan, 94(3), p.034706_1 - 034706_9, 2025/03

https://doi.org/10.7566/JPSJ.94.034706

3D visualization in rod bundle flow channel using deep learning-based bubble detection

Uesawa, Shinichiro; Ono, Ayako; Yoshida, Hiroyuki

Konsoryu, 39(1), p.61 - 71, 2025/03

https://doi.org/10.3811/jjmf.2025.004

Bubble visualization using a high-speed video-camera has been used as a measurement technique of bubble diameters and velocities. However, the bubble detection was difficult under the condition of the high void fraction because the overlapping bubbles for the sight direction of the camera increased with the void fraction. Additionally, the visualization for a system with objects, such as rod bundle flow channels, becomes more difficult. In this study, we applied a deep learning-based bubble detection technique with Shifted Window Transformer to bubble images shoot from two directions to identify the bubble size, three-dimensional (3D) positions of bubbles, 3D bubble trajectories in the rod bundle flow channel. Furthermore, we used perfluoroalkoxy alkane tubes with almost the same reflection as water in the channel to visualize the bubbly flow in the whole of the flow channel. We confirmed that the detection technique can segment individual bubbles in overlapping bubbles and bubbles behind the rod. By using the detection results, we estimated the diameter and velocity of each bubble and cross-sectional void fraction.