CIGMA experiments on integral phenomena related to thermal hydraulics in a reactor containment vessel and building during a severe accident

Abe, Satoshi; Hamdani, A.; Soma, Shu; Hangai, Ryosuke; Omori, Masashi; Owada, Akihiko; Omiya, Toshihito*; Shibamoto, Yasuteru

Nuclear Engineering and Design, 449, p.114787_1 - 114787_13, 2026/04

https://doi.org/10.1016/j.nucengdes.2026.114787

Temperature effect on radiolytically generated hydrogen yield from a plutonium nitric acid aqueous solution

Toigawa, Tomohiro; Hotoku, Shinobu; Kumagai, Yuta; Abe, Yuma*; Oyama, Kanichi*; Fukaya, Hiroyuki; Ban, Yasutoshi; Kida, Takashi; Hasegawa, Satoshi*; Nakano, Masanao*; et al.

Journal of Nuclear Science and Technology, 63(3), p.322 - 327, 2026/03

https://doi.org/10.1080/00223131.2025.2541067

The effect of temperature on hydrogen production generated from radiolysis was investigated to determine the associated implications for nuclear fuel reprocessing safety. The hydrogen yield from radiolysis of plutonium nitric acid solution was measured at temperatures up to the boiling temperature of the solution. The results showed no notable temperature dependence even under boiling conditions. The impact of solution agitation on hydrogen production was also assessed, which revealed minor differences in the hydrogen yield between static and agitated conditions at room temperature. These findings suggest that high temperatures or boiling the solution do not considerably enhance hydrogen generation, and provide crucial information for accurately modeling hydrogen risks under severe accidents.

Differences between radiation dose values under wearing and non-wearing geometries of personal dosimeters during sleep

Hirouchi, Jun; Takahara, Shogo; Watanabe, Masatoshi*

Health Physics, 130(1), p.87 - 93, 2026/01

https://doi.org/10.1097/HP.0000000000002001

With the return of residents after the Fukushima Daiichi Nuclear Power Station accident, some residents' radiation doses have been measured to ensure their safety. However, personal radiation doses vary depending on the behavior and residential environment of each subject even for a group of subjects living in the same region at the same time. In past studies, subjects were required to wear a personal dosimeter on their chest outdoors, while they were not required to wear the device indoors. This study investigated the difference between the dose values indicated for the wearing and nonwearing conditions of personal dosimeters during sleep. In particular, an adult human phantom was used to compare the indicated dose values when personal dosimeters were placed on the chest (reproducing the wearing condition) and near the head (reproducing the nonwearing condition) in two houses with a high radiation dose. Furthermore, to understand the reason for the difference in the indicated dose values, the radiation dose during sleep was calculated using a radiation transport calculation code. The dose values for the wearing condition were approximately 4% lower on the first floor and approximately 15% lower on the second floor than those for the nonwearing condition. In addition, the radiation doses and radiation dose ratios (head/chest) differed by approximately 30% and 20%, respectively, depending on the distance from the nearest window (1-3 m).

Frustrated random-singlet state with ice-type structural fluctuation in spinel titanates

Hanasaki, Noriaki*; Hattori, Takayuki*; Komoda, Takumi*; Minamoto, Kakuei*; Torigoe, Shuhei*; Yamashita, Satoshi*; Nakazawa, Yasuhiro*; Nakano, Takehito*; Yoshimi, Kohei*; Yashima, Mitsuharu*; et al.

Proceedings of the National Academy of Sciences of the United States of America, 123(1), p.e2517926123_1 - e2517926123_6, 2026/01

https://doi.org/10.1073/pnas.2517926123

Experimental analysis of non-condensable gas and steam distribution due to condensation in the CIGMA facility simulating the reactor building

Hamdani, A.; Soma, Shu; Abe, Satoshi; Shibamoto, Yasuteru

Nuclear Engineering and Technology, 9 Pages, 2026/00

https://doi.org/10.1016/j.net.2026.104219

Research plan on geosphere stability for long-term isolation of radioactive waste (Scientific program for fiscal year 2025)

Asamori, Koichi; Sueoka, Shigeru; Komatsu, Tetsuya; Ogata, Manabu; Uchida, Mao; Nishiyama, Nariaki; Tanaka, Kiriha; Kobayashi, Tomoharu; Mitsuguchi, Takehiro; Murakami, Osamu; et al.

JAEA-Review 2025-035, 29 Pages, 2025/10

JAEA-Review-2025-035.pdf:1.12MB

This report is a plan of research and development (R&D) on geosphere stability for long-term isolation of high-level radioactive waste (HLW) in Japan Atomic Energy Agency (JAEA), in fiscal year 2025. The objectives and contents of this research are described in detail based on the JAEA 4th Medium- and Long-term Plan (fiscal years 2022-2028). In addition, the background of this research is described from the necessity and the significance for site investigation and safety assessment, and the past progress. The plan framework is structured into the following categories: (1) Development and systematization of investigation techniques, (2) Development of models for long-term estimation and effective assessment, (3) Development of dating techniques.

On turbulence production in a jet passing through a grid-type obstacle; An LES approach

Abe, Satoshi; Hirose, Yoshiyasu; Shibamoto, Yasuteru

Results in Engineering (Internet), 27, p.106307_1 - 106307_16, 2025/09

https://doi.org/10.1016/j.rineng.2025.106307

The interaction of turbulent jets plays a critical role in heat and mass transfer across a variety of engineering applications. In particular, the merging phenomena of jets in nuclear reactors are important for validating the credibility of the core exit temperature (CET) as an indicator of core heat-up. CET serves as a key criterion for accident management and ensuring the safe operation of the reactor. Understanding the dynamics of these jet interactions is essential for improving reactor safety and efficiency. Furthermore, in the field of containment thermal hydraulics during severe accidents, the behavior of a jet interacting with a grid-type obstacle has attracted significant attention. This study utilized Large Eddy Simulation (LES) to gain a deeper understanding of the flow characteristics of a jet affected by such an obstacle, with particular focus on the turbulence production mechanism. To improve the accuracy of turbulence modeling, the Coherent Structure Smagorinsky Model (CSM) was employed. Validation against experimental data confirmed that LES is a valuable tool for investigating jet flow behavior. The vortex structure of the jet was visualized using the Q-criterion. Additionally, the impact of the grid-type obstacle on the turbulence kinetic energy production rate was analyzed through radial profiles at various locations and its cumulative production.

Validating terrestrial SER in 12-, 28-, and 65-nm SRAMs estimated by simulation coupled with one-time neutron irradiation

Takami, Kazusa*; Gomi, Yuibi*; Yasuda, Ryuichi*; Abe, Shinichiro; Ito, Masatoshi*; Kanda, Hiroki*; Fukuda, Mitsuhiro*; Hashimoto, Masanori*

IEEE Transactions on Nuclear Science, 72(8), p.2622 - 2628, 2025/08

https://doi.org/10.1109/TNS.2025.3534564

Neutron-induced soft errors in the terrestrial environment pose reliability issues for semiconductor devices. We have developed the new method for estimating terrestrial SER based on simulation coupled with one-time irradiation using a conventional neutron source. This method has been validated using 65-nm planar SRAMs. However, with the ongoing progression of process shrinkage and the increasing adoption of devices fabricated with newer processes, there is a growing demand for further experimental validation of the terrestrial SER estimation method for these advanced processes. In this work, we validated the estimation method for 12-nm 1-fin FinFETs and 28-nm planer SRAMs. The SERs estimated by our method were consistent with the SERs measured using a white neutron beam at RCNP within 28% error.

Seismic fragility evaluation of a thin-walled vessel in a sodium-cooled fast reactor

Watakabe, Tomoyoshi; Yamano, Hidemasa; Futagami, Satoshi

Transactions of the 28th International Conference on Structural Mechanics in Reactor Technology (SMiRT28) (Internet), 10 Pages, 2025/08

The fragility evaluation method of the RV in the SFR has been developed for both failure mode buckling and fatigue. The past studies showed the fragility using the RV of a loop-type SFR: radius R / thickness t is about 100. Recently, Japan Atomic Energy Agency together with Japanese industrial partners has been conducting a pool-type SFR design. The fragility of the pool-type RV has never been evaluated by the proposed method, which is based on the fatigue failure mode instead of the buckling. To investigate the basic behaviour of the pool-type RV under excessive seismic loading, this report investigated the fragility by using a simple cylindrical model with a wall thickness close to the design of the pool-type RV: R/t is 200, as the first trial.

Effects of inert gas addition on the characteristics of spherically expanding hydrogen-methane-air premixed flames in closed combustion vessels

Katsumi, Toshiyuki*; Zemba, Atsushi*; Kogishi, Makoto*; Iwanaga, Kohei*; Abe, Satoshi; Divart, P.*; Chaumeix, N.*; Kadowaki, Satoshi*

Journal of Thermal Science and Technology (Internet), 20(1), p.25-00103_1 - 25-00103_13, 2025/06

https://doi.org/10.1299/jtst.25-00103

In the serious accident at Fukushima Daiichi Nuclear Power Station, the presence of steam together with flammable organic compounds affected the hydrogen explosion. To investigate the effects of addition of inert gas, i.e. steam or nitrogen, on the explosion characteristics, we conducted the experiments of spherically expanding hydrogen-methane-air premixed flames in closed combustion vessels. Two types of vessels were used, and expanding flames were caught by Schlieren method. The flame propagation velocity depending on the flame radius was obtained by analyzing Schlieren images. When the flame radius was sufficiently small, smooth surface was found. The addition of inert gas to hydrogen-methane-air premixtures caused the decrease of propagation velocity of unstretched flame. When the flame radius was large, on the other hand, cellular surface generated by intrinsic instability was found. In this range, the flame acceleration was confirmed, which was induced by the evolution of cellular surface. We obtained the parameters of flame acceleration model and predicted the flame propagation velocity depending on the flame radius. The increment coefficient normalized by the propagation velocity of unstretched flame became larger at low equivalence ratios, which was due to stronger diffusive-thermal instability. Under the same equivalence ratio, the inert gas addition caused the increase of normalized increment coefficient. This denoted that the inert gas addition promoted the instability of premixed flames, which was due to the reduction of the effective Lewis number. The maximum pressure in a combustion vessel became lower in the case of inert gas addition. Moreover, the maximum pressure of experiments was lower than that of calculations under the adiabatic conditions, because of heat loss during premixed combustion. The obtained results were valuable knowledge to elucidate the hydrogen explosion at Fukushima Daiichi Nuclear Power Station.