Microstructural evolution and mechanical hardening of Cr-coated MDA cladding under high-dose Fe ion irradiation

Mohamad, A. B.; Chen, J.*; Ioka, Ikuo*; Suzuki, Eriko; Kondo, Keietsu; Abe, Yosuke; Yamashita, Shinichiro; Okubo, Nariaki; Nemoto, Yoshiyuki; Okada, Yuji*; et al.

Journal of Nuclear Materials, 625, p.156513_1 - 156513_9, 2026/04

https://doi.org/10.1016/j.jnucmat.2026.156513

Demonstration of the inherent safety feature of HTGRs through the loss-of-forced-cooling test in the HTTR

Nagasumi, Satoru; Hasegawa, Toshinari; Iigaki, Kazuhiko; Nakagawa, Shigeaki; Kubo, Shinji; Shimazaki, Yosuke; Nakajima, Kunihiro; Sakurai, Yosuke; Shinohara, Masanori; Saito, Kenji; et al.

Nuclear Engineering and Design, 446(Part A), p.114542_1 - 114542_14, 2026/01

https://doi.org/10.1016/j.nucengdes.2025.114542

To demonstrate HTGR's safety features, a loss-of-forced-cooling (LOFC) test was conducted using the HTTR. In this test, the forced cooling in the reactor core was intentionally lost by shutting down all helium gas circulators (HGCs) without reactor scram. During steady-state operation at 100% reactor power (30 MW), after the LOFC, the reactor power spontaneously decreased. This power reduction occurred due to the negative reactivity feedback effect triggered by an increase in core temperature. The power stabilized at a lower value of 1.2% after re-criticality. Additionally, the measured radioactivity concentration in the primary coolant remained nearly unchanged during this LOFC operation and during an immediately subsequent HTTR operation. This indicates no failure of the coated particle fuel, even after the increase in core temperature associated with the LOFC event. These results provide experimental evidence of the safety features of HTGRs.

Reactor response during thermal load fluctuation test using HTTR

Hasegawa, Toshinari; Nagasumi, Satoru; Kubo, Shinji; Iigaki, Kazuhiko; Shinohara, Masanori; Nakagawa, Shigeaki; Shimazaki, Yosuke; Nakajima, Kunihiro; Sakurai, Yosuke

Proceedings of 2025 International Congress on Advances in Nuclear Power Plants (ICAPP 2025) (Internet), 6 Pages, 2025/09

JAEA has planned a hydrogen production test using the High-Temperature Engineering Test Reactor (HTTR) to demonstrate hydrogen production utilizing the heat from a high-temperature gas-cooled reactor (HTGR). To realize the coupling of a hydrogen production facility with an HTGR, one of the key issues is to confirm the effect of thermal load fluctuations in the facility on the reactor. In this study, a thermal load fluctuation test was conducted during HTTR operation to investigate the reactor's response. The test was performed at 90% reactor power, during which the reactor inlet coolant temperature was increased by 11C to simulate a thermal load fluctuation. As a result, the reactor outlet coolant temperature remained almost unchanged, and the heat corresponding to the inlet temperature increase was absorbed by the core graphite blocks. Furthermore, due to the negative reactivity feedback effect associated with the rise in graphite block temperature, the reactor power decreased to 88% and stabilized without any control rod operation. These findings indicate that disturbances in the reactor inlet coolant temperature are mitigated by the heat storage capacity of the core graphite blocks.

Nondestructive 3D elemental imaging of Edo's archaeological artifacts via muonic X-ray measurements

Chiu, I.-H.; Osawa, Takahito; Ninomiya, Kazuhiko*; Takeda, Shinichiro*; Takahashi, Tadayuki*; Katsuragawa, Miho*; Watanabe, Shin*; Kubo, Kenya*; Saito, Tsutomu*; Mizumoto, Kazumi*; et al.

npj Heritage Science (Internet), 13, p.154_1 - 154_9, 2025/05

https://doi.org/10.1038/s40494-025-01741-8

Federated learning of creep rupture time and high temperature tensile strength prediction models

Sakurai, Junya*; Torigata, Keisuke*; Matsunaga, Manabu*; Takanashi, Naoto*; Hibino, Shinya*; Kizu, Kenichi*; Morita, Akira*; Inomoto, Masahiro*; Shimohata, Nobuaki*; Toyota, Kodai; et al.

Tetsu To Hagane, 111(5), p.246 - 262, 2025/04

https://doi.org/10.2355/tetsutohagane.TETSU-2024-124

Effects of Al addition on Vickers hardness increase by thermal aging of Fe-Cr-Al alloys; Evaluation by systematic experiments, machine learning modeling, and first-principles calculations

Abe, Yosuke; Tsuru, Tomohito; Fujita, Yohei*; Otomo, Masahide*; Sasaki, Taisuke*; Yamashita, Shinichiro; Okubo, Nariaki; Ukai, Shigeharu

Journal of Nuclear Materials, 606, p.155606_1 - 155606_12, 2025/02

https://doi.org/10.1016/j.jnucmat.2025.155606

We investigated the effect of Al addition on the formation of phase in Fe-Cr-Al model alloys by thermal aging. The Vickers hardness tests and a machine learning model indicate that the formation of the phase is promoted by low Al additions and suppressed by high Al additions. First-principles calculations, which indicate that Cr-Al-vacancy pairs are more stable than Cr-Cr pairs and that including Al atoms during phase nucleation may be energetically advantageous. On the other hand, the formation of Al-Al pairs was very unstable. The formation of Al-Al pairs near the interface can be avoided when the amount of Al addition is small. However, it is inevitable when the amount of Al addition is significant, leading to the instability of the phase.

Status of numerical tool development for the initiating phase in severe accidents of SFR in Japan

Fukano, Yoshitaka; Ishida, Shinya; Kubo, Shigenobu

IAEA-TECDOC-2079, p.163 - 175, 2025/01

https://doi.org/10.61092/iaea.xpz1-gu7q

In-situ detection of high-energy beta ray emitter Sr/Y inside the Fukushima Daiichi Nuclear Power Station Unit 3 reactor building using a liquid light guide Cherenkov counter

Terasaka, Yuta; Sato, Yuki; Furuta, Yoshihiro*; Kubo, Shin*; Ichiba, Yuta*

Nuclear Instruments and Methods in Physics Research A, 1070, Part 2 , p.170021_1 - 170021_9, 2025/01

https://doi.org/10.1016/j.nima.2024.170021

Systematic experimental and model-based evaluation of the synergistic effects of alloy composition and damage rate on the formation of Cr-rich precipitates in Fe-Cr-Al alloys under ion irradiation

Abe, Yosuke; Sasaki, Taisuke*; Yamashita, Shinichiro; Okubo, Nariaki; Ukai, Shigeharu

Journal of Nuclear Materials, 600, p.155271_1 - 155271_12, 2024/11

https://doi.org/10.1016/j.jnucmat.2024.155271

To investigate the formation behavior of Cr-rich precipitates (CrRP) in Fe-Cr-Al (ODS) alloys being developed as accident tolerant fuel cladding for light water reactors, 14 Fe-Cr-Al alloys with systematically varied Cr and Al compositions were irradiated with 10.5 MeV Fe at C at three damage levels. A three-dimensional atom probe analysis showed that the CrRP number density, volume fraction, and Cr concentration increase with increasing Cr composition, decreasing Al composition, and decreasing dose rate. The result of the multiple regression analysis on CrRP volume fractions indicates that in addition to the primary effects of these variables, there are several important interactions. It was also highlighted that to understand the dose rate effect on the CrRP formation behavior under neutron irradiation, it is useful to examine the irradiation time dependence, including the effective use of thermal aging data as a limit to the zero dose rate.

France-Japan collaboration on severe accident studies in sodium-cooled fast reactors, 4; Development of the SIMMER-V code with new physical models

Tagami, Hirotaka; Ishida, Shinya; Okano, Yasushi; Yamano, Hidemasa; Kubo, Shigenobu; Tobita, Yoshiharu*; Trotignon, L.*; Gubernatis, P.*; Dufour, E.*; Saas, L.*; et al.

Proceedings of International Conference on Nuclear Fuel Cycle (GLOBAL2024) (Internet), 4 Pages, 2024/10

JAEA has been developing the SIMMER-V code for severe accident simulations of future sodium-cooled fast reactors, including those with a unique, large-scale heterogeneous core. This paper describes the development framework of SIMMER-V in collaboration with CEA, representative new elements and an example of reactor test calculation.