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, 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.

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.

Achievement of safety demonstration tests using HTTR; Loss of forced cooling test at 100% reactor power (30 MW)

Nagasumi, Satoru; Hasegawa, Toshinari; Nakagawa, Shigeaki; Kubo, Shinji; Iigaki, Kazuhiko; Shinohara, Masanori; Saikusa, Akio; Nojiri, Naoki; Saito, Kenji; Furusawa, Takayuki; et al.

JAEA-Research 2025-005, 23 Pages, 2025/07

JAEA-Research-2025-005.pdf:2.68MB

A safety demonstration test under abnormal operating conditions using the HTTR (High Temperature Engineering Test Reactor) was conducted to demonstrate safety features of the HTGRs (High Temperature Gas-cooled Reactors). Under a simulation of a control rod shutdown failure, all primary helium gas circulators were intentionally stopped during a steady-state operation at 100% reactor thermal power (30 MW), temporal changes of the reactor power and temperatures around the reactor pressure vessel (RPV) were obtained after the complete loss of forced heat removal from the reactor core. After the event (primary coolant flow stopped), the reactor power quickly decreased due to the negative reactivity feedback associated with the core temperature rise, and then the reactor power spontaneously shifted to a stable state of low power (about 1.2%) even after a recriticality. Heat dissipation from RPV surface to a surrounding vessel cooling system (water-cooled panels) ensured the amount of heat removal required to maintain the reactor temperature constant in the low power state. In this way, the transition from the event occurrence to the stable and safety state, i.e., inherent safety features of HTGRs, were demonstrated in the case of core forced cooling loss without active shutdown operations.

Simulation of excavation and long-term permeability test at 350 m gallery in Horonobe Underground Research Laboratory based on large-scale three-dimensional coupled analysis

Suetake, Koya*; Ogata, Sho*; Yasuhara, Hideaki*; Aoyagi, Kazuhei; Inui, Toru*; Kishida, Kiyoshi*

Dai-16-Kai Iwa No Rikigaku Kokunai Shimpojiumu Koen Rombunshu (Internet), p.304 - 309, 2025/01

It is very important to predict the extent of EDZ (excavation disturbed zone) and the behavior of hydraulic conductivity after excavation of shafts and tunnels for geological disposal of radioactive waste. In this study, we attempted to reproduce the in-situ tunnel excavation test and the subsequent permeability test at the Horonobe Underground Research Laboratory (URL) using a three-dimensional tunnel excavation simulator. As a result, similar results to the in-situ tests were obtained for the extent of EDZ propagation by excavation and the results of permeability tests. The simulator is effective in predicting the mechanical effects of excavation, coupled phenomena such as deformation and seepage, and hydraulic conductivity in deep mudstones in Japan.

Annual report for research on geosphere stability for long-term isolation of radioactive waste in fiscal year 2023

Niwa, Masakazu; Shimada, Koji; Sueoka, Shigeru; Ishihara, Takanori; Hakoiwa, Hiroaki; Asamori, Koichi; Murakami, Osamu; Fukuda, Shoma; Ogita, Yasuhiro; Kagami, Saya; et al.

JAEA-Research 2024-013, 65 Pages, 2024/11

JAEA-Research-2024-013.pdf:4.22MB

This annual report documents the progress of research and development (R&D) in the 2nd fiscal year of the JAEA 4th Medium- and Long-term Plan (fiscal years 2022-2028) to provide the scientific base for assessing geosphere stability for long-term isolation of high-level radioactive waste. 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. The current status of R&D activities with previous scientific and technological progress is summarized.

Research plan on geosphere stability for long-term isolation of radioactive waste (Scientific program for fiscal year 2024)

Niwa, Masakazu; Shimada, Akiomi; Asamori, Koichi; Sueoka, Shigeru; Komatsu, Tetsuya; Nakajima, Toru; Ogata, Manabu; Uchida, Mao; Nishiyama, Nariaki; Tanaka, Kiriha; et al.

JAEA-Review 2024-035, 29 Pages, 2024/09

JAEA-Review-2024-035.pdf:1.24MB

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 2024. 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.

Short communication: Inverse correlation between radiation damage and fission-track etching time on monazite

Nakajima, Toru; Fukuda, Shoma; Sueoka, Shigeru; Niki, Sota*; Kawakami, Tetsuo*; Danhara, Toru*; Tagami, Takahiro*

Geochronology (Internet), 6(3), p.313 - 323, 2024/07

https://doi.org/10.5194/gchron-6-313-2024

In this study, we explored the impacts of radiation damage and chemical composition on the etching time of fission tracks in monazite. Despite the potential of monazite fission-track (MFT) dating as an ultralow-temperature thermochronology, the comprehensive effects of radiation damage and non-formula elements, especially on the etching rate of MFTs, remain unexplored, and established analytical procedures are lacking. We quantified the degree of radiation damage (dFHWM) in Cretaceous to Quaternary monazites distributed in the Japan arc through Raman spectroscopy and chemical composition analyses. Subsequently, MFT etching was performed to examine the correlation between these parameters and the etching time. Estimation of the degree of radiation damage showed an increase in radiation damage corresponding to the cooling age of each geological unit. For example, the Toya ignimbrite (ca. 0.1 Ma) and the Kurobegawa granodiorite (0.8 Ma), both of which are types of monazite from Quaternary geological units, have dFHWM values of 0.27 and 0.55 cm, respectively. In contrast, the Muro ignimbrite (ca. 15 Ma) has a dFHWM value of 4.01 cm, while the Kibe granite and the Sagawa granite, both of which are Cretaceous granitoids, yielded 7.35 and 6.31 cm, respectively. MFT etching of these samples according to the existing recipe (6M HCl at 90C for 60-90 min) was completed at 1200, 860, 210, 120, and 90 min for the Toya ignimbrite, Kurobegawa granodiorite, Muro ignimbrite, Sagawa granite, and Kibe granite, respectively.

Experimental visualization of water/ice phase distribution at cold start for practical-sized polymer electrolyte fuel cells

Higuchi, Yuki*; Yoshimune, Wataru*; Kato, Satoru*; Hibi, Shogo*; Setoyama, Daigo*; Isegawa, Kazuhisa*; Matsumoto, Yoshihiro*; Hayashida, Hirotoshi*; Nozaki, Hiroshi*; Harada, Masashi*; et al.

Communications Engineering (Internet), 3, p.33_1 - 33_7, 2024/02

https://doi.org/10.1038/s44172-024-00176-6

Thermochronology of hydrothermal alteration zones in the Kii Peninsula, southwest Japan; An Attempt for detecting the thermal anomalies and implications to the regional exhumation history

Sueoka, Shigeru; Iwano, Hideki*; Danhara, Toru*; Niwa, Masakazu; Kanno, Mizuho; Kohn, B. P.*; Kawamura, Makoto; Yokoyama, Tatsunori; Kagami, Saya; Ogita, Yasuhiro; et al.

Earth, Planets and Space (Internet), 75(1), p.177_1 - 177_24, 2023/12

https://doi.org/10.1186/s40623-023-01921-5

Fluid-inclusion and thermochronometric analyses have been applied to hydrothermal alteration zones and their host rocks outcropping in the Hongu area of the Kii Peninsula, southwestern Japan in an attempt to detect thermal anomalies related to hydrothermal events and quantify the thermal effects on the host rocks. Hydrothermal events at ca 150 deg. C and ca 200 deg. C were identified by fluid-inclusion microthermometry of quartz veins in the alteration zones. For the host rocks and alteration zones, in the youngest population zircon yielded U-Pb dates ranging between ca 74.7-59.2 Ma, fission-track dates of ca 27.2-16.6 Ma, and (U-Th)/He single-grain dates of ca 23.6-8.7 Ma. Apatite yielded pooled fission-track ages of ca 14.9-9.0 Ma. The zircon U-Pb dates constrain the maximum depositional ages of the sedimentary samples. However, the fission-track and (U-Th)/He dates show no clear trend as a function of distance from the alteration zones. Hence, no thermal anomaly was detected in the surrounding host rocks based on the thermochronometric data patterns. The fission-track and (U-Th)/He dates are rather thought to record regional thermal and exhumation histories rather than a direct thermal imprint of fluid flow, probably because the duration of such activity was too short or because fluid flow occurred before regional cooling events and were later thermally overprinted. Apatite fission-track ages of ca 10 Ma may reflect regional mountain uplift and exhumation related to the obduction of the SW Japan lithospheric sliver onto the Shikoku Basin, or the rapid subduction of the Philippine Sea slab associated with the clockwise rotation of the Southwest Japan Arc.

Research plan on geosphere stability for long-term isolation of radioactive waste (Scientific program for fiscal year 2023)

Niwa, Masakazu; Shimada, Koji; Sueoka, Shigeru; Fujita, Natsuko; Yokoyama, Tatsunori; Ogita, Yasuhiro; Fukuda, Shoma; Nakajima, Toru; Kagami, Saya; Ogata, Manabu; et al.

JAEA-Review 2023-017, 27 Pages, 2023/10

JAEA-Review-2023-017.pdf:0.94MB

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 2023. The objectives and contents in fiscal year 2023 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.