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Nakamura, Takemi; Ogasawara, Reira; Ushijima, Hiroki; Okada, Yuji; Yamaguchi, Atsushi; Horiguchi, Hironori; Hirane, Nobuhiko
JAEA-Technology 2026-008, 39 Pages, 2026/07
JAEA-Technology-2026-008.pdf:2.55MB
Irradiation studies targeting thermal neutrons to fast neutrons are being conducted at the JRR-3 research reactor, and to accurately evaluate the irradiation effect, it is necessary to consider the influence of the neutron spectrum. Fuels and cadmium wires used as combustible absorbers are burned by reactions with neutrons because the JRR-3 is a high-power 20MW reactor, and the fuel shuffling and replacement are performed periodically to ensure uniform burnup and long-term operation. Since it is necessary to accurately reproduce the burnup history both temporally and spatially, it is difficult to evaluate the neutron spectrum with high accuracy through computational analysis. Hence, we investigated a method that can easily and accurately evaluate neutron spectra at irradiation facility in equilibrium core of JRR-3 silicide fuel and verified the validity of the evaluation results. This method uses data of initial guess spectrum and response functions for each irradiation field calculated using MCNP code, reaction rate measured from neutron fluence monitors such as Au-Al and Ni wire to perform unfolding using SAND-II. We conducted characteristic measurements from fiscal year 2023 to 2025 and performed unfolding using actual measured values from neutron fluence monitors for seven cycles. As a result, in all cases, the C/E ratio of the reaction rate was 1.000, indicating good convergence, and the difference from the unfolding and measured neutron flux was within 0.3% for hydraulic irradiation facility HR-1, within 0.8% for pneumatic irradiation facility PN-2, and 0% for vertical irradiation facility RG-2. This suggests that the results of the unfolded spectrum are valid. The application of this method can meet a wide range of user needs, and it is expected to greatly contribute to the advancement of irradiation research with improved reliability of the JRR-3 irradiation field.
Tokunaga, Yo; Ishitobi, Takayuki; Sakai, Hironori; Kambe, Shinsaku*; Harima, Hisatomo*; Nakamura, Ai*; Maurya, A.*; Li, D.*; Homma, Yoshiya*; Honda, Fuminori*; et al.
Physical Review Letters, 137(4), p.046501_1 - 046501_6, 2026/07
Times Cited Count：1Yamano, Hidemasa; Futagami, Satoshi; Sasa, Kyohei*; Nakamura, Hironori*; Tokizaki, Minako*; Kubota, Ryuzaburo*
Proceedings of 2025 International Congress on Advances in Nuclear Power Plants (ICAPP 2025) (Internet), 12 Pages, 2025/09
This study examined the application of safety design criteria (SDC) and safety design guideline (SDG) developed in the Generation-IV international forum on the passive reactor shutdown capability to sodium-cooled fast reactors (SFRs) recently designed in Japan.
Hayakawa, Satoshi*; Hagiwara, Hiroyuki*; Imamura, Akira*; Onoda, Yuichi; Tanaka, Masaaki; Nakamura, Hironori*
Proceedings of 13th Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS13) (Internet), 8 Pages, 2024/11
In a sodium-cooled fast reactor, a cover gas region filled with argon gas is located above the sodium pool in the main vessel to prevent the hot sodium from contacting the structures. This region involves heat transportation by natural convection of the cover gas, radiation among liquid surface and structures, and sodium phase change between mist and vapor. In this study, the numerical evaluation method has been developed with a commercial CFD code, Fluent, incorporating the sodium mist transport and growth models, and the radiation scattering model. Simulations of a laboratory scale test with a cylindrical cover gas region was carried out for the validation of the method and showed that the temperature distribution and sodium mist concentration in the cover gas region are in good agreements with the test results. A simulation of a pool-type sodium cooled fast reactor has also conducted and the basic aspect of physical phenomena taking place in the cover gas region were evaluated.
Sogabe, Joji; Ishida, Shinya; Tagami, Hirotaka; Okano, Yasushi; Kamiyama, Kenji; Onoda, Yuichi; Matsuba, Kenichi; Yamano, Hidemasa; Kubo, Shigenobu; Kubota, Ryuzaburo*; et al.
Proceedings of International Conference on Nuclear Fuel Cycle (GLOBAL2024) (Internet), 4 Pages, 2024/10
In the frame of France-Japan collaboration, the calculational methodologies were defined and assessed, and the phenomenology and the severe accident consequences were investigated in a pool-type sodium-cooled fast reactor.
Koyama, Shinichi; Ikeuchi, Hirotomo; Mitsugi, Takeshi; Maeda, Koji; Sasaki, Shinji; Onishi, Takashi; Tsai, T.-H.; Takano, Masahide; Fukaya, Hiroyuki; Nakamura, Satoshi; et al.
Hairo, Osensui, Shorisui Taisaku Jigyo Jimukyoku Homu Peji (Internet), 216 Pages, 2023/11
In FY 2021 and 2022, JAEA perfomed the subsidy program for "the Project of Decommissioning and Contaminated Water Management (Development of Analysis and Estimation Technology for Characterization of Fuel Debris (Development of Technologies for Enhanced Analysis Accuracy, Thermal Behavior Estimation, and Abbreviated Analysis))" started in FY 2021. This presentation material summarized the results of the project, which will be available shortly on the website of Management Office for the Project of Decommissioning, Contaminated Water and Treated Water Management.
Kinjo, Katsuki*; Fujibayashi, Hiroki*; Matsumura, Hiroki*; Hori, Fumiya*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; et al.
Science Advances (Internet), 9(30), p.2736_1 - 2736_6, 2023/07
Times Cited Count：28 Percentile：91.86(Multidisciplinary Sciences)Tokunaga, Sho; Horiguchi, Hironori; Nakamura, Takemi
JAEA-Technology 2023-001, 37 Pages, 2023/05
JAEA-Technology-2023-001.pdf:1.39MB
The cold neutron source (CNS) of the research reactor JRR-3 converts thermal neutrons generated in the reactor into low-energy cold neutrons by moderating them with liquid hydrogen stored in the moderator cell. Cold neutrons generated by the CNS are transported to experimental instruments using neutron conduits, and are used for many studies of physical properties, mainly in life science, polymer science, environmental science, etc. Improvement of cold neutron intensity is essential to maintain competitiveness with the world's research reactors in neutron science, and we are developing a new CNS that incorporates new knowledge. The current moderator cell for the CNS of JRR-3 is a stainless-steel container which is a canteen bottle type, and the cold neutron intensity can be improved by changing the material and shape. Therefore, the basic specifications of the new moderator cell were changed to aluminum alloy which has a smaller neutron absorption cross section, and the shape was optimized using a Monte Carlo code MCNP. Since these changes in specifications will result in changes in heat generation and heat transfer conditions, the CNS of JRR-3 was re-evaluated in terms of self-regulating characteristic, heat transport limits, heat resistance and pressure resistance, etc., to confirm its feasibility in thermal-hydraulic design. This report summarizes the results of the thermal-hydraulic design evaluation of the new moderator cell.
-axis knight shift on UTe with 
= 2.1 KMatsumura, Hiroki*; Fujibayashi, Hiroki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; et al.
Journal of the Physical Society of Japan, 92(6), p.063701_1 - 063701_5, 2023/05
Times Cited Count：72 Percentile：98.71(Physics, Multidisciplinary)
Te-NMR on the uranium-based superconductor UTeFujibayashi, Hiroki*; Kinjo, Katsuki*; Nakamine, Genki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; et al.
Journal of the Physical Society of Japan, 92(5), p.053702_1 - 053702_5, 2023/05
Times Cited Count：12 Percentile：76.81(Physics, Multidisciplinary)
induced by magnetic fieldKinjo, Katsuki*; Fujibayashi, Hiroki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; Homma, Yoshiya*; et al.
Physical Review B, 107(6), p.L060502_1 - L060502_5, 2023/02
Times Cited Count：37 Percentile：91.85(Materials Science, Multidisciplinary)Nakamura, Hironori*; Hayakawa, Satoshi*; Shibata, Akihiro*; Sasa, Kyohei*; Yamano, Hidemasa; Kubo, Shigenobu
Proceedings of 12th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS12) (Internet), 7 Pages, 2022/10
In order to evaluate long-term coolablity of the debris-bed with decay heat, a three-dimensional calculation method coupled with the debris bed module was developed in this study. The coupled code calculation results show that natural circulation of the coolant between the hot pool and the cold pool is established through the four intermediate heat exchangers after the activation of the dipped direct heat exchangers. The cold pool with the debris-bed is continually cooled not only by the natural circulation flow, but also by heat transfer to the hot pool through the plenum separation plate between the hot pool and the cold pool. The effect of the three-dimensional flow field around the core catcher on the temperature in the debris-bed is about 20K under the current calculation condition.
Yamano, Hidemasa; Kubo, Shigenobu; Tokizaki, Minako*; Nakamura, Hironori*
Proceedings of International Conference on Topical Issues in Nuclear Installation Safety; Strengthening Safety of Evolutionary and Innovative Reactor Designs (TIC 2022) (Internet), 12 Pages, 2022/10
Specific design features of advanced sodium-cooled fast reactors (SFRs) designed in Japan are a passive reactor shutdown system, a passive decay heat removal system (DHRS), and an in-vessel retention (IVR) concept against an anticipated transients without scram (ATWS) in design extension condition (DECs). The present paper describes numerical analysis methodologies for event sequences studied in Japan and some numerical analyses of DECs to show the effectiveness of the passive shutdown system against a typical ATWS and severe accident mitigation measures for the IVR of molten core. For the passive shutdown capability, the numerical analysis has demonstrated the effectiveness of a self-actuated shutdown system against a severe ATWS event, for which the temperature response time was separately evaluated by a computational fluid dynamics (CFD) code. A recently developed debris-bed cooling analysis methodology coupled with a CFD code and a debris-bed module has successfully simulated a three-dimensional coolant flow field near the debris bed with the passive DHRS capability in order to demonstrate the debris-bed coolability on a core catcher.
Aoki, Dai*; Sakai, Hironori; Opletal, P.; Tokiwa, Yoshifumi; Ishizuka, Jun*; Yanase, Yoichi*; Harima, Hisatomo*; Nakamura, Ai*; Li, D.*; Homma, Yoshiya*; et al.
Journal of the Physical Society of Japan, 91(8), p.083704_1 - 083704_5, 2022/08
Times Cited Count：79 Percentile：97.92(Physics, Multidisciplinary) under pressure revealed by Te-NMRKinjo, Katsuki*; Fujibayashi, Hiroki*; Nakamine, Genki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; et al.
Physical Review B, 105(14), p.L140502_1 - L140502_5, 2022/04
Times Cited Count：13 Percentile：63.50(Materials Science, Multidisciplinary) determined by knight shift measurementFujibayashi, Hiroki*; Nakamine, Genki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; et al.
Journal of the Physical Society of Japan, 91(4), p.043705_1 - 043705_4, 2022/03
Times Cited Count：64 Percentile：96.98(Physics, Multidisciplinary)This study investigates the spin susceptibility in U-based superconductor UTe in the superconducting (SC) state by using Knight shift measurements for a magnetic field 
along the axis, which is the magnetic easy axis of UTe. Although a tiny anomaly ascribed to the SC diamagnetic effect was observed just below the SC transition temperature 
, the -axis Knight shift in the SC state shows no significant decrease, following the extrapolation from the normal-state temperature dependence. This indicates that the spin susceptibility is nearly unchanged below . Considering the previous Knight shift results for 
and 
, the dominant SC state is determined to be 
in the spin-triplet pairing, which is consistent with the spin anisotropy in the normal state. The present result shows that UTe is a spin-triplet superconductor with spin degrees of freedom.
Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Haga, Yoshinori; Tokiwa, Yoshifumi; Opletal, P.; Fujibayashi, Hiroki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; et al.
Journal of the Physical Society of Japan, 91(2), p.023707_1 - 023707_5, 2022/02
Times Cited Count：31 Percentile：89.90(Physics, Multidisciplinary)Te NMR experiments in field () applied along the easy magnetization axis (the -axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30
40 K, where the spin susceptibility along the hard magnetization axis (the 
-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in 
. We suggest that UTe would be located on the paramagnetic side near an electronic phase boundary, where either the magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations.
SR and NMR studiesHiga, Nonoka*; Ito, Takashi; Yogi, Mamoru*; Hattori, Taisuke; Sakai, Hironori; Kambe, Shinsaku; Guguchia, Z.*; Higemoto, Wataru; Nakashima, Miho*; Homma, Yoshiya*; et al.
Physical Review B, 104(4), p.045145_1 - 045145_7, 2021/07
Times Cited Count：7 Percentile：28.36(Materials Science, Multidisciplinary)Te NMR on UTeNakamine, Genki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; Homma, Yoshiya*; et al.
Journal of the Physical Society of Japan, 90(6), p.064709_1 - 064709_7, 2021/06
Times Cited Count：34 Percentile：86.85(Physics, Multidisciplinary) probed with Te-NMR measurementNakamine, Genki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; Homma, Yoshiya*; et al.
Physical Review B, 103(10), p.L100503_1 - L100503_5, 2021/03
Times Cited Count：61 Percentile：92.88(Materials Science, Multidisciplinary)