Study on severe accidents of oxide-fueled sodium-cooled fast reactors, 2; Identification of important phenomena related to recriticality elimination

Ishida, Shinya; Imaizumi, Yuya; Matsuba, Kenichi; Yamano, Hidemasa

Dai-30-Kai Doryoku, Enerugi Gijutsu Shimpojiumu Koen Rombunshu (Internet), 5 Pages, 2026/06

no abstracts in English

Sodium-cooled fast reactor and related multiphase flow phenomena, 4; Assessment of molten fuel-sodium interactions in severe accidents

Matsuba, Kenichi

Konsoryu, 40(2), p.121 - 124, 2026/06

https://doi.org/10.3811/jjmf.2026.T009

SFRs have the characteristic that reactivity is introduced when coolant boiling or movement of core constituents occurs within the core. Therefore, in the event of a severe accident involving core meltdown, there is a possibility of recriticality leading to mechanical energy release. To avoid this possibility, a safety design involves installing flow paths within the core to rapidly discharge molten fuel from the core, as well as a core catcher at the bottom of the reactor vessel to retain it. It is believed that molten fuel discharged outside the core interacts with sodium, and as a result of this molten fuel-sodium interaction, the molten fuel solidifies into fine particles and accumulates on the core catcher. This paper presents the current status of assessment of molten fuel-sodium interactions in severe accidents.

First freezing experiments with a molten mixture of boron carbide and stainless steel in core disruptive accidents of sodium-cooled fast reactors

Emura, Yuki; Matsuba, Kenichi; Kikuchi, Shin; Yamano, Hidemasa

Proceedings of 13th Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS13) (Internet), 8 Pages, 2024/11

Experimental study on the coolability of molten core materials discharged into a depth- and volume-limited sodium plenum

Matsuba, Kenichi; Kato, Shinya; Kamiyama, Kenji; Akaev, A. S.*; Vurim, A. D.*; Baklanov, V. V.*

Proceedings of 31st International Conference on Nuclear Engineering (ICONE31) (Internet), 7 Pages, 2024/11

https://doi.org/10.1115/ICONE31-135809

During a severe accident in sodium-cooled fast reactors, molten core materials could be discharged from the core region toward the lower sodium region of the reactor vessel through coolant channels, such as control rod guide tubes. Typical SFRs have a sodium plenum with limited depth and volume, such as the core inlet plenum located under the core region. Therefore, it is important to evaluate the coolability of molten core materials discharged into a depth- and volume-limited sodium plenum. In the present study, to deepen the understanding on the coolability of molten core materials discharged into such a sodium plenum, conditions under which molten core materials form solidified fragments were discussed based on an experiment discharging a molten fuel simulant (molten AlO) into a test vessel filled with liquid sodium.

New analysis model of solid body formation in particle method for jet impingement and solidification in severe accidents of SFRs

Imaizumi, Yuya; Kamiyama, Kenji; Matsuba, Kenichi

Annals of Nuclear Energy, 206, p.110658_1 - 110658_10, 2024/10

https://doi.org/10.1016/j.anucene.2024.110658

A Mechanism for spontaneous thermal fragmentation with coolant entrainment during the molten fuel-sodium interaction

Johnson, M.*; Emura, Yuki; Clavier, R.*; Matsuba, Kenichi; Kamiyama, Kenji; Brayer, C.*; Journeau, C.*

Nuclear Engineering and Design, 423, p.113165_1 - 113165_14, 2024/07

https://doi.org/10.1016/j.nucengdes.2024.113165

Experimental investigation of two interactions between molten jets and sodium, pertaining to severe accidents in a sodium-cooled fast reactor, have been undertaken at the JAEA's MELT facility. X-ray imaging and debris analysis reveal rapid formation of a crust at the melt coolant-interface, instigating thermal fragmentation events. Heat transfer calculations at the jet-coolant interface, supported by particle tracking velocimetry characterisation of the jet velocity, imply the formation of a solid crust within milliseconds of contact with the coolant. A mechanism for enhanced thermal fragmentation is proposed, inspired by observations from the X-ray imaging of coolant entrainment into the jet.

A Series of molten stainless steel-sodium interaction experiments to develop an evaluation methodology for jet breakup during core disruptive accidents in sodium-cooled fast reactors

Matsuba, Kenichi; Emura, Yuki; Kamiyama, Kenji

Proceedings of 2023 International Congress on Advanced in Nuclear Power Plants (ICAPP 2023) (Internet), 8 Pages, 2023/04

A series of Fuel-Coolant Interaction (FCI) experiments discharging about 1 kg of molten stainless steel (SS), one of prototypic materials, into a sodium pool (about 30 cm in diameter, about 1 m in depth) has been conducted at the MELT facility of Japan Atomic Energy Agency. X-ray visualization program confirmed that molten SS jets with diameters of about 1 to 2 cm were broken up by FCIs which occurred with sodium vapor expansion at depths near the initial sodium level. Responses of thermocouples suggested that the jets were rapidly cooled near the initial sodium level and became SS fragments before reaching the bottom of the sodium pool. Results of experiments with molten SS jets confirmed that an evaluation methodology for jet breakup under sodium-cooled conditions needs to be developed by taking into account effects of thermal parameters related to sodium vaporization as well as conventional parameters, such as density ratio of a jet and coolant, and jet diameter.

Measurements of pressure drop and void fraction of air-water two-phase flow in a sphere-packed bed

Yamamoto, Seishiro*; Odaira, Naoya*; Ito, Daisuke*; Ito, Kei*; Saito, Yasushi*; Imaizumi, Yuya; Matsuba, Kenichi; Kamiyama, Kenji

Proceedings of 12th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS12) (Internet), 4 Pages, 2022/10

Analysis on cooling behavior for simulated molten core material impinging to a horizontal plate in a sodium pool

Matsushita, Hatsuki*; Kobayashi, Ren*; Sakai, Takaaki*; Kato, Shinya; Matsuba, Kenichi; Kamiyama, Kenji

Proceedings of 13th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation and Safety (NUTHOS-13) (Internet), 9 Pages, 2022/09

During core disruptive accidents in sodium-cooled fast reactors, the molten core material flows through flow channels, such as the control rod guide tubes, into the core inlet plenum under the core region. The molten core material can be cooled and solidified while impinging on a horizontal plate of the inlet plenum in a sodium coolant. However, the solidification and cooling behaviors of molten core materials impinged on a horizontal structure have not been sufficiently studied thus far. Notably, this is an important phenomenon that needs to be elucidated from the perspective of improving the safety of sodium-cooled fast reactors. Accordingly, a series of experiments on discharging a simulated molten core material (alumina: AlO) into a sodium coolant on a horizontal structure was conducted at the experimental facility of the National Nuclear Center of the Republic of Kazakhstan. In this study, analyses on the sodium experiments using SIMMER-III as the fast reactor safety evaluation code were performed. The analysis methods were validated by comparing the results and experiment data. In addition, the cooling and solidification behaviors during jet impingement were evaluated. The results indicated that the molten core material exhibited fragmentation owing to the impingement on the horizontal plate and was, therefore, scattered toward the periphery. Furthermore, the simulated molten core material was evaluated to be cooled by sodium and subsequently solidified.

Development trends of small modular reactors and approaches of Japan Atomic Energy Agency to advanced reactor development

Matsuba, Kenichi; Shinohara, Masanori; Toyooka, Junichi; Inaba, Yoshitomo; Sumita, Junya

Enerugi, Shigen, 43(4), p.218 - 223, 2022/07

In the global trend toward decarbonization, Japan has a policy to pursue all options, including nuclear power, to achieve carbon neutrality by 2050. In order to meet the public requirements for nuclear power, it is important to promote the development of advanced reactors, including the Small Modular Reactor (SMR), as one of the promising options. This article describes the domestic and international trends of SMR development, introduces the activities of Japan Atomic Energy Agency (JAEA) for the development of advanced reactors including SMRs, and concludes with the future prospect for the introduction of advanced reactor including SMRs in Japan.