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Fukuda, Kodai
Proceedings of Nuclear Criticality Safety Division 2025 Conference (NCSD 2025) (Internet), p.191 - 194, 2025/09
Araki, Shohei; Arakaki, Yu; Maekawa, Tomoyuki; Kamikawa, Yutaka; Hasegawa, Kenta; Yoshikawa, Tomoki; Tada, Yuta; Sumiya, Masato; Seki, Masakazu; Aizawa, Eiju; et al.
Proceedings of Nuclear Criticality Safety Division 2025 Conference (NCSD 2025) (Internet), p.80 - 89, 2025/09
In order to investigate the effect on criticality characteristics of fuel debris containing concrete composition in the Fukushima Daiichi Nuclear Power Plant, critical experiments loaded concrete rods were conducted in the modified STACY. The experimental cores of the modified STACY were composed of light water, fuel rods, and concrete rods with grid plates with 15.0-mm lattice intervals in the core tank. Each fuel rod consisted of a zirconium alloy clad tube (9.5-mm outer diameter) and UO
pellets (8.2-mm diameter) with 4.98wt.% 235U enrichment. Each concrete rod was composed of concrete simulant pellets (7.0-mm diameter) and an aluminum alloy clad tube (9.5-mm outer diameter). The concrete simulant pellets were fabricated using an aggregate-free mortar in order to ensure a uniform composition of the pellets, and the composition was evaluated by chemical analysis. Three experimental cores were configurated with 9, 25 and 69 concrete rods. The critical water level data were obtained. An effect of composition uncertainty of the concrete simulant pellet was estimated by sensitivity analysis using MCNP6.2 with JENLD-4.0. The sensitivity analysis estimated the effect to be less than 3 pcm. The calculation results performed using MCNP6.2 with ENDF/B-VII.1, JENDL-4.0, and JENDL-5 libraries showed a tendency to overestimate the multiplication factor according to increasing the loading amount of the concrete rods.
Gunji, Satoshi; Araki, Shohei; Yoshikawa, Tomoki; Arakaki, Yu; Aizawa, Eiju; Seki, Masakazu; Ishii, Junichi; Izawa, Kazuhiko; Shiba, Shigeki*; Iwahashi, Daiki*
Proceedings of Nuclear Criticality Safety Division 2025 Conference (NCSD 2025) (Internet), p.90 - 99, 2025/09
Hasegawa, Kenta; Seki, Masakazu; Aizawa, Eiju; Sumiya, Masato; Yoshikawa, Tomoki; Maekawa, Tomoyuki; Ishii, Junichi; Araki, Shohei; Izawa, Kazuhiko; Gunji, Satoshi
Proceedings of Nuclear Criticality Safety Division 2025 Conference (NCSD 2025) (Internet), p.50 - 59, 2025/09
The STACY was modified from a homogeneous solution-fueled reactor to a heterogeneous-core reactor composed of fuel rods and a light water moderator. This modification was undertaken to support research on the criticality characteristics of fuel debris generated by the Fukushima Daiichi Nuclear Power Plant accident. To facilitate these studies, we developed simulant devices that can be inserted into the modified STACY core as components of fuel debris mock-up configurations. From April to June 2024, Authors conducted a series of pre-service inspections on the safety performance of STACY and subsequently received approval from the Nuclear Regulation Authority (NRA) to resume operation. Experimental operations involving the simulant devices were carried out from that point until the end of March 2025. The pre-service inspections included shutdown margin, one-rod stuck margin, and measurements of the reactivity addition rate. All results satisfied STACY's safety limits.
Ishii, Junichi; Seki, Masakazu; Aizawa, Eiju; Sumiya, Masato; Maekawa, Tomoyuki; Arakaki, Yu; Hasegawa, Kenta; Araki, Shohei; Izawa, Kazuhiko; Gunji, Satoshi
Proceedings of Nuclear Criticality Safety Division 2025 Conference (NCSD 2025) (Internet), p.39 - 48, 2025/09
The removal of fuel debris is one of the most important and challenging tasks in the decommissioning of power reactors damaged in the 2011 accident at Fukushima Daiichi Nuclear Power Plant (1F). The Japan Atomic Energy Agency (JAEA) has implemented a renewal program of the Static Experiment Critical Facility (STACY), transitioning it from a homogeneous solution system to a heterogeneous water-moderated system, in order to verify the criticality calculations that consider the fuel debris from the 1F accident. The first criticality of the modified STACY was achieved in April 2024. After a series of performance inspections, an experimental campaign for the investigation of fuel debris characteristics was started in August 2024. In this paper, we describe the main equipment of the modified STACY. In addition, the experimental equipment for the debris-simulated core configuration is also introduced.