Overview of the modified STACY for criticality research on Fukushima fuel debris

Maekawa, Tomoyuki; Araki, Shohei   ; Aizawa, Eiju; Ishii, Junichi; Izawa, Kazuhiko 

The Japan Atomic Energy Agency (JAEA) has modified its critical assembly, "STACY (STAtic experiment Critical facilitY)," replacing its solution-fueled core with a heterogeneous core consisting of UO fuel rods and reflector/moderator water to experimentally characterize the criticality of the fuel debris from the Tokyo Electric Power Company`s Fukushima Daiichi nuclear power plant accident. The modified STACY achieved its first criticality in April 2024. After successfully passing the pre-operational inspections, criticality experiments commenced in August 2024. The facility is equipped with two types of grid plates (their hole spacings are 1.50 cm and 1.27 cm) and can load samples simulating the composition of fuel debris, including concrete, steel, and zirconium. This presentation outlines presents the specifications of the modified STACY, the experimental samples that can be loaded, and the criticality experiments currently underway. These experiments are scheduled to continue until July 2025 and are actively being conducted at this time.