The Pre-service inspections for the Static Experiment Critical Facility (STACY)

Hasegawa, Kenta; Seki, Masakazu; Aizawa, Eiju; Sumiya, Masato; Yoshikawa, Tomoki ; Maekawa, Tomoyuki; Ishii, Junichi; Araki, Shohei  ; Izawa, Kazuhiko ; Gunji, Satoshi  

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.