Critical assemblies in JAEA and the role of new STACY

Sono, Hiroki ; Izawa, Kazuhiko ; Yoritsune, Tsutomu ; Suyama, Kenya; Tonoike, Kotaro 

Japan Atomic Energy Agency (JAEA) has constructed and operated nine critical assemblies. Of these nine facilities as of 2023, four have already been dismantled, four are under decommissioning, and only STACY is active but under temporally shutdown. STACY is scheduled to restart in 2024 after core modification from a "critical assembly using uranium nitrate solution fuel" to a "general-purpose critical assembly using uranium fuel rods and light-water moderator." The immediate objective of new STACY is to acquire criticality data for fuel debris removal from the damaged reactors in Fukushima-Daiichi Nuclear Power Plant. After the critical experiment program regarding fuel debris, the new STACY is expected to be used for various R&D on next-generation power reactors and others. In addition, the new STACY will serve as an educational and training reactor. These activities are useful not only for Japan but also for international collaborative research and joint use.