For the decommissioning of the Fukushima Daiichi Nuclear Power Station Units 1, 2, and 3, research and development activities have been pursued to retrieve fuel debris from the pressure and containment vessels of each reactor unit. In prepara­tion for the retrieval, however, there remain serious problems concerning the cooling water of fuel debris from the aspect of criticality safety.

To study the new criticality control measures for the fuel debris, the Japan Atomic Energy Agency has carried forward a project to modify the Static Exper­iment Critical Facility (STACY) and to pursue critical experiments regarding the fuel debris. STACY, a facility using solution fuel, is to be converted into a thermal critical assembly using fuel rods and a light water moderator. A series of critical experiments will be conducted in the modified STACY using simulated fuel debris samples. These samples are to be manufactured by mixing UO2 and reactor structural materials with various chemical compositions.

The license application for the STACY modification has been under safety review. The first criticality experiment in the modified STACY is scheduled for 2018. The modified STACY will provide benchmark data on criticality safety for fuel debris to validate the new criticality control measures applicable to the Fukushima Daiichi Nuclear Power Stations.

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