Criticality control of fuel debris / TMI-2 review and Fukushima expectation

Tonoike, Kotaro ; Yamane, Yuichi  

It is conceivable that a large amount of fuel debris has been produced in the reactors of Fukushima Daiichi Nuclear Power Station (1FNPS) due to the severe core damage and melting. The amount is far beyond minimum critical mass, and essentially subject to criticality control. This presentation will outline the criticality control of fuel debris in the Three Mile Island Nuclear Station, Unit 2 reactor (TMI-2). Then, criticality control problems of fuel debris during the 1FNPS decommissioning will be described contrasting with the TMI-2 case. Finally, technical expectations and research activities of the fuel debris criticality control for the 1FNPS will be presented. It is possible that the fuel debris will be retrieved under nonborated water at the risk of criticality. In this scenario, the risk control, mitigation measures in case of criticality, would be a key factor in safety of the decommissioning.