Criticality control system development for fuel debris removal in Fukushima Daiichi

In the Fukushima Daiichi Nuclear Power Station units 1, 2, and 3, it is estimated that the nuclear fuel bundles turned to non-uniform fuel debris and may remain at the locations such as the RPV bottom head and the pedestal floor. According to the results of observed plant parameters, the fuel debris at present is estimated as sub-critical. However, it is important to establish the criticality control techniques for future debris removal, in order to prevent re-criticality in case of the change in condition associated with removal. At International Research Institute for nuclear Decommissioning (IRID), criticality control techniques that correspond to the current-investigated methods for fuel debris removal and their applied element technologies are under development. (author)