[en] The decommission of the Fukushima Daiichi Nuclear Power Plant is an important issue and must be steadily forwarded, while the condition of the fuel in the core is not yet clearly known. So far we know that fuel debris are dispersed around in the vessel. The effect of geometrical change of the fuel debris on its neutron multiplication factor, k_eff, has been investigated for the criticality safety of the retrieval work in Fukushima Daiichi NPP. The calculated result showed that the size of the retrieved portion could be divided very roughly into 3 ranges: [0.1 - 1.0] cm in which the feedback reactivity was zero or very small, [x - 10] cm, the feedback reactivity was negative and [x - 100] cm, the feedback reactivity was large and positive, where x depends on the concrete volume ratio, R_c, and was between 2 and 10 cm. Those results indicate a rough image of the reasonable ranges in the size of the first sample to be obtained and the mean size of a piece of the fuel debris to be retrieved. Simulations following a postulated scenario have been made to compute the air dose rate risk for the workers during the retrieval operation.