PURPOSE: To reduce a quantity of consumption of the helium gas required when in a transfer of the fuel rod, by means of replacing only a cover gas, within an in- and-out guide tube, with the helium gas, in case of taking out the spent fuel.

CONSTITUTION: A spent fuel 7 and a transfer pot 8 are being contained in a rack 3 within a storage tank 1 of constitution comprising a Na-filled vessel 2, a cover gas like Ar and a plug 5. In a case when a transfer of the fuel 7 is operated to a following stage processing equipment, the work is to be operated through a fuel in-and-out guide tube 9. With a point end 9a of the guide tube 9 made lowering down to below a liquid level of Na, then with an operation alternately made between a cover gas suction exhaust system 12 and a He gas supply system 13, the atmosphere within the guide tube 9 is replaced with the He gas, with a taking out and/or a charging operated by the fuel in-out machine and the transfer machine. After a completion of transfer and charge work, the interior of the guide tube 9 is filled with Ar gas, as required, by a supply system 24.