[en] Purpose: To improve the efficiency for the recovery of radioactive corrosion products by utilizing sedimenting and subliming materials such as dry ice. Method: When a great amount of dry ice pieces each formed into an appropriate size are thrown into a pressure vessel of a nuclear reactor, the thrown dry ice pieces are sedimentated to the bottom of the pressure vessel and release CO₂ gas bubbles while moving around by the reaction force of the gas bubbles released from the dry ice pieces. As the result, deposited radioactive corrosion products are stirred and moved upwardly in the reactor core water. The radioactive corrosion products thus moved upwardly float in the reactor water and then recovered by a reactor water clean-up system or a provisional submerged cleaner. Accordingly, neither a particular device for agitating the water within the pressure vessel nor the operators therefor is required. Furthermore, since the dry ice pieces used are spontaneously released out of the vessel as the gas, recovery of them is unnecessary and no radioactive contamination wastes are resulted. (Yoshino, Y.)