PURPOSE:To allow a safe fuel exchange operation by forming coolant inflow holes in the respective rear surfaces of a fuel assembly duct and a fuel transfer pot and providing opening/closing means which communicate and shut off the two coolant inflow holes. CONSTITUTION:A projecting part 20 of the fuel assembly duct 16 and a hollow groove 28 of the fuel transfer pot 13 constitute the opening/closing means for communicating and shutting off the coolant inflow holes 19, 27 formed on the respective rear surfaces of the fuel assembly duct 16 and the fuel transfer pot 13. The used fuel assembly 18 existing in the fuel transfer pot 13 is taken out of a reactor 1 while the used fuel assembly 18 is held immersed in the coolant if the coolant inflow holes 19, 27 are shut off by the opening/closing means 20, 28 and when the fuel transfer pot 13 is hoisted straight upward.
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