of the transfer from the reactor core to the pocket therefore in accordance with the first embodiment it is possible to control the output of the reactor by controlling the position in the core at which each control rod 18 is inserted and to cause the emergency shutdown of the output of the reactor by inserting the control rods deeply into the core by adding the weight of the structure above each control rod 18 thereto regardless of the motors which are separated by the magnetic link mechanism 79 in which the function of the magnet might be shut off in an emergency facilitating the work of exchanging the core constituent elements since the control rods and the fuel assemblies which are core constituent elements can be handled and exchanged in the space where a conventional control rod drive mechanism is provided the space required for a conventional fuel exchanger can be removed so that the diameter of the small rotating plug 4 and the diameter of the large rotating plug surrounding the small rotating plug 4 can be greatly reduced therefore the size of the reactor vessel can be reduced and the amount of materials used in the reactor structure can also be reduced so that the reactor structure can be made more economical since in the second embodiment only the first elevating drive mechanism is changed from the first embodiment only the changed part is described below racks 45 are vertically fixed to.