Patent ID: 7215729

Claim:
A method for operating a nuclear reactor having a reactor vessel, a plurality of fuel assemblies loaded in a core arranged inside the reactor vessel, wherein each of said plurality of fuel assemblies includes a plurality of fuel rods and at least one water rod therein, and a pump which regulates a flow rate of coolant supplied to the core, the method comprising the steps of: raising a coolant surface formed between the coolant and a vapor in the at least one water rod by increasing the flow rate of the coolant supplied to the core based on increasing a number of revolutions of the pump during one period from a beginning of one fuel cycle, which one fuel cycle is an operation period of the nuclear reactor from when fuel assemblies in the nuclear reactor are replaced and operation of the nuclear reactor is started to when the nuclear reactor is stopped for renewing at least one of the fuel assemblies in the nuclear reactor, and before an end of the one fuel cycle; and further increasing the flow rate of coolant supplied to the core based on increasing the number of revolutions of the pump during another period after the one period to an end of the one fuel cycle in a state in which the at least one water rod is completely filled with the coolant.