PURPOSE: To prevent the generation of instability during a startup by withdrawing control rods after degassing operation for startup of natural circulation state, rising the temperature and pressure by nuclear heating and obtaining the rated power.

CONSTITUTION: A steam condenser 7 is evacuated with a vacuum pump 11 and the inside of the reactor is degassed by opening the main steam drain valve 9 with the main steam isolation valve 8 closed. At this moment, the incore pressure is lowered to approximately 0.3 atmosphere. Then control rods 3 are withdrawn to start nuclear heating. The power is suppressed to 1% of the rated power. As the cooling water temperature at this state is the room temperature and the core inlet subcooling is ca. 50°C, the geysering does not occur. This is because coolant heatup by the nuclear heating still does not begin at this moment. Therefore, it is not necessary after this time to keep the power at 1% of the rated power for escaping the geysering and so the reactor power can be maximized within the coolant heatup rate of 55°C/hr and the startup time is shortened.