BWR type reactor and start-up method therefor

The present invention provides a method of ensuring a recycling flow rate by static water head difference between the inside and the outside of a reactor core of a BWR type reactor. That is, water is charged to a primary coolant system upon start-up thereof. The inside of a pressure vessel is pressurized by a pressurizing tank and control rods are withdrawn. Then, cooling water is heated to a high temperature in a state of a single phase flow. Next, the pressurization is cancelled. The pressure in the pressure vessel is approached to a saturated pressure corresponding to the temperature of coolants. Then, coolants are transited to a two phase flow state, thereafter, coolants are put to nuclear heating in a two phase flow state. As a result, predetermined nuclear reactor operation temperature and pressure are obtained. According to the method described above, flow change and lowering of reactor core stability caused by occurrence of instable phenomenon of low temperature two phase flow can be prevented upon start-up. In addition, the reactor start-up time can be shortened to improve economy and reliability. (I.S.)