Method of starting BWR type reactor

Relocation of loops in materials due to neutron irradiation is suppressed to reduce the irradiation brittlement, by entering power-up operation directly after confirming the elevation of the material temperature to a predetermined level. Increase in the vacuum degree in the condensator is started by the vacuum pump and then the vacuum degree is further increasred switching to a starting air extraction vessel. Then, control rod withdrawal is started by control rod drives to attain a critical state in the nuclear reactor. Then, both of recycling pumps are operated and the reactor water temperature is elevated to an appropriate level higher than about 190 deg C and lower than the rated operation temperature by Joule heat generated. The temperature for the material in the reactor and the pressure vessel is monitored so that the temperature increase rate of the material does not exceed 55 deg C/h due to the restriction of thermal stresses. After confirming the reaching of the material temperature to a range higher than about 190 deg C and lower than the rated operation temperature, reactor power is increased by the control rod withdrawal and the recycling flow rate control to a rated power level and rated operation are conducted. (S.T.)