BWR type reactor shut down method

In the present invention, the radioactivity concentration of reactor water is kept low to reduce the amount of radioactive materials accumulated on the surface of structures in contact with reactor water thereby suppressing operator's exposure dose. An operation method effective to reduce the amount of radioactive materials of ion ingredients deposited on the inner surface of pipelines of an after heat removing system is to start the operation of the after heat removing system within a range of a reactor water temperature at 130degC to 115degC (the value as measured by a reactor inner pressure gauge: 2 - 0.5kg/cm2). In addition, in order to suppress peeling of radioactive materials of crud ingredients, the temperature of reactor water is lowered automatically by controlling the opening degree of a turbine bypass valve based on data of a reactor water temperature lowering rate by a method of blowing steams from a turbine bypass system to a condensator upto a reactor water temperature of 130degC or lower. If the method is performed, the dose rate of the after heat removing system can be kept low, and it can contribute to the shortening of the shut down period by automatic control of the turbine bypass valve. (I.S.)