Internal pump system

An internal pump in an advanced BWR type light water cooled reactor comprises an electric motor for a pump impeller, a heat exchanger and a power source/control section. Streams from diffuser exits of two adjacent pumps cause pressure pulsation by the interference between the impeller blades and the diffuser blades. When rotational phase angles are made identical, the pulsating pressure is increased, to increase the fluid vibration force applied to a control rod guide tube. Then, in the present invention, the rotation phase angles of the two adjacent pumps are so controlled as not to agree with each other by additionally providing a rotation phase angle detector and a rotation phase angle control function to a rotational number control device of an internal pump. The phases of the fluctuation pressure of the streams discharged from the two pumps are made different, to thereby decrease the fluctuation of streams joining and flowing into the reactor core and, to decrease the vibration force applied to the reactor structures, which can improve the safety all the more. (N.H.)