Internal pump of nuclear reactor

A sealed hole has been formed to a reactor pressure vessel for taking out output signals of a rotation detector disposed for measuring attenuated rotation number of an internal pump shaft. However, this deteriorates the sealing reliability of the reactor pressure vessel. In the present invention, a residual voltage detection section is disposed on the side of the power source of a driving electric motor for detecting the reduction rate of the number of rotation of the internal pump after interruption of the power source. With such a constitution, the attenuation property of the internal pump after interruption of the power source can be inspected without deteriorating the reliability of the reactor pressure vessel. (N.H.)