Reactor power measuring device and measuring method therefor

In the present invention, device for measuring a neutron flux level in a power region of a BWR type reactor is simplified and its life is elongated, to make a transfer mechanism unnecessary. Namely, an ultrasonic waveguide rod has cavities for heat insulation relative to coolants on every predetermined distance in the longitudinal direction, and is inserted into coolants in the reactor core. An ultrasonic wave transmitting/receiving device is electrically connected to the ultrasonic waveguide rod and placed outside of the reactor. A signal processing device converts the difference of transmitting time of ultrasonic wave pulses of the ultrasonic wave guide rod measured at the ultrasonic wave transmitting/receiving device into a reactor power. With such procedures, since the ultrasonic wave guide rod is inserted into the reactor core, the measurement can be conducted without moving the ultrasonic wave guide rod as a detector. As a result, a transfer device and transfer path are made unnecessary. In addition, since the ultrasonic wave guide rod is made of a single material, it is never broken. Since the ultrasonic wave transmitting/receiving device is placed at the outside with no high temperature, no high pressure and no high dose rate, the working life of the reactor can be extended. (I.S.)