Method of Monitoring Reactor Bottom Area, Reactor Bottom Area Monitoring Apparatus and Nuclear Reactor

An ultrasonic sensor has a piezo-electric element attached at an end surface outside a reactor pressure vessel (RPV) of a sensor leading edge portion. The sensor leading edge portion passes through a bottom head of the RPV and is installed on the bottom head. Ultrasonic waves generated by the piezo-electric element are propagated to the sensor leading edge portion and are propagated to reactor water in the RPV from the sensor leading edge portion. When water surface of the reactor water in the RPV exists below a core support plate, the ultrasonic waves propagated inside the reactor water are reflected on the water surface. Ultrasonic waves reflected on the water surface are propagated into the reactor water, enter the sensor leading edge portion, and are received by the piezo-electric element. Using the ultrasonic waves received by the piezo-electric element, the water level in the RPV is obtained.