Flow rate measuring device in reactor core

A temperature detector is disposed in a flow channel of coolants which is a mixture of recycling coolants in a reactor pressure vessel and a feed water, for measuring the reactor core flow rate at a high accuracy to confirm the state of flow rate. That is, a temperature measuring portion receives the output of the temperature detector and calculates the temperature of the coolants. A difference pressure measuring device receives the output from the difference pressure detector disposed at each of the portions in the reactor and calculates the pressure in the reactor. A mathematical processing section calculates the coolant flow rate in the reactor core based on the coolant temperature and the reactor pressure and transfers it to the reactor core flow rate control system. Further, the result of the calculation is informed and recorded in an output power device. With such a constitution, the temperature of coolants which as a mixture of the recycling coolants and the feedwater are directly detected and the flow rate at the inlet of the reactor core can be calculated based on the differential pressure at each of the positions in the lower portion of the reactor core. Accordingly, it is possible to accurately recognize the flow rate and confirm the state of the flow rate of coolants in the reactor. As a result, operation controllability and safety in the reactor can be improved. (I.S.)