PURPOSE: To obtain a device which can measure the flow rate in the core of a nuclear reactor accurately by providing an internal pump in the coolant passage from the lower part in the clearance between a pressure vessel and a moderator tank to a lower plenum and providing a measuring device for flow rate of coolant near said pump.

CONSTITUTION: A perpendicular clearance 16 is provided between a pressure vessel 10 having a core part in the central part and a moderator tank 11, and the coolant is circulated from the passage to a lower plenum as shown by an arrow by means of an internal pump 10 having an impeller 18 provided in the coolant passage 17 below said clearance 16. A temp. measuring part 38 and a comb-tooth-like sensing part 36 of Pitot tube construction consisting of full pressure sensing ports 39 and static pressure sensing ports 40 are provided in the inflow port of the impeller 18. The outputs thereof are converted and calculated through a pressure transmitting part 37, whereby the temperature-corrected flow rate of the coolant is determined. The parts 36 are provided in plurality in the circumferential direction of the vessel 10. Since the device is constituted in such a way, the flow speed are determined with the parts 36 and further from the distributions in the flow speed in the respective sensing parts, the distributions of the flow rates of the coolant flowing in the core part are determined.