Patent Number: 048428060
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a device for measuring the recirulating flow rate of the primary coolant (reactor coolant) in a boiling water reactor (BWR) in which an internal pump is installed on the bottom of a reactor pressure vessel (called hereinafter an internal pump type reactor), particularly for measuring the flow rate of the primary coolant precisely and easily. In one typical example of a known device for measuring primary coolant recirculating flow rate of a BWR, differential pressure detectors are installed on the upstream and downstream sides of a core support plate which supports a fuel assembly in the reactor, and the pressure difference caused by the core support plate is detected by a pressure difference transmitter as a pressure difference at the core support plate thereby to measure the recirculating flow rate (core flow rate). In the recirculating flow rate measuring device of this prior type, the core support plate constitutes a flow path resistance, and the flow rate is obtained or calculated from the pressure difference between the upstream side and the downstream side of the core support plate. With a device of the type described above, however, since the pressure difference at the core support plate is measured as a pressure including a pressure difference in the interior of the fuel assembly, when two phase flow conditions of the liquid phase and the gas phase in the fuel assembly vary in accordance with the reactor output, the resisting value also varies, and accordingly, the flow rate cannot be exactly measured. In another recirculating flow rate measuring device which has been proposed in this field, differential pressure detectors are located on the upstream and downstream sides of an internal pump to detect the pressure difference caused by the location of the internal pump by a pressure difference transmitter as a pressure difference at the internal pump, and in which the rotational speed of the internal pump is detected by a pulse converter. The recirculating flow rate (pump flow rate) is then measured by the combination of the pressure difference at the internal pump and the rotational speed thereof. With the flow rate measuring device of the latter mentioned type, however, it is generally necessary to preliminarily determine the relationship between the pressure difference at the internal pump and the rotational speed thereof and the flow rate, but this relationship is often different with the internal pump used. Moreover, even with the same internal pump, the relationship for determining the flow rate may be different depending on the operational conditions thereof. Furthermore, with a device of this character, since the pressure difference at the internal pump and the rotational speed thereof are taken as factors to be inputted, the construction of the device is made complicated, and the accurate measuring of the flow rate cannot be attained. As still another recirculating flow rate device, the prior art provides another type in which a flow meter of general type such as a Venturi tube is installed in the reactor (for example, refer to Japanese Patent Laid-Open Publication No. 144594/1980), but a device of this type involves drawbacks of complicated structure and increasing of the flow resistance of the primary coolant. SUMMARY OF THE INVENTION An object of this invention is to overcome the problems encountered in the prior art technique and to provide a device of simple construction for accurately measuring the recirculating flow rate of the primary coolant in a nuclear reactor. Another object of this invention is to provide a device for measuring the recirculating flow rate of the primary coolant recirculated in a reactor pressure vessel by an internal pump, the device comprising, in specific combination, flow path resistance means, differential pressure detectors, a differential pressure transmitter, and a flow rate computing device. A further object of this invention is to provide a recirculating flow rate measuring device additionally provided with an auxiliary equipment for measuring the recirculating flow rate of the primary coolant in a reactor core. These and other objects can be attained according to this invention by providing a device for measuring the recirculating flow rate of the primary coolant recirculating in a reactor pressure vessel by means of an internal pump located at the bottom of the reactor pressure vessel, the device comprising a flow path resistance means disposed in the recirculating flow path of the primary coolant in the reactor pressure vessel and adapted to cause a pressure difference between an upstream part and a downstream part of the primary coolant caused to flow at constant flow rate, detectors positioned at an upstream position and a downstream position of the flow path resistance means so as to detect the pressure difference between the upstream and downstream positions of the primary coolant flow, a transmitter operatively connected to the detectors for converting the pressure difference value into an electric signal representing the pressure difference and transmitting the thus converted signal, and a device operatively connected to the transmitter for computing the recirculating flow rate of the primary coolant in response to the signal from the transmitter. In a preferred embodiment of the invention, the flow passage resisting means comprises a plurality of support legs which are disposed between the bottom of the reactor pressure vessel and a lower end of a cylindrical shroud disposed within the reactor pressure vessel to partition the interior thereof and adapted to support the lower end of the shroud, and the differential pressure detectors are installed respectively at the upstream and downstream sides of the shroud support legs which are interposed therebetween. In one modification of the preferred embodiment of the invention, the flow path resistance means comprises a rectifying lattice positioned at an upstream position of the internal pump and adapted to rectify the suction flow of the internal pump, and the differential pressure detectors are disposed respectively at the upstream and downstream sides of the rectifying lattice which is interposed therebetween. Furthermore, in another preferred embodiment of the invention, the device further comprises an auxiliary measuring means for measuring the core flow rate of the primary coolant, the auxiliary measuring means comprising detectors disposed at upstream and downstream sides of a core support plate and adapted to detect the pressure difference of the primary coolant therebetween, a transmitter operatively connected to the detector and adapted to convert the detected pressure difference value into an electric signal and transmitting the thus converted signal, and a device operatively connected to the transmitter and adapted to compute the core flow rate in response to the signal from the transmitter. These and other objects, features, and advantages of this invention will become clear from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.