Patent Number: 048428060
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the space in a reactor pressure vessel 11 is divided into inner and outer spaces by a cylindrical shroud 13 supported at the lower end thereof by a plurality of shroud support legs 12, and a fuel assembly, not shown, constituting a reactor core is arranged in the inner space which is inside the shroud 13. The fuel assembly is supported by a core support plate 14. A plurality of internal pumps 15, each provided with impellers 15a and a motor 15b for driving the impeller 15a in rotation are mounted at the bottom portion of the pressure vessel 11 which is outside the shroud 13. A primary coolant (reactor coolant) is recirculated in the pressure vessel 11 in the directions shown by arrows in FIG. 1 by the operation of the internal pumps 15. The shroud support legs 12 are disposed at a plurality of spaced apart positions between the lower end of the shroud 13 and the bottom portion of the pressure vessel 11 thereby to support the lower portion of the shroud 13, thus improving the earthquake-proof strength of the shroud. The primary coolant flows through the spaces between the respectively adjacent shroud support legs 12 towards the reactor core. Differential pressure detectors 16a and 16b provided with pressure receiving surfaces are provided, respectively, on the upstream and downstream sides of the shroud support legs 12. The differential pressure detectors 16a and 16b are operatively connected to a pressure difference transmitter 17 which detects any pressure difference caused by the shroud support legs 12 as the pressure difference at the shroud support leg, converts this pressure difference value into an electrical signal representing the pressure difference and outputs the thus obtained signal. The outputted signal from the pressure difference transmitter 17 is inputted into a flow rate computing means 18 which thereupon computes the recirculating flow rate (pumping flow rate). The other differential pressure detectors 19a and 19b are respectively positioned at upstream and downstream positions of the core support plate 14 to detect the pressure difference caused by the core support plate 14 as a pressure difference at the core support plate. The differential pressure detectors 19a and 19b are operatively connected to a pressure difference transmitter 21 which converts the detected pressure difference value into a signal representing the pressure differencee at the core support plate and then outputs the thus obtained signal. The electric output signal generated from the pressure difference transmitter 21 is inputted into a flow rate computing means 22 in which the recirculated flow rate (core flow rate) can be computed. The recirculating flow rate measuring device of the organization described above for measuring the core flow rate is installed for the purpose of attaining an auxiliary function of the recirculating flow rate measuring device for measuring the pumping flow rate. The circulating flow rate measuring device of the organization described hereinbefore operates as follows. When the internal pump 15 operates, the primary coolant flows in the pressure vessel 11 in the arrow direction. At this time, the shroud support legs 12 act as flow path resistance means thereby to create the pressure difference between the upstream and downstream positions of the shroud support legs 12. The pressure difference is detected by the pressure difference detectors 16a and 16b, and the detected values are converted by the pressure difference transmitter 17 into the electric signals representing the pressure differences. The converted signals are inputted into the flow rate computing means 18 in which the recirculating flow rate of the primary coolant is computed and measured. During these operations, since the shroud support legs 12 are located at positions apart from the fuel assembly, not shown, constituting the reactor core, the primary coolant passing through the respective shroud support legs 12 constitutes a single phase, i.e., a liquid phase, whereby the coolant flow is evenly maintained. In addition, in such a case, it is experimentally found that the relationship between the pressure difference between the upstream and downstream positions of the flow resistance and the flow rate passing these positions is maintained constant. In other words, as shown in FIG. 3, the relationship between the flow rate and the pressure difference is represented by the following equation. ##EQU1## in which: Q designates the flow rate; .DELTA.p is the pressure difference; .gamma. is the specific gravity of the fluid; and .alpha. is the coefficient of flow rate. The coefficient .alpha. is experimentally known to be constant with no relation to the operational conditions of the reactor and the individual characteristics of the internal pumps used. As described above, according to this embodiment of the invention, the recirculating flow rate (pumping flow rate) of the primary coolant can be measured easily and accurately merely by measuring the pressure difference of the primary coolant between the upstream and downstream sides as it passes in liquid phase between the respective shroud support legs 12. In addition, the flow rate measurement can be performed more accurately by combined use of a recirculating flow rate measuring device which measures the flow rate passing the reactor core. FIG. 2 is a vertical sectional view of another embodiment of the recirculating flow rate measuring apparatus according to this invention, in which a rectifying lattice 23 adapted to rectify the suction flow of the internal pump 15 is installed between the pressure vessel 11 and the shroud 13 on the upstream side of the internal pump 15. The rectifying lattice 23 is constituted by combining, in the form of a lattice, fine elongated plate members, by a flat plate with a number of perforations, or by a combination of the members of these two types. Differential pressure detectors 24a and 24b are disposed at positions on the upstream and downstream sides of the rectifying lattice 23 disposed therebetween to detect the pressure difference between the upstream and downstream sides of the rectifying lattice 23 caused thereby. The differential pressure detectors 24a and 24b are operatively connected to a pressure difference transmitter 25 which converts the detected pressure difference into an electrical signal which is then outputted therefrom. The pressure difference transmitter 25 is further connected to a flow rate computing means 26 which computes the recirculating flow rate (pumping flow rate) in response to the transmitted electric signal representing the pressure difference. Similarly as in the preceding embodiment of the invention, the recirculating flow rate measuring device accomplishes an auxiliary function, being provided with pressure detectors 19a and 19b disposed at upstream and downstream positions of the core support plate 14 so as to detect the pressure difference caused thereby, a pressure difference transmitter 21 for converting the detected pressure difference into an electrical signal, and a flow rate computing means 22 for computing the core flow rate. As described hereinbefore, according to this embodiment of the invention, since the rectifying lattice 23 is positioned apart from the fuel assembly (not shown) constituting the reactor core, and the primary coolant passing through the rectifying lattice 23 constitutes a single phase, i.e., a liquid phase, a relationship as shown in FIG. 3 is established between the pressure difference and the flow rate, and the relationship represented by the equation (1) is also applicable. The recirculating flow rate (pumping flow rate) can thus be measured easily and accurately merely by measuring the pressure difference of the primary coolant in the liquid phase between the upstream and downstream sides of the rectifying lattice 23. As will be understood from the foregoing description with respect to preferred embodiments of this invention, since the position of the flow passage resisting means in the recirculating flow path of the primary coolant in a reactor pressure vessel causes a pressure difference of the primary coolant between the upstream and downstream sides of the flow resisting means, the relationship between the pressure difference and the flow rate can be determined constantly. Accordingly, the detection of this pressure difference makes possible easy and exact measurement of the recirculating flow rate of the primary coolant. In another aspect, according to this invention, the shroud support legs or a rectifying lattice, which are ordinarily located in the reactor pressure vessel, are utilized as a flow passage resisting means, so that special members or equipment are not required additionally for measuring the flow rate of the primary coolant. Thus the measuring means is compact and easily installed in an existing nuclear reactor.