Patent Number: 043158005
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, a boiling-water reactor incorporating therein the improvement provided by the present invention is generally designated by the numeral 2. The reactor 2 comprises a reactor core 4, a shroud 6 enclosing the reactor core 4, and a pressure vessel 8 surrounding the shroud 6, with a vertical annular space 10 being defined between the shroud 6 and pressure vessel 8. Installed in a lower portion of the vertical annular space 10 is means 12 for forcing a coolant to circulate within the reactor 2 through the reactor core 4 comprising a plurality of internal circulation pumps 14 arranged annularly. Each pump 14 includes, as shown on an enlarged scale in FIG. 2, a motor 16 located outside the pressure vessel 8. The drive force of each motor 16 is transmitted via a shaft 18 penetrating the pressure vessel 8 to an impeller 20 located within the pressure vessel 8. The impeller 20 is enclosed in a casing 22 serving concurrently as a diffuser which is sealingly secured to an opening 26 of an annular partition plate 24 sealingly secured to a lower portion of the vertical annular space 10. The numerals 28, 30 and 32 designate a main steam conduit, a feedwater conduit and control rod housings, respectively. The construction described hereinabove is commonly installed in a boiling-water reactor of the type provided with means for forcing the coolant to circulate within the reactor through the reactor core. The steam produced is passed through the main steam conduit 28 to a turbine, not shown, and returned to the pressure vessel 8 via the feedwater conduit 30 after doing work in the turbine. As indicated by arrows in FIGS. 1 and 2, the coolant returned to the pressure vessel 8 flows downwardly in the vertical annular space 10 and is forced to flow into below the reactor core 4 by the internal circulation pumps 14, and then flows between the control rod housings 32 into the reactor core 4 so that the coolant flows through the reactor core 4 in forced circulation. According to the principle of the invention as above mentioned, the coolant forced circulation means 12 further comprises guide means 34 for vertically guiding a flow of a coolant being sucked into each pump 14 to increase the effective inertia of the coolant flow. The guide means 34 of the embodiment shown comprises a plurality of tubular guide members each located adjacent a suction port of one of the pumps 14 and extending substantially coaxially with the pump 14 in a vertical direction. More specifically, each tubular guide member 34 is circular in shape and includes an upper end 36 and a lower end 38 having an inner diameter D which is substantially equal to the diameter of an inlet 40 of the casing 22 serving as the suction port of the pump 14. The tubular guide member 34 is secured at portions thereof near its upper end 36 to the shroud 6 and pressure vessel 8 by welding or other suitable means through a pair of supports 41 and at its lower end 38 to the upper end of the casing 22 by welding or other suitable means. In case the pumps 14 are of the type having a diffuser mounted separately from the casing 22 between the impeller 20 and casing 22, the tubular guide member 34 may be secured to either the casing or the diffuser. In this case also, the inner diameter of the upper end and the lower end of the tubular guide member 34, particularly of its lower end, is preferably substantially equal to the diameter of the suction port of each pump 14, in order that the coolant may flow smoothly and no adverse influences may be exerted on the enhancement of the effective inertia of the coolant flow effected by the coolant forced circulation means. Preferably, the tubular guide member 34 has a length L which is about two (2) to ten (10) times the diameter D of the suction port of the pump 14 or the inlet 40 of the casing 22 when the length Lo of the fluid guide or the casing 22 is added to the length of the tubular guide member 34. Generally, the lower limit of the length of the tubular guide member 34 may be determined by the degree to which the tubular member 34 of a certain length can accomplish the object of increasing the effective inertia of the coolant flow as desired and the upper limit thereof may vary depending on whether or not the tubular guide member 34 of a certain length physically interferes with other structures in the reactor. In the embodiment shown and described herein, the tubular guide members 34 are all in the form of a venturi tube. Each venturi tube 34 is tapped at 46 and 48 in a portion thereof near its upper end 36 and in a throat 44 and connected to outlet tubes 50 and 52 respectively. The outlet tubes 50 and 52 are sealingly fitted, in the same manner as is usually done in other measurement systems of a nuclear reactor, to nozzles 54 and 56 which are in turn sealingly welded to the pressure vessel 8 at 58 and 60 respectively. The outlet tubes 50 and 52 are connected, at ends thereof which are not shown, to a measurement system outside the reactor. Thus it is possible to measure the delivery made by each circulation pump 14 based on the presure differential between the taps 46 and 48, hence it is possible to determine the flow rate of the coolant flowing through the reactor core 4 by calculating the total of the deliveries made by all the internal circulation pumps 14. The tubular guide member 34 may, of course, be in the form of a straight tube when there is no need to assign the function of flow rate measurement thereto. In the embodiment shown and described hereinabove, the tubular guide member 34 of the coolant forced circulation means 22 performs, according to the principle of the invention as above mentioned, the function of increasing the effective inertia of the coolant flow being sucked into the pump 14 by substantially increasing the length of the flow guide of each pump 14, to thereby enhance the stability of the flow conditions of the coolant flowing through the reactor core 4 in the reactor 2. FIG. 3 is a graph showing a time constant governing the dynamic characteristics of a flow of a fluid in relation to the fluctuation reduction ratio X.sub.2 /X.sub.o. Our analysis shows that whereas the time constant of the flow of a coolant flowing through each internal circulation pump when the coolant forced circulation means of the prior art having only internal circulation pumps is used is about 0.06, the corresponding value in the present invention in which the coolant forced circulation means further includes tubular guide members attached to the internal circulation pumps is about 0.12 to 0.6 when the length L of the tubular guide member is 2 to 10 times of the diameter D of the suction port of the pump including the length Lo of the fluid guide of the pump. Thus it has been ascertained that the fluctuation reduction ratio is greatly reduced and the flow conditions of the coolant flowing through the reactor core is stabilized even if disturbance occurs. From the foregoing, it will be understood that according to the invention, the provision of means for vertically guiding over a predetermined distance a flow of the coolant being sucked into each internal circulation pump permits increase in the effective inertia of such coolant flow so as to increase the time constant of the flow, thereby to enhance the stability of the flow conditions of the coolant flowing through the reactor core, which provides a nuclear reactor of high operation characteristics. It will be also understood that the use of venturi tubes for the guide means permits the provision of useful and effective means for measuring the flow rate of the coolant flowing through the reactor core.