Patent Number: 043158005
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors, and more particularly to the improvement of a nuclear reactor of the type which is provided with internal pumps for forcing a coolant to circulate within the reactor through a reactor core. The aforesaid type of nuclear reactor comprises a shroud enclosing the reactor core, and a pressure vessel surrounding the shroud, with a vertical annular space being defined between the shroud and the pressure vessel. Installed in a lower portion of the vertical annular space are a plurality of internal circulation pumps for forcing a coolant in the vertical annular space to flow into below the reactor core so that the collant flows through the reactor core in forced circulation. One of the problems encountered with regard to the aforesaid type of reactor is that the flow conditions of the coolant flowing through the reactor core tend to become unstable. For example, if the flow of the coolant in the reactor core is influenced by some disturbance, the flow rate of the coolant will tend to fluctuate, and once fluctuation of the flow rate occurs the flow rate repeatedly increases and decreases before stable flow conditions are restored after a prolonged period of fluctuation. Reactors belonging to the aforesaid type include those of different constructional arrangements, and in some of them difficulties have been experienced in obtaining measurements of the flow rate of a coolant flowing through the reactor core due to the positional relation of equipment arranged in the pressure vessel and elsewhere. SUMMARY OF THE INVENTION A principal object of this invention is to improve the stability of the flow conditions of a coolant flowing through a reactor core of a nuclear reactor of the type which is provided with internal pumps for forcing a coolant to circulate within the reactor through the core. Another object is to provide, in a nuclear reactor of the type described, means for effectively measuring the flow rate of the coolant flowing through the reactor core. The present invention is based on the following principle. In a nuclear reactor of the type provided with internal pumps for forcing the coolant to circulate within the reactor through the core, as above mentioned, it has been experienced that the flow conditions of the coolant flowing through the core is apt to become unstable, which is a phenomenon inherent in this type of nuclear reactor. It has been assumed that the phenomenon is attributed to a reduction in time constant T and an increase in gain C in the following equation (1) representing dynamic characteristics of the flow of a coolant through an internal pump which are caused by some disturbance affecting the flow of the coolant: ##EQU1## where .DELTA.W: change in flow rate. .DELTA.P: change in pressure. PA1 C: gain. PA1 T: time constant. PA1 S: Laplace arithmetic unit. PA1 .DELTA.Lo/A: effective inertia of coolant being sucked into internal pump. PA1 Lo: length of fluid guide of internal pump. PA1 A: sectional area of suction portion of internal pump. PA1 K: constant. Of all the factors governing the dynamic characteristics of the coolant in equation (1), we have observed the time constant T and carried out its analysis. As a result, it has been found that the time constant T is in inverse proportion to a pressure loss occurring when the coolant flows through each internal circulation pump and in proportion to the effective inertia of the coolant being sucked into each pump. In other words, it has been found that the time constant T in equation (1) can be expressed by equation (2) as follows: ##EQU2## where P.sub.p.o : pressure loss in internal pump section. Thus it has been turned out that in order to stabilize the flow conditions of a coolant flowing through the core of a nuclear reactor provided with internal pumps for forcing the coolant to circulate within the reactor through the core, one has only to increase the length Lo of the fluid guide of each internal circulation pump, to thereby increase the effective inertia of the coolant being sucked into the internal pump. Based on this principle, the invention provides, in a nuclear reactor of the type described, means for vertically guiding over a predetermined distance the flow of a coolant being sucked into each internal pump, to thereby substantially increase the length of the fluid guide of the pump and increase the effective inertia of the flow of the coolant. According to the invention, there is provided, in a nuclear reactor comprising a reactor core, a shroud enclosing the reactor core, a pressure vessel surrounding the shroud and defining a vertical annular space between the vessel and the shroud, and a plurality of internal circulation pumps disposed in a lower portion of the vertical annular space for forcing a coolant in the vertical annular space to flow into below the reactor core so that the coolant flows through the reactor core in forced circulation, the improvement comprising means for vertically guiding over a predetermined distance a flow of the coolant being sucked into each internal circulation pump to increase the effective inertia of the flow of the coolant, to thereby enhance the stability of flow conditions of the coolant flowing through the reactor core. Preferably, the predetermined distance referred to hereinabove is about two (2) to ten (10) times the diameter of a suction port of the pump when the length of the fluid guide is added to the predetermined distance. Preferably, the guide means includes a tubular guide member disposed adjacent the suction port of each internal circulation pump and extending substantially coaxially therewith in a vertical direction. Preferably, the inner diameter of upper and lower ends of the tubular guide member is substantially equal to the diameter of inlet of a casing of the internal circulation pump, and the tubular guide member is securely connected at its lower end to upper end of the casing. Preferably, the tubular guide member is a venturi tube and the improvement further comprises flow measurement means associated with the venturi tube for measuring the flow rate of the coolant flowing through the reactor core.