Patent Number: 043137946
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention Broadly, the present invention relates to a self-actuating and self-locking flow cutoff valve. It particularly relates to use of such a valve in a nuclear reactor of the type which utilizes a plurality of fluid supported absorber elements to provide for the safe shutdown of the reactor. 2. Prior Art There are numerous applications wherein there is a need for a self-actuating, self-locking flow cutoff valve. The need is particularly great in the case of nuclear reactors of the type which utilize a plurality of fluid supported neutron absorber elements to ensure the capability for a safe shutdown of the reactor. More particularly, heretofore nuclear reactors were typically shut down by control rods which were introduced through the top of the core and raised from or lowered into the core by mechanical means such as a motor which operates via clutch gears or the like. In an emergency, the clutch would be disengaged and the control rods allowed to fall into the core to shut down the reactor. Such a system had certain disadvantages. Specifically, there is a possibility that a mechanical device such as the clutch could not be disengaged or that some fault may have occurred which would distort the passage through which the control rods have to pass causing them to bind and preventing full insertion of the rods into the core. In such instance, it would not be possible to shut down the reactor. Accordingly, considerable interest has been generated in the use of a plurality of fluid supported neutron absorbing elements which would fall under the influence of gravity into the core in the event of a loss of fluid flow. Thus the reactor could be shut down by the simple expedient of shutting off the flow and further, in the event of an unforeseen loss of fluid flow, the reactor also would be shut down automatically. U.S. Pat. No. 3,228,847 suggests a reactor control system which includes a control assembly for controlling neutronic flux. The control assembly comprises an inner tube extending from a nonactive region of the reactor into the active region, and an outer tube surrounding the inner tube and spaced therefrom. The outer tube has a closed end and the inner tube has an open end adjacent and spaced from the closed end of the outer tube. Neutron absorbing particles are positioned between the inner and outer tube for movement along the tube under the force of flow. The neutron absorbing particles are moved out of the active region of the reactor by fluid flow and fall back into the active region under the influence of gravity when the flow is shut off. U.S. Pat. No. 3,257,286 suggests a ball-type control for a nuclear reactor. A number of elongated conduits are positioned in the nuclear reactor so that the first section of the conduit is located within the core and an adjoining second section is located exteriorly of the core. Each conduit holds a number of individual bodies, each of which contains a high neutron absorption cross-section material. The movement of the neutron absorber bodies within the conduits is achieved by providing a source of pressurized fluid available to each end of the conduit for selectively positioning the neutron absorber bodies within the first and second sections of the conduit. It is stated that a fission reactor can be started up, shut down, or reactivity controlled during reactor operations by varying the location of the absorber bodies. U.S. Pat. No. 3,347,747 discloses a control organization and method for a nuclear reactor. The reactor is provided with a number of laterally spaced vertical passageways in the region of the core and distributed throughout the area thereof. The passageways include a lower portion which extends generally throughout the height of the core and an upper portion which extends above the core into the reactor vessel. Positioned within and confined in each passageway is a movable means which contains a poison and which is movable from a lower position within the region of the core to an upper position in the passageway, where it is generally above the core. The poison-containing means is moved by gravity to its lower position and is moved from its lower to its upper position by means of a fluid which is directed upward in the passageway. In U.S. Pat. No. 4,076,583 there is disclosed another control method for a nuclear reactor which comprises a plurality of elongated conduits extending through and above the core of a reactor. A plurality of neutron absorber elements are located within the conduit, and during normal operation form a stacked bed in the portion of the conduit extending above the core. That section of the conduit in which the stacked bed is formed is provided with a fluid bypass means, it having been found that such bypass means ensures the capability of reliably maintaining all of the absorber elements in the stacked bed and out of the core during normal operations and further minimizing the pressure drop of fluid flowing through the stacked bed during normal operation. While all of the foregoing suggested techniques appear to offer advantages over reliance solely on a control rod system, there is still room for improvement. More particularly, in all of these systems where gravity is relied upon to cause the absorber elements to move into the core, any residual fluid flow, even though it may be below the minimum for safe operation of the reactor, acts to retard the fall of the absorber elements. For example, in the event of a complete power failure, the inertia of a centrifugal pump would be sufficient to continue providing some flow after the loss of power and after the flow rate of fluid had dropped below the point at which the reactor should be shut down. Thus, clearly it would be advantageous to have a self-actuating flow cutoff valve in the fluid stream such that once the fluid flow dropped below a predetermined point, the valve would automatically close and substantially reduce the time required for the neutron absorbing elements to fall into the reactor core and safely shut it down. Further, in the event that there might be some erratic flow or surge of pressure or flow subsequent to it having declined below the safe level, such valve advantageously would be self-locking to prevent an inadvertent startup of the reactor by a resumption of fluid flow. SUMMARY OF THE INVENTION The present invention provides a flow cutoff valve which is self-actuating and once in a closed position is self-locking. The present invention is particularly applicable to a nuclear reactor of the type which utilizes a plurality of fluid supported absorber elements to provide for the safe shutdown of the reactor. Broadly, the invention comprises a substantially vertical elongated housing having an apertured plate located therein, the apertures providing for fluid flow from one end of the housing to the other. A substantially vertical elongated nozzle member also is located in and affixed to the housing. The nozzle member has an opening in its bottom end for receiving fluid and apertures adjacent the top end for discharging fluid, and two sealing means comnprising radially outwardly and downwardly extending sealing surfaces, one located above and the other below the apertures. The nozzle member is surrounded by the walls of an elongated flow cutoff sleeve having a fluid opening adjacent its upper end. The sleeve also includes two sealing means comprising radially inwardly and upwardly extending sealing surfaces affixed to it, one below the flow opening and one adjacent the lower end of the sleeve. The sleeve is movable from an upper open position wherein the nozzle apertures are substantially unobstructed to the flow of fluid therethrough and a closed position wherein the sleeve and nozzle sealing surfaces are mated and the mated sealing surfaces and the walls of the sleeve obstruct the flow of fluid through the apertures. In addition, the nozzle and sleeve sealing means cooperatively act together to provide for the exposure of a greater area for fluid pressure to exert a force in a downward direction than is exposed to fluid pressure to exert a force in an upward direction whereby once said valve is in a closed position, any increase in fluid pressure will act to maintain the valve in a closed position. The valve further includes a balance member located above and attached to the flow cutoff sleeve. The balance member contacts the apertured plate when the sleeve is in an open position and obstructs the flow of fluid through a predetermined flow area of the apertures in the apertured plate to produce a pressure drop across the apertured plate and balance member; the pressure drop being just sufficient to support the balance member and flow cutoff sleeve at a predetermined minimum fluid flow. Thus, when the fluid flow drops below a predetermined value, the pressure drop across the balance member will be insufficient to maintain the flow cutoff sleeve in an open position and it will move under the influence of gravity to a closed position. In one embodiment of the invention, the cutoff valve further includes a piston member extending upwardly from the top end of the nozzle member and into the flow cutoff sleeve a sufficient distance such that when the sleeve moves from an open to a closed position, the uppermost portion of the piston member moves past the flow opening in the flow cutoff sleeve and the fluid trapped therein provides a dampening force on the closure of the flow cutoff sleeve to prevent or minimize any hydraulic shock. In yet another embodiment of the invention, the valve further includes a mechanical means for moving the flow cutoff sleeve between an open and closed position. The various features of the novelty which characterize the invention are pointed out with particularity in the claims which form part of this specification. For a better understanding of the invention, its operating advantage and specific objects attained by its use, reference should be made to the accompanying drawings and descriptive matter in which are illustrated and described, respectively, certain preferred embodiments of the invention.