Patent Number: 043137946
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1, therein is depicted a hydraulically supported neutron absorber element column designated generally as 10. The column depicted incorporates the valve of the present invention in a particularly preferred application, namely, in a nuclear reactor of the type which utilizes a plurality of hydraulically supported neutron absorbing elements. The column will have an outside configuration substantially the same as the fuel elements which are placed in the core of the nuclear reactor. The reactor core generally will comprise a plurality of fuel elements, absorber columns, and control rods, which also are made up of neutron absorbing material. Column 10 comprises a housing 12 containing therein lower and upper grates or aperture plates 14 and 16, respectively, which define a retention zone 18 which contains a body or stacked bed of neutron absorbing elements 20, which are depicted in their position during normal reactor operation. Specifically, as described more fully in U.S. Pat. No. 4,076,583, the stacked bed of neutron absorbing elements is supported by hydraulic fluid above and out of the reactor core zone. Also located in housing 10 and retention zone 18 is a fluid bypass tube 22 provided with a plurality of apertures or openings 24 to permit a portion of the fluid passing into the stacked bed of neutron absorbing elements 20 to bypass the bed thus ensuring that the elements are reliably maintained out of the core zone and minimizing the pressure drop through the stacked bed. Located above the neutron absorbing elements is the valve of the present invention which is circled as detail 2 and which will be described with more particularity with reference to FIG. 2 which shows the valve in an enlarged view and in greater detail. The fluid passing through the valve, shown in its open normal operating position, passes upwardly through housing 12 and out the upper end thereof. Also included in housing 12 is a weighted number 26 for mechanically moving the valve of the present invention from an open to a closed position or vice versa. Weighted member 26 is retained in position by an electromagnet 28 and a curie point alloy magnet 29 which are affixed to an elongated rod 30, the two acting cooperatively together with rod 30 during normal operation to move weighted member 26 and open or close the valve of the present invention. In the event that the fluid passing therethrough exceeds a certain maximum desired temperature, such as in the event of a transient overpower, curie point alloy magnet 29 will automatically release weighted member 26 which will in turn close the valve of the present invention causing the neutron absorbing elements to rapidly drop into the reactor core shutting down the reactor. Alternatively, electromagnet 28 can be deenergized to release weighted member 26. Located adjacent a lower end of bypass tube 22 and below aperture plate 14 there advantageously also is provided a self-lifting flow cutoff valve 32 provided with a drag plate 34. During normal operation, the fluid flowing past valve 32 acts upon drag plate 34 to lift the valve from an open position to a closed position whereby substantially all the fluid flows through aperture plate 12. In the event the fluid flow drops below a predetermined point, the valve drops back to an open position permitting fluid flow through apertures (not shown) in the lower end of bypass tube 22. This in turn permits fluid displaced by the descending column of neutron absorbing elements to flow in through the bypass tube and out through the bottom apertures (not shown) whereby neutron absorbing elements fall into the core zone more rapidly than they would without the addition of flow valve 32. The required area of the drag plate is, of course, a function of the desired actuation flow rate and weight of the valve. The area may be determined mathematically or through experimentation. In a preferred embodiment, the sealing arrangement in flow valve 32 is substantially an inverted image of the sealing means of the valve of the present invention which will be described more fully in the following paragraphs. Referring now to FIG. 2, therein is depicted the valve of the present invention in an enlarged sectional view. For convenience in understanding the invention, in this view the valve is shown in a closed position, for example, as if having been moved to the closed position by weighted member 26 acting along the dotted lines. In the embodiment depicted, housing 12 forms a part of the valve of the present invention. Located within housing 12 is a substantially vertical elongated nozzle assembly 36 comprising an inlet member 38 affixed to the housing 12 by, for example, a plurality of threaded fasteners 40. Nozzle assembly 36 further includes a substantially vertical nozzle housing 42 provided with a plurality of apertures 44 adjacent its top end. Nozzle assembly 36 includes sealing means located above and below the apertures 44 such as the downwardly outwardly extending sealing surfaces 46 on nozzle housings 42 and 48 on adjustment ring 50, respectively, the latter of which is threadedly attached to nozzle housing 42 to provide for vertical adjustment. Also located in housing 12 is flow cutoff sleeve assembly 52 having walls 54 surrounding nozzle housing 42. The upper portion of walls 54 are provided with at least one fluid flow opening 56. Flow cutoff sleeve assembly 52 further includes two sealing means 58 and 60, one located below the flow opening 56 and the other located adjacent the lower end of wall 54. The two sealing means comprise radially inwardly and upwardly extending surfaces. It will be seen from the drawing that in accordance with the present invention when the valve is in a closed position, sealing means 46 and 58 and sealing means 48 and 60 act cooperatively to provide for the exposure of a greater area for fluid pressure to exert force in a downward direction than is exposed for fluid pressure to exert force in an upward direction, whereby once the valve is in a closed position, an increase in fluid pressure will act to maintain said valve in a closed position. Cutoff sleeve assembly 52 also includes a balance member 62 which, when said sleeve is in the open position, contacts an apertured plate 64 having a plurality of apertures 66 to provide a flow area for the flow of fluid therethrough. When in contact, balance member 62 obstructs or covers a sufficient flow area such that when fluid is flowing through the housing there is provided a pressure drop across the balance member, said pressure drop being just sufficient to maintain the cutoff sleeve in an open position at a predetermined minimum flow rate. The flow area which should be obstructed by balance member 62 is readily determinable in accordance with the following equation: ##EQU1## where W is the weight of the sleeve assembly, a is the unobstructed flow area through the apertures, Q is desired minimum flow rate at which the valve should close and C.sub.f is the nozzle coefficient of the apertures. The nozzle coefficient will be a function of, among other things, the shape of the apertures and the thickness of the plate. Its value is readily determinable by one skilled in the art through routine experimentation. Advantageously, the valve of the present invention further includes some mechanical means for moving the flow cutoff sleeve between an open and closed position. An exemplary type of mechanical means illustrated in FIG. 2 is depicted as link member 68, which is slidingly connected to weighted member 26 to provide mechanical movement between an open and closed position. The weighted member has sufficient weight to overcome the retention force provided by the pressure drop across balance member 62. In a preferred embodiment of the invention, there also is provided a piston member 70 which is located above and attached to nozzle housing 42. In operation as flow cutoff sleeve assembly 52 moves from an upward open position to a downward closed position, the fluid contained therein is readily displaced out through flow openings 56 until such time as the uppermost surface of piston member 70 is past the uppermost opening, at which time the flow area for displaced fluid is reduced to a desired minimum value determined by the clearance between piston member 70 and inside diameter of sleeve walls 54 of flow cutoff assembly 52. A valve in accordance with the present invention was constructed and installed in a test neutron absorber column subassembly, substantially as depicted in FIGS. 1 and 2. The invention was tested by reducing the flow of fluid through the subassembly at various rates and measuring the time required for substantially all of the neutron absorbing elements to drop into the core zone, both with and without the valve of the present invention. It was determined that the time for all of the elements to fall into the core zone was substantially constant (from about 4 to 6 seconds) using the valve of the present invention. Without the valve, the time varied considerably depending upon the rate at which flow was reduced. Indeed, in some instances, where the flow was slowly reduced to zero, the time required for all of the elements to drop into the core zone was as long as 20 seconds or more. Thus, this example clearly demonstrates the utility of the present invention. In addition, it is difficult to design a neutron absorber column subassembly in such a manner that the elements will consistently drop into the core zone until the flow is reduced below about 50% of the design operating flow rate. With the present invention, it is readily possible to design the balance member such that the valve will close at any desired minimum flow rate. Further, the closing point is highly repeatable, thus further demonstrating that the present invention provides a reliable self-actuating valve. While the invention has been described with reference to a particular preferred embodiment, it will be readily apparent that it would have utility in other areas wherein a self-actuating, self-locking valve is desired. In addition, in some instances it may be desirable to provide some resilient material on the sealing surfaces to ensure substantially zero leakage around those surfaces. However, in the particularly preferred embodiment, some leakage is not only acceptable but actually is desired. Specifically, when the invention is used in conjunction with a reactor, there may be a considerable amount of decay heat by the absorber elements and it is desirable to maintain some fluid flow through the column even though the valve is in a closed position. In such instance it is readily feasible to design the sealing surfaces such that there is a desired amount of leakage therethrough. Numerous other advantages and variations of the invention will be readily apparent to those skilled in the art. Accordingly, the scope of the invention should be determined not by the illustrative embodiments depicted, but rather by the appended claims.