Patent Number: 052672859
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1-3, there is shown a typical nuclear reactor 2. Reactor 2 includes reactor vessel 4 having a generally cylindrical portion 6 and a hemispherical portion 8 on one end thereof. Disposed within reactor vessel 4 is main core support 10, which is positioned with respect to the interior side walls of reactor vessel 4 and secured into position by brackets 12. Reactor core 14 is disposed above main core support 10 and is supported thereby. Lower head, or lower plenum 16, is defined between the underside of main core support 10 and the inner surface of hemispherical portion 8 of reactor vessel 4. A secondary core support 18 is located in a lower, generally central portion of lower plenum 16. Secondary core support 18 includes secondary core support plate 20 and a plurality of secondary core support columns 22 extending downwardly from main core support 10 to secondary core support plate 20. In a preferred embodiment, four secondary core support columns are provided. Secondary core support 18 is intended to prevent reactor core 14 from impacting the bottom of reactor vessel 4 in the event of a failure of main core support 10. Generally annular downcomer 24 surrounds reactor core 14. The lower end of downcomer 24 extends around the outer edge of main core support 10 and is in communication with lower plenum 16. During operation of the reactor, coolant fluid, typically water, is pumped into the reactor through one or more cold legs 28. Each cold leg 28 is in communication with downcomer 24. The coolant fluid flows downward through downcomer 24 and into lower plenum 16. The coolant fluid flowing into lower plenum 16 circulates generally downwardly near the lower end of downcomer 24. The circulation becomes generally upwardly directed near the center of lower plenum 16. The coolant circulating within lower plenum 16 flows into a plurality of reactor core coolant inlet openings 30 located on the underside of main core support 10. Reactor core coolant inlets 30 extend through main core support 10 and are in communication with core 14. Coolant inlets 30 transmit the coolant fluid into core 14 to cool fuel assemblies 32. Upon passing through core 14, the heated coolant fluid is discharged from reactor vessel 4. During normal operation, the reactor coolant system described above will be completely filled with coolant fluid. The fresh coolant flowing into the system through cold legs 28 maintains the coolant flow rate and pressure within the system. It is important that an evenly distributed coolant flow and uniform coolant pressure be provided to reactor core coolant inlets 30. As discussed above, non-uniform coolant flow and pressure across reactor core coolant inlets 30 may result in uneven cooling of fuel assemblies 32 during operation. The apparatus of this invention includes generally planar vortex-suppressing plate 40 which is suspended below main core support 10 in lower plenum 16. Plate 40 is oriented generally parallel to main core support 10. A plurality of support columns 42 connect plate 40 to the underside of main core support 10. Support columns 42 extend from the upper surface of plate 40 to the underside of main core support 10. The ends of core support columns 42 are preferably connected to plate 40 and to main core support 10 by bolting or welding, however, any suitable fastening means may be used. In a preferred embodiment, eight support columns 42 are provided. The eight columns 42 are preferably angularly spaced around an outer circumferential portion of plate 40 and attach to main support 10 between reactor core coolant inlets 30. The adjacent support columns 42 are preferably substantially equally spaced about the circumference of plate 40. In a preferred embodiment, the distance between adjacent support columns 42 is about 25.5 to 37.7 inches. As shown in FIGS. 2 and 3, plate 40 includes a plurality of openings 44, 47 therethrough. Openings 44, 47 permit the coolant fluid to flow through plate 40. In a preferred embodiment, four angularly spaced openings 44 are provided about a circumferential portion of plate 40 and at least one opening 47 is provided in a central portion of plate 40. In a preferred embodiment, plate 40 includes a generally circular inner ring portion 46 having at least one opening 47 therein, a plurality of spaced spokes 48 extending generally radially outwardly from inner ring 46, and outer ring portion 50 connected to the outer ends of spokes 48. Outer ring 50 is separated from inner ring 46 by the spokes 48. Openings 44 are preferably defined between outer ring 50 and inner ring 46 and are separated from one another by the width of spokes 48. The ends of support columns 42 connected to plate 40 at outer ring 50. Secondary core support columns 22 are preferably secured to inner ring 46 of plate 40. The connection of secondary core support columns to ring 40 braces columns 22 against undesired movement thereof caused by the action of the circulating coolant fluid on columns 22. Holes 52 may be provided through inner ring 46 through which secondary core support columns 22 are received. Secondary core support columns may be secured to plate 40 using bolts, welding or any other suitable means. Referring again to FIG. 1, in a preferred embodiment plate 40 is positioned within lower plenum 16 about 2 to 4 feet below the underside of main core support 10. In addition, the perimeter of plate 40 is preferably positioned about 0.25 to 1 feet from the inner wall of reactor vessel 4. It has been found that this position will result in plate 40 intersecting vortices that may form in lower plenum 16 at or below the midpoint of such vortices, thereby maximizing the vortex-suppressing effect of plate 40. The intersection of plate 40 with such vortices will disrupt the rotational flow pattern of the vortices, thereby precluding the formation of continuous low pressure regions at the center lines of the vortices. As discussed above, suppression of vortex formation assists in maintaining uniform coolant flow and pressure at reactor core coolant inlets 30. It will be appreciated that while the above-described position of plate 40 within lower plenum 16 is the preferred location, the position may be varied in order to maximize vortex suppression within the lower plenum. For example, plate 40 is may be positioned in lower plenum 16 such that it is at a height above the central portion of lower plenum 16 that is equal to about 55 to 75 percent of the radius of curvature of hemispherical end 8 of vessel 4 which defines lower plenum 16. Referring now to FIGS. 2 and 3, in a preferred embodiment, where the interior diameter of reactor vessel 4 is preferably 12 to 16 feet, the total diameter of plate 40 is about 7 to 9 feet. Inner ring 46 preferably has a diameter of about 3 to 6 feet a width of about 6 to 12 inches and opening 47 has a diameter of about 11/2 to 5 feet. Each spoke 48 is about 1 to 2 feet long and about 6 to 10 inches wide. The distance between the outer perimeter of inner ring 46 and the inner perimeter of outer ring 50 is preferably substantially equal to the length of spokes 48. Outer ring 50 preferably has a width of about 6 to 12 inches. Plate 40 is preferably about 2 to 4 inches thick. It has been found that these dimensions provide optimum vortex suppression action in a nuclear reactor as described herein. Moreover, it is been found that a plate having these dimensions only minimally contributes to coolant pressure drop within the reactor vessel. However, it will be appreciated that the dimensions of plate 40 may be varied in order to provide optimum vortex suppressing action in any desired installation. For example, the total diameter plate 40 may be about 50 to 60 percent of the diameter of vessel 4 at cylindrical portion 6 and about 50 to 90 percent of the diameter of plate 40, as measured across openings 44, may be open. Plate 40 is preferably made of steel using conventional fabrication techniques. However, it will be appreciated that plate 40 may be made of any suitable material. Referring again to FIG. 1, the preferred method of this invention includes providing a vortex-suppressing plate 40 as described herein. Plate 40 is suspended in lower plenum 16 and nuclear reactor 2 such that is below and generally parallel to main core support 10. The preferred distance from main core support 10 to plate 40 is about 2 to 4 feet, however, it will be appreciated that this distance may be varied, as discussed above, to maximize the vortex-suppressing action of plate 40. The preferred method of this invention also includes circulating reactor coolant fluid in the lower plenum 16 for distribution to reactor core coolant inlets 30. As discussed herein, the presence of plate 40 within lower plenum 16 will suppress formation of vortices in the circulating coolant fluid because of the intersection of plate 40 with such vortices. Whereas particular embodiments of this invention have been described for purposes of illustration, it will be evident to those skilled in the art that numerous variations in the details may be made without departing from the invention as defined in the appended claims.