Patent Number: 051981858
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows a portion of an existing type of reactor 10. The portion shown is interior to a reactor vessel, namely, a portion of a reactor 10 having a plenum 12 and a shield 14. Coolant, as indicated by the arrows, flows into plenum and circulates among a plurality of penetrations 16. There is one penetration 16 for each fuel element 18. Only three penetrations 16 are shown although a greater number exists, perhaps 600. The left side of FIG. 1 represents a typical penetration 16 near the center of the reactor core; the right side, the reactor periphery; the center, an intermediate penetration 16. The purpose of penetrations 16 is to provide a path for coolant to flow down through shield 14 and into each element 18 to remove the heat of fission and radioactive decay. Penetration 16 comprises a slotted tube 24 having a plurality of vertical slots 26, a universal sleeve housing 28 within slotted tube 24 that extends from just above plenum 12 downward, surrounding fuel element 18. Universal sleeve housing 28 has an upper portion 30 and a lower portion 32 defined essentially by plenum 12 so that lower portion 32 is generally below plenum 12 and upper portion 30 is even with plenum 12. Universal sleeve housing 28 has a plurality of holes 38 in upper portion 30 arranged in rows and columns to allow the passage of coolant from plenum 12 through slots 26 and thence into interior 40 of sleeve 28. In lower portion 32 are orifice plates 44, 46, 48 which have holes 50. Orifice plates 44, 46, 48 are to restrict flow to fuel elements 18. Plates 44, 46, 48 will restrict more for peripheral positions than core interior positions, as suggested by the relative number of holes 50 in FIG. 1, which number is illustrative only. The number of holes 50 in orifice plates 44, 46, 48 varies from sleeve to sleeve. Slots 26 are lined up with holes 38 by a conventional keying arrangement between slotted tube 24 and housing 28. Toward the center of the core, orifice plates will have more holes than toward the periphery; for example, orifice plate 44 will have more holes 50 than orifice plate 46, which is at an intermediate position, and orifice plate 48, near the periphery of the core, will have the fewest holes 50. The number of holes 50 determines the amount of restriction in the flow of coolant to fuel elements 18. The fewer the number of holes 50, the lower the flow of coolant through orifice plates, 44, 46, 48. FIG. 2 shows a portion of a reactor 56 corresponding to that shown in FIG. 1. Reactor 56, however, incorporates the present invention. Reactor 56 has a plenum 58 and a shield 60. As with reactor 10 of FIG. 1, reactor 56 has penetrations 62 for its fuel elements 64. Penetrations 62 also have a slotted tube 70 with slots 72. However, each fuel element 64 is not surrounded with a universal sleeve housing as in reactor 10 of FIG. 1. Reactor 56 has a set of housings, generally similar to universal sleeve housing 28, but each different with respect to each other. FIG. 2 shows three housings 74, 76, 78, with housing 74 located toward the center of the reactor core, housing 76 located farther from the core center and housing 78 located near the core periphery. Housings 74, 76, 78 have holes 84 that allow coolant from plenum 58 to flow into the interiors 86, 88, 90, of housings 74, 76, 78, respectively, through slots 72 of slotted tube 70. Each housing 74, 76, 78, will have an upper portion 80 and a lower portion 82. Upper portion 80 is defined by plenum 58; that is, lower portion 82 is the part of housings 74, 76, 78 below plenum 58. Although holes 84 are shown in FIG. 2 to be of the same diameter and arranged in rows and columns, it is not necessary that the holes be of the same size or so arranged, although it is preferable to do so. It is important, however, to vary (and to control) the amount of coolant admitted to interiors 86, 88, 90, admitting more coolant to those interiors of penetrations near the center of the core and less to those near the periphery of the core. The amount may be varied by changing the total area of the holes of the housings through which coolant flows by varying the diameter of holes 84, by changing the number of holes 84, or, in fact, by changing the shape of the holes, to slots or ovals for example. The amount of coolant flowing to fuel elements 64 should be greatest toward the center of the reactor core and less farther out, least to the peripheral fuel elements 64. Although three housings (74, 76, 78) are shown in FIG. 2, a reactor core can be divided into an arbitrary number of zones (housings 74, 76, 78 representing three different zones) in the form of rings from the center of the core outward, with the amount of coolant entering the interior of the housings of fuel elements in each zone being equal and each outwardly laying zone receiving less coolant than those located in the immediately adjacent, inward zone. There are two design parameters that affect the amount of flow of coolant into fuel elements 64: the flow area of holes 84 and the elevation of that area relative to the coolant level in plenum 58. Therefore, in addition to holes 84 being all the same diameter and arranged in rows and columns, all housings 74, 76, 78 will preferably have rows beginning at the bottom of upper portion 80 and continuing up toward the top of upper portion 80. The number of rows of holes 84 will then be fewest in housing 78 and greatest for housing 74. There are no orifice plates in reactor 56. Although orifice plates also serve to restrict flow as does a reduction in the number of holes 84, the impact on the flow of fewer rows of holes in housings 76 and 78 than in 74 results in greater coolant flow to housings 74 under accident conditions. In reactor 56, the peripheral fuel elements 64 receive less coolant during normal operation than the interior fuel elements 64. FIGS. 3 and 4 illustrate the effect on flow to fuel elements 64 graphically. FIG. 3 is in particular a graph of several series of data points, each series corresponding to a number of zones. The graph shows the minimum flow of coolant to fuel elements during a LOCA versus the height of holes 84 beginning with a row on the bottom of upper portion 80. If the top rows of holes in the outermost zones are eliminated, the minimum flow of coolant increases. If the number of rows eliminated is five from the housings in the outer three zones of a particular reactor, minimum flow increases from 10.3 to 12.6 gallons per minute, a 22% increase. Greater increases are seen as more zones are revised to have five fewer rows. For five zones, the increase is 56% more coolant. In FIG. 4 the minimum coolant flow is graphed versus the number of zones having fewer holes. The different curves illustrate the change in flow versus the number of affected zones when different numbers of rows are eliminated. It will be seen that the larger the number of rows eliminated and the more zones that are affected, the greater will be the increase in the minimum flow of coolant during a LOCA. At some point, however, normal operation becomes affected by restricted flow. The precise number of holes or flow area in each zone depends on a great many reactor parameters including the power rating of the reactor, its coolant flow rate, its power density, average burnup of the fuel, accident assumptions, other measures taken to mitigate a LOCA, and so forth. However, restriction of flow to the peripheral assemblies by reduction of the flow area to the interiors of the fuel elements housings and lowering the elevation of the flow area relative to the plenum liquid level will generally result in a significant improvement in the flow of coolant to the core during LOCA without reducing flow during nominal conditions. It will be apparent to those skilled in the art that many changes and substitutions can be made to the preferred embodiment herein described without departing from the spirit and scope of the present invention which is defined by the appended claims.