Patent Number: 041464303
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, there is shown in FIGS. 1 and 2 a typical nuclear reactor vessel 10 including the vessel head 12. The vessel 10 encloses a reactor core 14 which includes a plurality of elongated fuel assemblies 16 oriented adjacent one another. The assemblies 16 are supported by a lower core plate 18 which is perforated to allow passage of coolant and which in turn is supported by a core barrel 20. The core barrel 20 is supported from a ledge 22 of the reactor vessel 10, and is restrained in lateral movement by a radial support system 24 affixed to the vessel 10. The main flow of reactor coolant fluid typically enters the vessel 10 through one or more inlet nozzles 26, passes downward about the outer periphery of the core barrel 20 and about the affixed neutron shields 28, is turned one hundred and eighty degrees in a lower plenum 30, passes upward through the lower core plate 18 and core 14, and exits through outlet nozzles 32. It is of prime importance that the flow of coolant is carefully controlled into and about the fuel assemblies 16 of the core 14. Baffling of coolant flow about the core 14 has typically been performed by a baffle plates 34 and formers 36 assembly, through which a small bypass flow of reactor coolant is also passed. This bypass flow must be minimized since it decreases the thermal efficiency of the reactor, but must be large enough to adequately cool the surrounding components. This flow experiences a pressure drop as it passes through the region, the major portion of which occurs at each former 36 elevation. In order to also minimize bypass flow between the outermost fuel assemblies 16, which are typically operating at a lower power density than more central assemblies, and the baffle plates 34, the plates 34 are oriented close to the assemblies 16. A typical fuel assembly 16 is shown in FIG. 3, and includes a plurality of nuclear fuel rods 38 bounded by an upper 40 and a lower nozzle 41. Shown removably inserted within the assembly is a control rod element 44. Spaced along the assembly 16 length are a plurality of grid structures 46 which provide lateral support of the fuel rods 38 while allowing axial growth. Grids 46 of assemblies 16 within a given core are positioned at the same elevations and also serve as the contact points among adjacent assemblies 16. As reactor coolant passes through and along an assembly, it experiences a pressure drop, most particularly at the grid 46 elevations. This invention, a preferred embodiment of which is shown in FIG. 4, utilizes the flow resistance phenomenon at the grid 46 elevations to assist in flow baffling control. It has been found that coolant flow can be controlled, even with elimination of the baffle plates, by orientation of formers 48 at about the grid 46 elevations. The formers 48 are affixed to the core barrel 20, and extend toward the respective grid 46. This arrangement places the areas of high flow resistance at the same elevations, with the result that the flow of coolant is primarily vertical, with only small amounts of vortex flow between consecutive formers 48. If the formers 48 and grids 46 are not at about the same elevations, large undesirable vortex and cross flows would result. Excessive cross flows can result in undesirable vibration of the assemblies 16. In this embodiment, the formers are preferably extended to within one-tenth of an inch of the peripheral fuel assembly grids 46, and are within about one inch of the grid elevation. The effect of such an arrangement is shown in FIG. 5, which is an illustration of the results of water table tests which simulated the invention. A sufficient amount of coolant flows in the area between the test formers 49, while the main stream passes vertically. The test apparatus also includes a test assembly 51 with test grids 53. The total amount of the peripheral flow in a reactor is preferably about one-half percent of the total coolant flow, and should not exceed two percent to maintain an acceptable thermal efficiency. In a similar test, illustrated in FIG. 6, an additional test former 49 was placed between test grid 53 elevations. As shown, the vortex flow was significant, and caused excessive impingement on the test fuel assembly 51. It should here be noted that, with this invention, if there is former 48 to assembly 16 contact in a core 14, it will occur at the grid 46. The load is thus distributed through the assembly, and not only received locally by a few fuel rods 38. This alleviates the potential for fuel damage as a result of vibration impact, fretting, or local rod overheating. In addition to the basic inventive embodiment, maintaining flow baffling control by orientation of the formers 48 at about the grid 46 elevations, alternate embodiments may be utilized to provide additional beneficial functions. FIG. 7 shows an embodiment where intermittent baffle plates 50 are positioned closely adjacent the fuel assembly grids 46 at the edge of formers 52 which are at about the grid elevations. This embodiment not only provides the flow baffling function, but can also be utilized to provide transverse support for peripheral fuel assemblies 16, as the load will be transmitted through the grid 46. The baffle plates 50 in this embodiment are preferably segmented about the core periphery to reduce the differential expansion loadings between the baffles 50 and formers 52. The baffles 50 are preferably positioned to extend over the local upstream and downstream pressure and flow effects of the grids 46. Another embodiment, shown in FIG. 8, includes a non-structural neutron reflector 54 positioned between consecutive formers 52, which may also include the intermittent baffle plates 50. This embodiment may be utilized to improve the nuclear characteristics of the operating core 14 and increase the reactor efficiency. Similar to the baffle plates 50, the reflector 54 may also be segmented about the core periphery. It may also be utilized only at selected peripheral locations, such as the "corners" where the core 14 is closest to the core barrel 20. An ideal neutron reflector starts with a layer of water and alternates layer of metal and water. Another embodiment, incorporating such reflectors 56, is shown in FIGS. 9 and 10. A plurality of reflectors 56 are affixed between consecutive formers 58. The formers 58 may be provided with grooves 60 to receive the reflectors 56. To simplify the manufacturing process the grooves 60 may extend over the entire surface of the formers 58, as shown. They also can be sized specifically to receive the reflectors 56. As discussed above, the reflectors 56 may be utilized only at selected locations, such as the "corners" of the core 14. Also, the reflectors 56 may be utilized only at selected core elevations, such as increased use of reflectors about the core midplane as contrasted with the core upper and lower extremities. It should here be noted that the inventive embodiments are not inconsistent with orienting flow holes vertically through any of the formers, although they are not necessary. Also, the baffles, if utilized, need not be of the same size and may, for example, be smaller at the upper portion of the core. Further, the affixed baffle plates need not be affixed at their centers to the respective formers. FIG. 11, for example, shows a single piece combined baffle-former 64. To assist in maintaining a proper baffling function of flow through the core 14, the lower portion of the vertical segment may be provided with a leading tapered edge 66 that deflects coolant toward the core 14. Yet another embodiment is shown in FIG. 12, which shows a fuel assembly 16 seated on the lower core plate 18. This embodiment utilizes sealing means such as a flexible metallic spring 68, to act as a flow baffle. The sealing spring 68, or series of springs, extends across the area between a former 70 and its respective grid 46, and contacts the grid. It therefore maintains a zero gap, minimizing the bypass flow. The sealing spring 68 may be affixed to the former 70 or a reflector 72 if it is utilized. The sealing spring 68 also provides transverse support for the peripheral fuel assemblies 16, and may function to dampen assembly vibrations. The spring may be of varying configurations, but should be sized and shaped only to contact the assembly 16 through the grid, and the contact surface is preferably minimized. Such contact does not raise significant concerns about fuel rod 38 overheating or fretting. Also shown is a former 74 at the lowermost grid 46 elevation. It is sized to be closely adjacent, or abutting against, the fuel assembly lower nozzle 42. It may also include a beveled edge 76 so as not to interfere with the lowermost grid 46. In this embodiment coolant will flow through the perforated lower core plate 18 and up through the core 14 including the peripheral assemblies. The main stream of coolant will flow up through and about these assemblies 16, with a minor portion flowing in the areas between consecutive formers 70 and sealing springs 68, such that no stagnant areas exist. It will therefore be recognized that utilization of this invention will function to baffle flow through a nuclear reactor core, to provide coolant to the components surrounding the core, and to minimize coolant bypass flow. It alleviates the problems associated with prior art baffling arrangements, and is compatible with many desirable alternative arrangements and additional functions. It will also be apparent that many modifications and variations are possible in view of the above teachings. It therefore is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.