Patent Number: 
Section: description

FIG. 1 is a sectional view, with parts cut away, of a boiling water nuclear reactor pressure vessel (RPV) 10. RPV 10 has a generally cylindrical shape and is closed at one end by a bottom head 12 and at its other end by a removable top head 14. A side wall 16 extends from bottom head 12 to top head 14. A cylindrically shaped core shroud 20 surrounds a reactor core 22. Shroud 20 is supported at one end by a shroud support 24 and includes a removable shroud head 26 at the other end. An annulus 28 is formed between shroud 20 and side wall 16. Heat is generated within core 22, which includes fuel bundles 36 of fissionable material. Water circulated up through core 22 is at least partially converted to steam. Steam separators 38 separates steam from water, which is recirculated. Residual water is removed from the steam by steam dryers 40. The steam exits RPV 10 through a steam outlet 42 near vessel top head 14. The amount of heat generated in core 22 is regulated by inserting and withdrawing control rods 44 of neutron absorbing material, such as for example, hafnium. To the extent that control rod 44 is inserted into fuel bundle 36, it absorbs neutrons that would otherwise be available to promote the chain reaction which generates heat in core 22. Control rod guide tubes 46 maintain the vertical motion of control rods 44 during insertion and withdrawal. Control rod drives 48 effect the insertion and withdrawal of control rods 44. Control rod drives 48 extend through bottom head 12. Fuel bundles 36 are aligned by a core plate assembly 50 located at the base of core 22. A top guide 52 aligns fuel bundles 36 as they are lowered into core 22. Core plate 50 and top guide 52 are supported by core shroud 20. Fuel bundles 36 have a substantially square cross section. In alternative embodiments, fuel bundles can have a rectangular or other polygon cross section. FIG. 2 is a top schematic view of an F-lattice configuration of core 22 of reactor pressure vessel 10. Substantially standard size fuel bundles 36 and large control rods 44 are utilized in core 22. Each large control rod 44 is sized to provide poison control for sixteen conventional size fuel bundles 36. Conventional size fuel assemblies 36 and large control rods 44 are arranged in an F-lattice configuration 54 to facilitate minimizing the number of control rod drives and control rods. F-lattice configuration 54 has large control rods 44 in staggered rows 56 with sixteen conventional fuel bundles 36 surrounding each large control rod 44. FIG. 3 is a top schematic view of core plate assembly 50 for F-lattice core configuration 54, and FIG. 4 is a top sectional schematic view of core plate assembly 50. Referring to FIGS. 3 and 4, core plate assembly 50 includes a flat plate 58 supported by a plurality of support beams 60. Flat plate 58 includes a plurality of control rod guide tube openings 62 sized to receive cruciform shaped control rod guide tubes 46. Each guide tube opening 62 has a cruciform shape and includes slots 64, 66, 68, and 70 extending radially from a central portion 72 at right angles to each other. Slots 64, 66, 68, and 70 define four fuel bundle receiving areas 74. Core plate assembly 50 also includes four fuel bundle supports 76 located in each fuel bundle receiving area 74. Each fuel bundle support 76 extends through flat plate 58 and includes a coolant flow outlet 78. FIG. 5 is a sectional side view of a known fuel bundle support 80 extending through a core plate 82. Fuel support 80 includes a coolant flow inlet 84 and a coolant flow outlet 86. A bore 88 extends from inlet 84 to outlet 86. An orifice plate 90 is located inside bore 88. Coolant flows into flow inlet 84, through bore 88 and flow outlet 86 and into fuel bundle 36. Coolant flow inlet 84 and coolant flow outlet 86 are coaxial and centerline 92 passes through the center of both inlet 84 and outlet 86. Coolant flow outlet 86 is configured to receive a lower tie plate 94 of a fuel bundle 36. Because of the geometry of F-lattice core configuration 54, a core plate support beam 96 obstructs coolant flow inlet 84 of about 50% of fuel bundle supports 80 located on core plate 82. The obstruction of flow inlet 84 caused by support beam 96 can create flow separation and bi-stable flow which can influence the coolant flow pattern at both coolant flow inlet 84 and within fuel bundle 36. FIG. 6 is a sectional side view of a fuel bundle support 98, in accordance with an embodiment of the present invention, extending through flat plate 58 of core plate assembly 50. Fuel support 98 includes a coolant flow inlet 100, a coolant flow outlet 102 sized to receive lower tie plate 94 of a fuel bundle 36. A coolant flow bore 104 extends between coolant flow inlet 100 and coolant flow outlet 102. Coolant flow inlet 100 is offset from coolant flow outlet 102 so that a centerline 106 of coolant flow inlet 100 is parallel to a centerline 108 of coolant flow outlet 102. Coolant flow inlet 100 includes an orifice plate 110. Coolant flow inlet 100 is positioned adjacent to a support beam 60 of core plate assembly 50. FIG. 7 is a top schematic view of core plate assembly 50. Core plate assembly 50 includes a plurality of fuel bundle supports 98 and a plurality of cruciform shaped control rod guide tube openings 62 arranged in an F-lattice core configuration 54. Four fuel bundle supports 98 are located in each fuel bundle receiving area 74. Because of the offset configuration of coolant flow inlet 100 and coolant flow outlet 102 in fuel bundle supports 98, each coolant flow inlet 100 is positioned adjacent a core plate support beam 60, and therefore, there are no obstructions of coolant flow inlets 100. FIG. 8 is a top schematic view of a core plate assembly 112 that includes a plurality fuel bundle supports 114 in accordance with another embodiment of the present invention. FIG. 9 is an enlarged top view of fuel bundle support 114, and FIG. 10 is a cross sectional view of fuel bundle support 114 through line Axe2x80x94A. Core plate assembly 112, similar to core plate assembly 50 described above, includes a flat plate 116 supported by a plurality of support beams 118, a plurality of control rod guide tube openings 120, and a plurality of fuel bundle receiving areas 122. Each fuel bundle support 114 supports four fuel bundles 36 (see FIG. 6) and includes four coolant flow inlets 124 and four coolant flow outlets 126. Each fuel bundle receiving area 122 contains one fuel bundle support 114. Each coolant flow inlet 124 has a corresponding coolant flow outlet 126 and a bore 128 extending from coolant flow inlet 124 to corresponding coolant flow outlet 126. Coolant flow inlet 124 is offset from corresponding coolant flow outlet 126 so that a centerline 130 of coolant flow inlet 124 is parallel to a centerline 132 of corresponding coolant flow outlet 128. An orifice plate 134 is located in each coolant flow inlet 124. Additionally, coolant flow inlets 124 are located in fuel bundle support 114 so that each coolant flow inlet 124 is the same distance from a support beam 118. Particularly, a distance xe2x80x9cBxe2x80x9d from coolant flow inlet 124 is the same for all coolant flow inlets 124 in fuel bundle support 114. The above described core plate assembly 50 with fuel bundle supports 98 and core plate assembly 112 with fuel supports 114 provide unobstructed coolant flow inlets and therefore identical flow entrance conditions for all fuel assemblies 36. FIG. 11 is a top schematic view of a core plate assembly 136 that includes a plurality fuel bundle supports 138 in accordance with another embodiment of the present invention. Core plate assembly 136, similar to core plate assembly 112 described above, includes a flat plate 140 supported by a plurality of support beams 142, a plurality of control rod guide tube openings 144, and a plurality of fuel bundle receiving areas 146. Each fuel bundle receiving area 146 includes one fuel bundle support 138, and each fuel bundle support is configured to support one large fuel bundle (not shown). Each large fuel bundle is approximately 1.5 times the size of a standard fuel bundle 36. Fuel support 138 includes a coolant flow inlet 148 and a coolant flow outlet 150. A coolant flow bore (not shown) extends between coolant flow inlet 148 and coolant flow outlet 150. Coolant flow inlet 148 is offset from coolant flow outlet 150. Coolant flow inlet 148 is positioned adjacent to a support beam 142 of core plate assembly 136. While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.