Bottom nozzle plate for a nuclear reactor fuel assembly

Abstract: A bottom nozzle plate for a fuel assembly in a nuclear reactor is formed from a rigid substantially square plate (24) having legs to be positioned on the lower core plate of the reactor to support the fuel assembly above coolant inlet openings (22) in the core plate. The rigid square plate (24) has two types of coolant flow holes (34 and 42) therethrough. First flow holes (34) are grouped in clusters of four to define a square pattern (35) relative to the sides of the plate (24) such that the width of the sections (36) of the plate between the flow holes in each cluster is less than the width of the sections (38) of the plate between the clusters of four holes. Second flow holes (42) are formed in an irregular pattern to define a substantially clover leaf shape. The second flow holes (42) are positioned so as to be above the coolant inlet openings (22) in the core plate when the bottom nozzle plate is installed in the reactor.

Description: The invention relates generally to a nuclear reactor fuel assembly and in particular to a bottom nozzle plate of such a fuel assembly. Commercial nuclear reactors include multiple fuel assemblies. Each fuel assembly may comprise a plurality of fuel rods radially spaced apart in a parallel array by grid assemblies spaced along the length of the fuel rods. Each grid assembly is formed in an eggerate design by multiple metal strips that criss-cross at right angles to form individual cells for each of the fuel rods. The strips are provided with tabs that project into the cells against the fuel rods. The tabs serve the purpose of holding the fuel rods in their respective radial position. Control rod guide thimble tubes also extend through selected cells in the grid assembly and are attached at their upper and lower ends respectively to an upper end fitting and a lower end fitting. The upper and lower end fittings are also commonly referred to in the industry as nozzle plates since they are rigid plates that provide structural integrity and load bearing support to the fuel assembly and are provided with flow apertures therethrough for coolant flow. The lower end fitting or nozzle plate is positioned directly above openings in the lower portion of the reactor where coolant flows up into the reactor to the core. The ligaments between apertures in the end fittings coincide with the ends of the fuel rods and limit upward or downward movement of the fuel rods. Debris such as metal particles, chips, and turnings is generated during manufacture, installation, and repair of the reactor, piping, and associated cooling equipment. The size and complexities of the equipment prevent location and removal of all such debris before operations are commenced. Also, some of this debris may not become loose matter in the system until the system is put into operation. It has been recognised that this debris presents a greater problem to the system than previously thought. These small pieces of debris have been found to lodge between the walls of the grid cells and the fuel elements. Movement and vibration of the lodged debris caused by coolant flow results in abrasion and removal of cladding on the fuel rods. This in turn leads to detrimental effects such as corrosion of the fuel rods and failure to retain radioactive fission gas products. Such damage, although not critical to safety of the surrounding environment, can reduce operating efficiency by the need to suspend operation while replacing damaged fuel rods. It can be seen that a need exists for a debris filter capable of filtering debris of a size which may lodge between the grid cell walls and the fuel rods. An important consideration besides that of filtration is that a substantial coolant pressure drop across the filter must be avoided in order to maintain an adequate coolant flow over the fuel rods for heat removal therefrom. Specifications US-A-4,684,495 and US-A-4,684,496 disclose debris traps formed from a plurality of straps aligned with one another in a criss-cross arrangement and defining a plurality of interconnected wall portions which form a multiplicity of small cells each having open opposite ends and a central channel for coolant flow through the trap. Patent Specification US-A-4,828,791 discloses a debris resistant bottom nozzle which is a substantially solid plate having cut-out regions in alignment with inlet flow holes in the lower core plate. Separate criss-cross structures fixed to the plate extend across the cut-out regions to act as a debris trap. Patent Specifications US-A-4,664,880 and US-A-4,678,627 disclose debris traps mounted within a bottom nozzle that define a hollow enclosure with an opening so as to form a debris capturing and retaining chamber. Patent Specification US-A-4,652,425 discloses a trap for catching debris disposed between the bottom nozzle and the bottom grid. The structure forms multiple hollow cells that receive the fuel rod lower end plugs with dimples in each cell for catching debris carried into the cells by the coolant flow. Patent Specification US-A-4,900,507 discloses a nuclear fuel assembly debris filter bottom nozzle formed from a rigid plate having a plurality of flow holes. The holes are individually 4.8 mm +/- 0.2 mm (0.190 inches +/- 0.008 inches) in diameter which is smaller than the predetermined maximum cross-sectional dimension of each of the unoccupied spaces through the lowermost grid. The sections of the plate extending between the flow holes have a maximum dimension of 2.5 mm (one-tenth inch). Although a variety of approaches have been taken to the need for filtration of primary coolant, it is felt that the problem has not been adequately addressed. While certain designs such as those utilizing only small flow holes may provide adequate filtration of damaging debris, the problem of an unacceptable pressure drop may still exist. According to the invention there is provided a bottom nozzle plate for a nuclear reactor fuel assembly having legs to be positioned on a lower core plate of the reactor to support the fuel assembly above coolant inlet openings in the core plate, the bottom nozzle plate comprising: a rigid substantially square plate with a plurality of first coolant flow holes extending through the square plate, each of the first flow holes being circular in cross section and 4.57 mm +/- 0.13 mm (0.180 inches +/- 0.005 inches) in diameter with the majority of the first flow holes grouped in clusters of four to define a square pattern relative to the sides of the square plate such that the width of the sections of the square plate between the first flow holes in the square pattern is less than the width of the sections of the square plate between the clusters of four holes, and a plurality of second coolant flow holes extending through the square plate and positioned so as to be above the coolant inlet openings in the core plate when the bottom nozzle plate is installed in the reactor, each of the second flow holes being of an irregular pattern to define a substantially clover leaf shape. Such a nuclear fuel assembly bottom nozzle plate (lower end fitting) can prevent the passage of damage-inducing debris while minimizing pressure drop of coolant flow across the plate. The bottom nozzle plate serves to support the guide thimbles and fuel rods as well as acting as a filter. The spacing between square clusters corresponds with the spacing of the fuel rods in the fuel assembly such that the chamfered ends of fuel rods that are allowed to rest on the ligaments of the plate do not block the flow holes. The spacing of the holes and the number of wider ligaments between clusters is determined by the fuel array, such as a 17 x 17 or 15 x 15, with which the nozzle plate is to be used. The second flow holes are formed by providing an additional smaller aperture through the nozzle plate substantially at the centre of some of the clusters of the first flow holes such that the four holes of the original cluster are all interconnected to form an irregular shaped flow hole. The second flow holes are positioned so as to be above the core plate holes that pass coolant flow to the nozzle plate when installed in the reactor. The invention is diagrammatically illustrated by way of example in the accompanying drawings in which: Figure 1 is a partial sectional view of a typical fuel assembly for a nuclear reactor; Figure 2 is a side view of a bottom nozzle plate for a nuclear reactor fuel assembly according to the invention; Figure 3 is a plan view corresponding to Figure 2; Figure 4 is an underneath plan view corresponding to Figure 2; Figure 5 is an enlarged view of a second kind of flow hole used in a bottom nozzle plate according to the invention; Figure 6 is an enlarged view of an alternate embodiment of the second kind of flow hole used in a bottom nozzle plate according to the invention; Figure 7 is an enlarged cross sectional view of one of a first kind of flow hole used in a bottom nozzle plate according to the invention; Figure 8 is a top view illustrating the relative positioning of fuel rods and the second flow hole; and Figure 9 is a view taken on line 9-9 of Figure 8. Referring to the drawings, Figure 1 shows a typical nuclear reactor fuel assembly 10 used in a pressurized water reactor and generally comprises a plurality of fuel rods 12, grid assemblies 14, guide tubes 16, upper end fitting 18, and lower end fitting or bottom nozzle plate 20. The fuel rods 12 are maintained in an array spaced apart by the grid assemblies 14. The guide tubes 16 extend through the grid assemblies 14 and are attached to the upper end fitting 18 and the bottom nozzle plate 20 and, in addition to providing structural integrity to the entire assembly, also serve as guides for control rods (not shown). The upper end fitting 18 and the bottom nozzle plate 20 provide structural and load bearing support for the fuel assembly 10 and are also provided with openings therethrough to allow coolant to flow vertically through the fuel assembly 10. The bottom nozzle plate 20 rests on a lower core support plate (not shown) of the reactor and directly above coolant inlet openings 22 (shown in broken lines in Figure 3) in the core support plate that direct coolant upwardly to the fuel assembly 10. Dimples or tabs in the walls of the grid assemblies 14 radially position the fuel rods to allow maximum surface area contact of the fuel rods 12 with the coolant as it flows upwardly therethrough. The bottom nozzle plate 20, seen in Figures 2 to 4, is formed from a rigid substantially square plate 24. Legs 26 may be separate parts attached to the plate 24 by any suitable means such as welding or may be integral therewith. Openings 28 provided on two diagonally disposed legs 26 are used to position the bottom nozzle plate 20 with the lower core plate to prevent movement of the bottom nozzle plate 20 and the fuel assembly 10 during reactor operations. The bottom nozzle plate 20 is provided with two kinds of holes that do not pass coolant flow therethrough. A plurality of guide thimble holes 30 are provided and spaced across the plate in a pattern corresponding to the guide thimble tubes of the fuel assembly. The threaded ends of the guide thimble tubes 16 extend through the guide thimble tube holes 30 and receive threaded nuts so that the guide thimble tubes are attached to the bottom nozzle plate 20. An instrumentation tube hole 32 is substantially at the centre of the bottom nozzle plate 20 and receives an instrumentation tube used to monitor core conditions during reactor operations. Two types of coolant flow holes are provided through the bottom nozzle plate 20 to pass coolant received from the coolant inlet openings 22 in the core plate and to filter debris of a size known to cause damage to the fuel rods 12. A plurality of a first kind of flow holes 34 are provided through the nozzle plate 20 with each first flow hole 34 circular in cross section and 4.6 mm +/- 0.13 mm (0.180 +/- 0.005 inches) in diameter. As can be seen in Figures 3 and 4, the majority of the first flow holes 34 are grouped in clusters of four holes arranged to define a square pattern 35 relative to the sides of the nozzle plate 20. This provides an arrangement of flow holes where the width of the section or ligaments of the plate between the flow holes within the cluster is less than the width of the ligaments of the plate between the clusters. The narrower ligaments within the clusters are indicated by the numeral 36 while the wider ligaments between clusters are indicated by the numeral 38. In the preferred embodiment, the spacing of the flow holes 34 to define the square pattern 35 is determined by the number of the fuel rods 12 in the fuel assembly above the bottom nozzle plate 20. For example, in a 17 x 17 array such as that illustrated, the number of square clusters is arranged so as to provide 15 of the wider ligaments 38 across the bottom nozzle plate 20 in addition to the boundary ligaments. In this manner, the end of the fuel rod 12 contacts the bottom nozzle plate 20 at the wider ligament 38. This provides stable support to the fuel rod and more importantly does not result in any of the flow holes being blocked by the fuel rod which would cause a pressure drop and reduction of coolant flow. On a projected basis the fuel rods 12 do overlap the holes, however the rods are chamfered at the lower end to prevent direct blockage. This is best shown in Figures 8 and 9. As can be seen in the cross sectional view of Figure 7, each of the first flow holes 34 is provided with a chamfered lower edge 40. As indicated by the letter A, the angle of the chamfered lower edge 40 is 45 degrees plus or minus 2 degrees. This provides a diameter of 5.5 mm +/- 0.2 mm (0.216 +/- 0.008 inches) (as indicated by the letter B) at the lower edge 40 in the preferred embodiment. The chamfer decreases the hydraulic resistance of coolant flow through the flow holes and thus reduces the pressure drop of coolant flow. As seen in the drawings, the presence of the guide thimble tube holes 30 and the instrumentation tube hole 32 prevent all of the first flow holes 34 from being arranged in groups of four. In order to maintain structural integrity of the bottom nozzle plate 20 around the holes 30 and 32, it is necessary to eliminate one of the first holes 34 in each group around the holes 30 and 32 to prevent overlap of holes. Second kind of flow holes 42 are preferably positioned so as to be above the coolant inlet openings 22 in the core plate when the bottom nozzle plate 20 is in its installed position. As best seen in Figure 5, each of the second flow holes 42 is an irregular shaped hole that may be described as a modification of the four first flow holes 34 in the square pattern 35 to define a substantially clover leaf shape in plan or underneath plan. The modification to obtain the second flow hole 42 comprises interconnecting the four flow holes in the square pattern. This is accomplished by removing a section of the bottom nozzle plate 20 that is centrally located between the four flow holes such that an equal amount of each flow hole in the pattern is included by the removed section. In the preferred embodiment, a central portion 44 of each second flow hole 42 defines a square having its corners intersect the four flow holes of the square pattern. As indicated by the letter C in Figure 5, the extended distance along a plane from the centre of the central portion 44 to the centre of each of the four holes originally forming the square pattern is 2.86 mm (0.1125 inches) in the preferred embodiment. As indicated by the letter D, the width of the central portion 44 is 2.97 mm (0.117 inches). As indicated by the letter E, each corner of the central portion 44 intersects a width of 0.64 mm +/- 0.38 mm (0.025 +/- 0.015 inches) of each of the four flow holes originally forming the square pattern. The channel width E of 0.64 mm (0.025 inches) is chosen to be less than the nominal spacing of 1 mm (0.041 inches) between bottom nozzle plates in the reactor core. This ensures that the most open passage for debris to enter the fuel rod region is not through the flow holes that lead directly to the spaces between the fuel rods and grid assemblies that are more likely to trap damaging debris. In the preferred embodiment, the second flow holes 42 are located on bottom nozzle plate 20 so as to be evenly distributed over all the coolant inlet openings 22 in the core plate. For a 17 x 17 array such as that illustrated, there are a total of sixteen second flow holes 42, four above each coolant inlet opening 22 in the core plate. This provides a flow area of 5465 sq mm (8.47 square inches) above the coolant inlet openings 22 in the core plate. In addition to minimizing pressure drop of coolant flow, the number and positioning of the second flow holes 42 are also selected so as not to have a detrimental effect on the structural integrity of the bottom nozzle plate 20. The structural loads that act on the bottom nozzle plate 20 during operations are transmitted through the guide thimble tubes and the fuel rods 12 and are reacted out at the legs 26. Due to the large number of load points (24 guide thimble tubes and 264 fuel rods in a 17 x 17 array), the bottom nozzle plate 20 behaves as a uniformly loaded plate fixed at the corners. Under these loading conditions the maximum stresses occur near the centre of the plate and near the fixed boundary. The second flow holes 42 are located midway between these areas in a generally low stressed area of the nozzle plate 20. An alternate embodiment of the second flow hole 42 is illustrated in Figure 6 and indicated by the numeral 142. A central portion 144 is circular instead of square and is 3.6 mm (0.142 inches) in diameter. This provides a flow area of 5497 sq mm (8.52 square inches) above the coolant inlet openings 22 in the core plate. The dimensions C and E are the same as for the second flow hole 42. In operation, the bottom nozzle plate 20 supports the fuel assembly 10 by means of the guide thimble tubes 16 attached through the guide thimble tube holes 30. The bottom nozzle plate 20 is positioned on the core plate by dowels extending through the openings 28 in the legs 26. The first and second flow holes 34, 42 are sized and spaced apart so as to filter damage inducing debris from the coolant while minimizing the pressure drop of coolant flow. For a 17 x 17 array fuel assembly, the square plate 24 measures 214 mm (8.425 inches) along each side and is 16 mm (0.637 inches) thick. The thickness of the plate is required to maintain structural integrity as it supports the fuel assembly and is subject to the pressures of coolant flow. The spacing of the flow holes in the square pattern 35 provides the ligaments 38 between square clusters that are 2.3 mm (0.090 inches) and the ligaments 36 between holes in the square clusters that are 1.15 mm (0.045 inches). After allowing sufficient space for the guide thimble tube holes 30 and the instrumentation tube hole 32, this provides eight-hundred and sixty of the first flow holes 34 and sixteen of the second flow holes 42 for a total of eight-hundred and seventy-six flow holes. For the standard square plate used in a 17 x 17 array as mentioned above, this provides a flow hole density of approximately 1.9 holes per sq cm (12.34 holes per square inch). Naturally, the number of flow holes will vary in direct proportion to the size of the bottom nozzle plate used with the flow hole size and spacing taught herein.

Claims: A bottom nozzle plate (20) for a nuclear reactor fuel assembly (10) having legs (26) to be positioned on a lower core plate of the reactor to support the fuel assembly (10) above coolant inlet openings (22) in the core plate, the bottom nozzle plate (20) comprising: a rigid substantially square plate (24) with a plurality of first coolant flow holes (34) extending through the square plate (24), each of the first flow holes (34) being circular in cross section and 4.57 mm +/- 0.13 mm (0.180 inches +/- 0.005 inches) in diameter with the majority of the first flow holes (34) grouped in clusters of four to define a square pattern (35) relative to the sides of the square plate (24) such that the width of the sections (36) of the square plate (24) between the first flow holes in the square pattern is less than the width of the sections (38) of the square plate (24) between the clusters of four holes, and a plurality of second coolant flow holes (42, 142) extending through the square plate (24) and positioned so as to be above the coolant inlet openings (22) in the core plate when the bottom nozzle plate (20) is installed in the reactor, each of the second flow holes (42, 142) being of an irregular pattern to define a substantially clover leaf shape. A bottom nozzle plate according to claim 1, wherein each of the first flow holes (34) has an inlet chamfer (40) of 45 degrees plus or minus 2 degrees. A bottom nozzle plate according to claim 1, wherein the width of the sections (36) of the square plate (24) between the first flow holes in the square pattern is approximately 1.13 mm (0.045 inches) and the width of the sections (38) of the square plate between the clusters of four holes is approximately 2.29 mm (0.090 inches).

Inventors: KING RAYMOND ALLEN, KING, RAYMOND ALLEN
Applicants: B & W FUEL CO, B&W FUEL COMPANY
Application year: 1991
Main classification: G21C19
IPC codes: G21C3, G21C19