Patent Number: 
Section: description

Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. As shown in the accompanying drawings, a lips-type multi-purposed spacer grid 50 according to the present invention receives and supports a plurality of elongated nuclear fuel rods 8 at positions spaced at regular intervals in a nuclear fuel assembly, and comprises a plurality of two types of inner strips 12 and 13. The two types of inner strips 12 and 13 intersect each other at right angles in accordance with a designed array, thus forming the spacer grid 50 with a plurality of four-walled unit cells for receiving and supporting the elongated nuclear fuel rods 8, as shown in FIGS. 2, 4a and 4b. Each of the inner strips 12 and 13 is fabricated, as shown in FIGS. 3a and 3b, by integrating a plurality of unit strip parts 45 into a linear strip, and each of the unit strip parts 45 is comprised of a water strider-type spring 16, an upper dimple 15 and a lower dimple 17, as will be described later herein. The inner strips 12 and 13 each have a plurality of notches at the junctions of the unit strip parts 45 such that each notch having a predetermined length vertically extends downward or upward. In the nuclear fuel assembly, one fuel rod 8 is received and supported within one cell. As shown in FIGS. 5 to 7d, each of the unit strip parts 45 constituting the inner strips 12 and 13 is comprised of a frame used as a support frame of the unit strip part 45, the water strider-type spring 16 formed in an opening defined at the middle part of the frame, and the upper and lower dimples 15 and 17 provided at the upper and lower parts of the frame, respectively. That is, the upper and lower dimples 15 and 17 are formed at positions above and under the water strider-type spring 16 in each unit strip part 45. The unit strip part 45 also has a mixing blade 14. The mixing blade 14 extends upward to a predetermined length from a side of the upper edge of the upper dimple 15, as best seen in FIG. 5. In the spacer grid 50 according to the present invention, the spring 16 has a shape similar to the profile of a water strider that is an aquatic insect of the family Gerridae, having six slender legs fringed with hairs, enabling the insect to dart about on the surface of water. Of the six legs of the water strider, four long legs except for two relatively short forelegs extend from the body in diagonal directions in a manner similar to that of the spring 16, so the spring 16 is a so-called xe2x80x9cwater strider-type springxe2x80x9d. The frame of each unit strip part 45 is comprised of two vertical support columns 40 and two horizontal support beams 25. The two support columns 40 are vertically disposed in parallel while being spaced apart from each other at a predetermined interval. The two horizontal support beams 25 horizontally extend between the two columns 40 at vertically symmetrical upper and lower positions, thus connecting the two columns 40 to each other and defining a rectangular middle opening between the beams 25 and the columns 40. An upper opening which is open upward is defined between the two columns 40 and the upper beam 25, and a lower opening which is open downward is defined between the two columns 40 and the lower beam 25. The water strider-type spring 16 is formed in the rectangular middle opening of the frame. As shown in FIGS. 6a to 6c, the water strider-type spring 16 is comprised of an equiangular curved part 23, two side extensions 36 and 37, and four spring legs 28. The equiangular curved part 23 is axially formed along the central axis of the spring 16, and has a predetermined width while being curved within the direction of the width at a radius of curvature allowing the equiangular curved part 23 of the spring 16 to come into surface contact with the cladding of a fuel rod 8. The two side extensions 36 and 37 extend outward to a predetermined width while being bent at a predetermined angle in opposite directions from both sides of the equiangular curved part 23. The four spring legs 28 diagonally extend from upper and lower comers of the two side extensions 36 and 37. The four spring legs 28 are connected to the inside edges of the frame at the four corners of the rectangular middle opening of the frame. The four spring legs 28 are thus integrated with the frame of the unit strip part 45 at four points, so that the water strider-type spring 16 has a four point support structure in which the spring 16 supports a fuel rod at the four points. The spring 16 is also projected in a direction from a vertical surface formed by the frame. The upper and lower dimples 15 and 17 are formed at positions above and under the water strider-type spring 16 in each unit strip part 45. As shown in FIGS. 7a to 8b, each of the upper and lower dimples 15 and 17 is comprised of a curved dimple part 20 and two side dimple extensions 21. The curved dimple part 20 is axially formed along the central axis of each dimple 15, 17, and has a predetermined width while being curved within the direction of the width at a radius of curvature allowing the curved dimple part 20 to come into surface contact with the cladding of the fuel rod 8. The two side dimple extensions 21 extend outward in opposite directions from both sides of the curved dimple part 20 to a predetermined width is while being curved at a predetermined angle. The upper dimple 15 is curved along the lower edge thereof to form an arc-shaped lower edge 32, while the lower dimple 17 is curved along the upper edge thereof to form an arc-shaped upper edge 31. The upper and lower dimples 15 and 17 are also projected from the vertical surface formed by the frame in a direction opposed to the projecting direction of the water strider-type spring 16. The radius of curvature of the equiangular curved part 23 of the water strider-type spring 16 is determined to be slightly larger than that of the cladding of each fuel rod 8, so that the equiangular curved part 23 comes into close surface contact with the cladding of the fuel rod 8 and soundly supports the fuel rod 8 even when the fuel rod 8 vibrates or is impacted by external force. That is, the radius of curvature of the equiangular curved part 23 is designed to be slightly larger than that of the cladding of each fuel rod 8 before the fuel rod 8 is installed in a unit cell of the spacer grid. However, after the fuel rod 8 is installed in the unit cell of the spacer grid, the radius of curvature of the equiangular curved part 23 becomes equal to that of the cladding because the cladding pushes the spring 16 in a direction opposed to the projecting direction of the spring 16. An axial opening 29 is formed along the central axis of the equiangular curved part 23 to have a slender appearance, so that coolant is completely collected in the gap between the cladding of the fuel rod 8 and the equiangular curved part 23. Therefore, it is possible to prevent disturbance of heat transfer at a part of the cladding-due to the coolant remaining at the gap between the cladding and the equiangular curved part 23, so that the spacer grid does not cause nucleate boiling at the claddings of fuel rods 8. The four spring legs 28 of the water strider-type spring 16 may have bent parts 24 at which the spring legs 28 are bent in a direction opposed to the bent direction of the two side extensions 36 and 37. In such a case, the bent parts 24 of the spring legs 28 are projected from the vertical surface formed by the frame in a direction opposed to the projecting direction of the equiangular curved part 23. In addition, it is possible to control the fuel rod support force of the water strider-type spring 16 by adjusting the bent angle of the bent parts 24. The upper and lower edges of the water strider-type spring 16 are curved to form arc-shaped edges 38 which are symmetrical with respect to a horizontal axis of the spring 16. That is, the upper edge of the spring 16, formed by the upper edges of the equiangular curved part 23, the two side extensions 36 and 37 and the two upper spring legs 28, is downwardly curved to form an arc-shaped edge. In the same manner, the lower edge of the spring 16, formed by the lower edges of the equiangular curved part 23, the two side extensions 36 and 37 and the two lower spring legs 28, is upwardly curved to form an arc-shaped edge. The arc-shaped upper and lower edges of the water strider-type spring 16 are symmetrical with respect to the horizontal axis of the spring 16. Therefore, when the spacer grid of the present invention fabricated by the intersecting inner strips 12 and 13 is sectioned along a horizontal direction as shown in FIG. 4b, each of the water strider-type springs 16 is viewed in the form of a lower lip, while each of the upper and lower dimples 15 and 17 is viewed in the form of an upper lip. In each unit strip part 45 of the spacer grid, the water strider-type spring 16 is projected in a direction opposed to that of the upper and lower dimples 15 and 17, so that the spring 16 and the dimples 12 and 13 of each unit strip part 45 support different fuel rods 8, separately. In addition, when the spring 16 and the dimples 12 and 13 of each unit strip part 45 are viewed from the top or the bottom of the spacer grid, they form a pair of lips. As shown in FIGS. 9a to 9d, the mixing blade 14 extends upward to a predetermined length from the upper edge of one side dimple extension 21 of the upper dimple 15 while being smoothly curved in the same direction as the projecting direction of the water strider-type spring 16. The mixing blade 14 thus has a spoon-shaped configuration which is concave at a side surface thereof facing the fuel rod 8. It is preferable to determine the bent angle of the mixing blade 14 relative to a vertical surface of the unit strip part to 90xc2x0 or less. That is, the mixing blade 14 is curved such that an acute angle is formed between a normal line at the uppermost end of the mixing blade 14 and an axial line of the side dimple extension 21 of the upper dimple 15. In addition, the upper edge of each mixing blade 14 is placed along a circle which has a radius larger than that of the cladding of a fuel rod 8, as shown in FIG. 9b, so that the mixing blades 14 do not scratch or damage the cladding of the fuel rod 8 during a process of installing fuel rods 8 in the spacer grid while producing a nuclear fuel assembly. In order to space the mixing blades 14 apart from the cladding of the fuel rod 8, the upper edges of the mixing blades 14 are designed such that they are placed along a circle having a diameter larger than that of the cladding. As shown in FIGS. 4a and 4b, the lips-type multi-purposed spacer grid 50 according to the present invention is fabricated by the two types of inner strips 12 and 13 which are each comprised of a plurality of unit strip parts 45 integrated into a linear strip, and which intersect each other at right angles to form a plurality of four-walled unit cells in the spacer grid 50 for receiving and supporting the elongated nuclear fuel rods 8 such that one fuel rod 8 is received and supported within one cell. The water strider-type spring 16 of each unit strip part 45 is projected from a vertical surface formed by the frame of the unit strip part 45 in a direction opposed to the projecting direction of the upper and lower dimples 15 and 17. Therefore, within each four-walled unit cell of the spacer grid 50 defined by four unit strip parts 45, the two water strider-type springs 16 of two neighboring unit strip parts 45 meeting each other at a right angle are projected toward the center of the unit cell, and the upper and lower dimples 15 and 17 of the remaining two unit strip parts 45 are projected toward the center of the unit cell. The fuel rod 8, installed within the four-walled unit cell of the spacer grid 50, is thus supported at six points by the two springs 16 and the four dimples 15 and 17. In addition, the mixing blade 14 of each unit strip part 45 extends upward from the upper dimple 15 while being smoothly curved in the same direction as that of the water strider-type spring 16 of the unit strip part 45. Therefore, the mixing blades 14 of unit strip parts 45,are directed toward fuel rods 8 installed in neighboring unit cells. When viewing the spacer grid 50 of the present invention from the top as shown in FIG. 4a, the upper dimples 15 and the water strider-type springs 16 support the fuel rods 8 while being curved at their fuel rod contact surfaces at the same radius of curvature as that of the claddings of the fuel rods 8, and the mixing blades 14 extending from the upper dimples 15 are outwardly curved to be directed over neighboring unit cells. When viewing the spacer grid 50 from the bottom as shown in FIG. 4b, the lower dimples 17 and the: water strider-type springs 16 support the fuel rods 8 while being curved at their fuel rod contact surfaces at the same radius of curvature as that of the claddings of the fuel rods 8. FIGS. 9a and 9b show a four-walled unit cell 18 of the spacer grid according to the present invention, with a fuel rod 8 supported within the cell 18. Particularly, these drawings show the surface contact between the fuel rod 8 and the water strider-type springs 16 and the upper and lower dimples 15 and 17 of unit strip parts 45 defining the four-walled unit cell 18. FIG. 9d shows currents of coolant guided by the mixing blades 14 of the intersecting strips 12 and 13. Due to the mixing blades 14, the spacer grid 50 forcibly mixes coolants flowing through the coolant passages of the spacer grid, thus enhancing the fuel rod cooling efficiency of the nuclear fuel assembly. Since the upper dimple 15 has the arc-shaped lower edge 32, and the lower dimple 17 has the arc-shaped upper edge 31, it is possible to reduce pressure loss inside the spacer grid 50. In addition, the spacer grid 50 of the present invention does not have any horizontal support beam at a position under the lower dimple 17, different from conventional spacer grids, so that the inventive spacer grid 50 effectively removes impurities from coolant when the coolant having the impurities flows into the spacer grid 50 through the lower end of the spacer grid 50. The equiangular curved part 23 of each water strider-type spring 16 is axially formed along the central axis of the spring 16 such that the curved part 23 has a substantial length. Therefore, the fuel rod support surface of the springs 16 is enlarged, and the fuel rod support force of the springs 16 is increased. Soundness of the spacer grid 50 supporting the fuel rods 8 within a nuclear fuel assembly is thus improved. Another advantage of the spacer grid 50 according to the present invention resides in that load, applied to the water strider-type spring 16 of each unit strip part 45 from a fuel rod 8 through the equiangular curved part 23, is effectively distributed to the entire structure of the unit strip parts 45 through the four spring legs 28. FIG. 10 shows an inner strip constituting a lips-type multi-purposed spacer grid for nuclear fuel assemblies according to a second embodiment of the present invention. As shown in the drawing, the water strider-type spring 16xe2x80x2 provided in each unit strip part of the inner strip constituting the spacer grid according to the second embodiment is designed such that the width of the axial opening formed along the central axis of the equiangular curved part 23 is enlarged to reduce the width of the two side extensions 36 and 37. The spacer grid according to the second embodiment of the present invention improves heat transfer efficiency thereof, thus more effectively transferring heat from the claddings of fuel rods to coolant. In this embodiment, the size of the axial opening of the equiangular curved part 23 may be adjusted in an effort to control the fuel rod support force of the water strider-type spring 16xe2x80x2, thus improving the fuel rod support force of the springs 16xe2x80x2 and soundness of the spacer grid supporting the fuel rods in a nuclear fuel assembly. FIGS. 11a and 11b show an inner strip according to a third embodiment of the present invention, and a lips-type multi-purposed spacer grid fabricated by intersecting the inner strips, respectively. As shown in the drawings, each unit strip part of the inner strip according to the third embodiment is designed such that two water strider-type springs 16xe2x80x3, each having a short equiangular curved part 23, two short side extensions 36 and 37, and four short spring legs 28, are formed at upper and lower portions inside the rectangular middle opening of the unit strip part. Therefore, a fuel rod 8, installed within a four-walled unit cell of the spacer grid, is supported at eight points by four water strider-type springs 16xe2x80x3 and four dimples, so that the spacer grid more stably supports the fuel rods inside a nuclear fuel assembly. In addition, even though one of the two springs 16xe2x80x3 of each unit strip part is broken by, for example, impurities, the remaining spring 16xe2x80x3 effectively supports the fuel rod, so that soundness of the spacer grid supporting the fuel rods is improved. As described above, the present invention provides a lips-type multi-purposed spacer grid for nuclear fuel assemblies. In the spacer grid of the present invention, fuel rods are in contact with dimples and water strider-type springs in an equiangular surface contact manner. The spacer grid thus soundly supports the fuel rods even when the fuel rods are excessively loaded in any direction due to a variation in operational conditions of a nuclear reactor. Particularly, the fuel rod support surface of the springs is enlarged, and the fuel rod support force of the springs is increased. Soundness of the spacer grid supporting the fuel rods within a nuclear fuel assembly is thus improved, and the spacer grid reduces possible fretting wear of the fuel rods due to hydraulic vibration of the fuel rods. The springs of the inventive spacer grid each have four spring legs designed in the form of four long slender legs of a water strider. In addition, the four legs of the water strider-type spring each have a bent part which appropriately controls the fuel rod support spring force of the water strider-type spring, and enlarges the allowable elastic range of the spring. The upper and lower edges of the water strider-type spring are curved to form arc-shaped edges which are symmetrical with respect to a horizontal axis of the spring. Due to the arc-shaped upper and lower edges of the water strider-type spring, the torsion applied to the spring from a fuel rod through the equiangular curved part is effectively distributed to the entire structure of the unit strip part, so that the spacer grid effectively supports the fuel rods during an operation of a nuclear reactor. An axial opening is formed in the equiangular curved part of the water strider-type spring, so that it is possible to prevent disturbance of heat transfer at a part of the cladding due to coolant remaining at a gap between the cladding and the equiangular curved part. Therefore, the spacer grid does not cause nucleate boiling at the claddings of the fuel rods. It is also possible to control the fuel rod support spring force of the spacer grid to a desired level by appropriately changing the size of the axial opening formed at the equiangular curved part of the water strider-type spring. It is thus not necessary to impose excessive force on the fuel rods when installing or removing the fuel rods in or from the spacer grid, so that the claddings of the fuel rods are less likely to be scratched or damaged by the springs of the spacer grid. The claddings of the fuel rods are thus prevented from corrosion caused by such scratched or damaged parts. This results in an extension of life spans of the fuel rods. In addition, a mixing blade extends upward from the upper edge of an upper dimple while being smoothly curved to have a spoon-shaped configuration, so that the mixing blade changes the axial flow of coolant to a lateral flow within each unit four-walled cell of the spacer grid, thus effectively mixing the coolant within the spacer grid. Since the upper dimple has an arc-shaped lower edge, and the lower dimple has an arc-shaped upper edge, it is possible to reduce pressure loss inside the spacer grid. In addition, the flow direction of coolant flowed in from at the lower end of the spacer grid is changed, and the debris of coolant is guided to the gaps between the dimples and the water strider-type springs of the spacer grid, so that debris are effectively captured at the gaps. The spacer grid thus minimizes damage to the fuel rods due to such debris. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.