Patent Number: 
Section: description

FIG. 5 is a perspective view of a spacer grid according to the preferred embodiment of this invention. FIG. 6a is a plan view of the above spacer grid. FIG. 6b is a side view of the spacer grid. FIG. 7 is an enlarged perspective view of a unit strip included in the spacer grid of this invention. FIG. 8 is a graphic diagram showing the force applied from a fuel rod as a function of the displacement of a grid spring for the spacer grid of this invention. FIG. 9a is a plan view of a fuel rod cell of the spacer grid according to this invention, with one fuel rod placed and supported by grid springs and dimples within the cell. FIG. 9b is a perspective view of the fuel rod cell. FIG. 10 is a view, showing a deformation of a grid spring due to the external force applied from the fuel rod in the spacer grid of this invention. As shown in the drawings, the spacer grid 1 according to the present invention is fabricated with a plurality of inner strips 10, which are arranged while intersecting each other at right angles prior to being encircled with four perimeter strips 12, thus forming an egg-crate pattern. The inner and perimeter strips 10 and 12 have the same height, and are each fabricated with a plurality of unit strips 15 arranged in parallel. Due to the above-mentioned strip arrangement of the spacer grid 1, a plurality of unit fuel rod cells 17 are regularly defined in the spacer grid 1 such that each cell 17 is formed by four unit strips 15. A fuel rod 5 is fitted into each cell 17 while being isolated from the other fuel rods, thus forming a desired fuel assembly. The construction of the unit strips 15 for the inner strips 10 is different from that of the unit strips 15 for the perimeter strips 12 as follows: That is, each unit strip 15 for the inner strips 10, referred to simply as xe2x80x9cfirst unit stripxe2x80x9d in the following description, has one grid spring 20, which is vertically formed on the central portion of the first unit strip 15 while projecting in a direction as best seen in FIG. 7. Two dimples 25 are formed on the first unit strip 15 at positions above and under the grid spring 20 while projecting in a direction opposite to the spring 20. However, each unit strip 15 for the perimeter strips 12, referred to simply as xe2x80x9csecond unit stripxe2x80x9d in the following description, has only one grid spring 20 without having any dimples 25 different from the first unit strip 15. The grid spring 20 of each second unit strip 15 for the perimeter strips 12 is formed in the same manner as that described for the first unit strip 15 of the inner strips 10. In the above spacer grid 1 of this invention consisting of the intersecting inner strips 10 and the perimeter strips 12 surrounding the intersecting structure of the inner strips 10, the inner strips 10 isolate the unit fuel rod cells 17 from each other and support the fuel rods 5 set within the cells 17, and so the first unit strip 15 for the inner strips 10 has one grid spring 20 and two dimples 25 which project from the unit strip 15 in opposite directions. However, the perimeter strips 12 do not support any fuel rod 5 outside them, and so the second unit strip 15 for the perimeter strips 12 is not provided with any dimples 25, different from the first unit strip 15 for the inner strips 10. Even though the first and second unit strips 15 for the inner and perimeter strips 10 and 12 are different from each other in their structure, the grid springs 20 formed on the first and second unit strips 15 have the same shape and construction. The construction of the grid springs 10, commonly formed on the first and second unit strips 15 for the inner and perimeter strips 10 and 12, is included in the most important characteristics of the present invention. That is, the grid spring 20 of each unit strip 15 comprises a vertical support part 40 and a fuel rod support part 50. In order to form the vertical support part 40, an opening 30 is formed at the central area of each unit strip 15, with upper and lower base parts 32 and 34 extending downward and upward from the central portions of the top and bottom edges of the opening 30. The upper base part 32 also branches symmetrically into two bridge parts 36 and 38, which extend symmetrically downward until they are united at the lower base part 34. The fuel rod support part 50 is provided at the central portion of the spring 20. This fuel rod support part 50 comprises a conformal support part 45, which has a specifically bent shape and comes into surface contact with the external surface of a fuel rod 5 to elastically support the fuel rod 5, and two transverse connection parts 47, which extend outward from opposite outside edges of the conformal support part 45 while being specifically bent until the parts 47 are integrated with the central portions of the inside edges of the two bridge parts 36 and 38 into a single structure. The conformal support part 45 has the same radius of curvature as that of the fuel rod 5, thus being brought into surface contact with the external surface of the fuel rod 5. In the present invention, it is preferable to shape the profile of the contact surface of the conformal support part 45 as a circular or elliptical profile. Such a circular or elliptical profile of the conformal support part 45 is suitable for enlarging the surface contact area of the part 45 relative to the fuel rod 5, in addition to accomplishing a desired uniform contact pressure distribution and reducing the peak stress of the part 45. The two dimples 25 of each first unit strip 15 have the same radius of curvature as that of the fuel rod 5 in the same manner as that described for the conformal support part 45 of the grid spring 20. In addition, the contact surface of the dimples 25 is also appropriately curved at an angle suitable for accomplishing conformal contact of the dimples 25 with the fuel rod 5. As best seen in FIGS. 9a and 9b, the conformal support part 45 of the grid spring 20 and the conformal dimples 25 are brought into surface contact with the external surface of a fuel rod 5 when the rod 5 is set within a fuel rod cell 17 having the springs 20 and dimples 25. In order to form the vertical support part 40 of the grid spring 20, the upper and lower base parts 32 and 34 commonly project from the unit strip 15 in the same direction through a bending process while extending downward and upward from the central portions of the top and bottom edges of the opening 30. The vertical support part 40 is also bent along the junction lines of the two base parts 32 and 34 and the two bridge parts 36 and 38. The vertical support part 40 is further bent at two or more positions on each of the two bridge parts 36 and 38 so as to generally project toward the fuel rod 5, thus elastically supporting load applied from the fuel rod 5 thereto through the conformal support part 45 coming into direct contact with the fuel rod 5. In the preferred embodiment shown in the drawings, the upper and lower base parts 32 and 34 of the vertical support part 40 bend in the diverging direction, when the plane of each unit strip is datum plane. However, the two bridge parts 36 and 38 appropriately bend at several positions in the converging direction, when the plane of each unit strip is still datum plane. Therefore, the vertical support part 40 has a generally arc-shaped profile of the side face. Since the vertical support part 40 is shaped through a bending process as described above, its strength is relatively lower than that of the fuel rod support part 50 having the conformal support part 45. Therefore, the vertical support part 40 is elastically deformed prior to a deformation of the conformal support part 45 in the case of an application of a force from the fuel rod 5 to the grid spring 20. When the fuel rod 5 exerts a force to the fuel rod support part 50 of the grid spring 20, the two bridge parts 36 and 38 of the vertical support part 40 primarily absorb the force transmitted from the fuel rod support part 50. In such a case, as the two connection parts 47 of the fuel rod support part 50 are integrated with the central portions of the inside edges of the two bridge parts 36 and 38 into a single structure, a twisting moment 52 is applied to the inside edge of each of the two bridge parts 36 and 38 in a direction of FIG. 10. However, the two bridge parts 36 and 38 are also integrated with the unit strip 15 while converging to the upper and lower base parts 32 and 34, and so another twisting moment 54, opposite in direction to that of the twisting moment 52, acts around each bridge part 36, 38 at the junctions of the bridge parts and the base parts, thus somewhat offsetting the twisting moment 52. That is, a balance is formed between the two types of twisting moments 52, 54 having opposite directions each other, and so the twisting moment-induced deformation of the vertical support part 40 is minimized, but the spring 20 is mainly affected by a bending moment 55. The spacer grid 1 of this invention is designed such that the inside edges of the two bridge parts 36 and 38 included in the vertical support part 40 of the grid spring 20 are less likely to be deformed by a twisting moment. The spacer grid 1 thus almost completely overcomes the problems, which have been experienced in the conventional spacer grids due to twisting moments causing both a divergence of the vertical support parts of the grid springs and a reduction in the radius of curvature of the conformal support parts of the springs, and making the conventional spacer grids fail to maintain their surface contact with fuel rods. In the spacer grid 1 of this invention, the grid springs 20 are designed to reduce a slide movement of the fuel rods 5 relative to the conformal support parts 45 of the springs 20, thus stably supporting the fuel rods in the fuel assembly, and thereby improving the soundness of the fuel assembly, in addition to reducing a fretting wear of the external surface of the fuel rods. The vertical support part 40 of the grid spring 20 is bent at a plurality of positions, and has an enlarged elastic range, thus effectively and stably supporting the fuel rod while almost completely preventing a formation of gap between the fuel rod 5 and grid spring 20 irrespective of variable fuel rod support conditions inside a nuclear reactor. In addition, the grid springs 20 maintain their spring force capable of effectively supporting the fuel rod within the elastic limit of the springs 20. FIG. 8 is a graphic diagram showing the force, applied from a fuel rod 5, as a function of the displacement of a grid spring 20 for the spacer grid 1 of this invention. In the present invention, it should be understood that the base parts 32, 34 and the two bridge parts 36, 38 of the vertical support part 40 included in the grid spring 20 may be somewhat freely changed in their widths and the positions and angles of their bent portions as desired to obtain more optimized characteristic curves in the graphic diagram. From FIG. 8, it is apparent that the grid spring 20 of this invention has a larger elastic range than that of the conventional grid spring, effectively supports the fuel rod within the elastic limit of the grid spring 20. In the spacer grid 1 of this invention, as the upper and lower edges of the dimples 25 and the conformal support parts 45 of the grid springs 20 are bent back as well, the spacer grid 1 thus minimizes a fretting wear of the fuel rods 5 regardless of a sliding movement of the fuel rods relative to the conformal support parts 45 during an insertion of the fuel rods into the fuel rod cells 17 of the spacer grid or during an axial movement of the fuel rods within the cells 17 due to a lengthwise thermal growth of the fuel rods. The grid spring 20 of this invention was invented after studying design factors capable of optimizing the restoring force of the grid springs, stress and contact pressure applied to the springs, which are the most important factors required to accomplish a desired stable support for the fuel rods 5 inside a nuclear reactor fuel assembly. In order to optimize the restoring force of the grid springs, the springs are designed such that they are not plastically deformed, but remain within their elastic range even though they are somewhat excessively deformed to reach a maximum allowable displacement resulting from an addition of a displacement preset by a safety factor to a normal displacement of the grid springs during an insertion of the fuel rods into the spacer grids. The spacer grid of this invention is thus optimized with respect to the restoring force of its grid springs, and stably supports the fuel rods 5 while preventing vibration of the fuel rods during the effective life of the fuel rods. The stress of the grid springs is optimized to minimize the equivalent stress generated in the grid springs due to a displacement of the springs during an insertion of fuel rods into the spacer grids of a nuclear fuel assembly or an axial slip of the fuel rods relative to the springs in the case of a lengthwise thermal growth of the fuel rods. The contact pressure is optimized in terms of geometric variables to allow a uniform contact pressure distribution at the contact surface of the fuel rods and the conformal support parts 45 of the grid springs. That is, in a conventional spacer grid, the contact surface of the fuel rods and the conformal support parts of the grid springs does not have a uniform contact pressure distribution, thus sometimes causing a fretting wear of the fuel rods at the contact surface. The shape of the conformal support part 45 of the grid spring according to the present invention is designed in accordance with the optimized spring force, stress and contact pressure of the grid springs. In order to accomplish the optimized spring force, stress and contact pressure of the grid spring 20 of the spacer grid, the vertical support part 40 of the grid spring is appropriately bent at a plurality of positions through a bending process, with two base parts 32 and 34 and two bridge parts 36 and 38 integrated into a single structure to form a desired vertical support part 40. FIGS. 11 and 12 are FEM diagrams, each showing a contact pressure distribution in the case of 0.4 mm displacement of a grid spring analyzed through a finite element method (FEM). Of the diagrams, FIG. 11 shows an FEM diagram of a conventional H-type grid spring, while FIG. 12 is an FEM diagram of the grid spring according to this invention. From the comparison of the two diagrams, FIGS. 11 and 12, it is apparent that the contact pressures in the spacer grid of this invention are uniformly distributed over a wide area different from the contact pressure distribution of the conventional spacer grid. FIG. 13 is a graphic diagram, comparatively showing the characteristic curves of equivalent stress and strain relative to a displacement of the conventional H-type grid spring and present grid spring. As shown in the diagram, the stress varies in a similar manner in the conventional grid spring and the present grid spring. However, in the case of strain, it is apparent that the grid spring of this invention is excellent than the conventional grid spring. The spacer grid of this invention has the following operational effect. Since the vertical support part 40 of the grid spring 20 according to this invention is appropriately bent at several positions through a bending process, the strength of the vertical support part 40 becomes lower than that of the fuel rod support part 50 of the grid spring coming into direct contact with a fuel rod. Therefore, when the fuel rod exerts a force to the grid spring, the vertical support part 40 supports the force exerted from the fuel rod while being elastically deformed. In such a case, the vertical support part 40 is designed to minimize its deformation caused by a twisting moment, and is mainly deformed due to a bending moment. Therefore, it is possible to almost completely prevent a variation in the radius of curvature of the conformal support part 45 of the grid spring 20, in addition to minimizing the sliding movement of the fuel rod 5 relative to the conformal support part 45. The spacer grid 1 having such grid springs 20 stably supports the fuel rods 5 regardless of a variation in the fuel rod support conditions of the nuclear reactor fuel assembly, thus improving the soundness of the fuel assembly in addition to minimizing the fretting wear of the fuel rods. In another embodiment of the present invention, as shown in FIG. 7, the grid spring 20 of the present invention have upper and lower opening 52 and 54 surrounded by the vertical support part 40 and the fuel rod support part 50. In addition, as shown in FIG. 14, the grid spring 20 of the present invention further include upper and lower extending part 56 and 58 each extending from center of upper/lower base part 32/34 of the vertical support part 40 and partially enclosing said upper/lower opening 52/54. Consequently, efficiency of the flow mixing between adjacent fuel rod cells in the spacer grid is increased and the high frequency vibration of the fuel rods and strips due to the cross flow is restrained with effect. As described above, the present invention provides a spacer grid for nuclear reactor fuel assemblies with a grid spring capable of maintaining conformal contact with a fuel rod and having an enlarged elastic range. The object of the present invention is to overcome the problems, which have been experienced in the conventional spacer grids, and in which the vertical support parts of each grid spring are affected by both a bending moment and a twisting moment in the case of an exertion of a force from the fuel rod to the grid spring, thus allowing the grid spring to lose its desired conformal contact with the fuel rod in accordance with a displacement of the spring caused by the force exerted from the fuel rod and fail to stably support the fuel rod within the fuel assembly. In the spacer grid for nuclear reactor fuel assemblies according to this invention, the grid springs are designed to maintain conformal contact with fuel rods and have an enlarged elastic range. Even when the fuel rods exert a force to the grid springs so as to deform the springs, the conformal support parts of the springs continuously maintain their conformal contact with the fuel rods, while the vertical support parts of the springs are deformed by the force transmitted from the fuel rods through the conformal support parts. In such a case, the vertical support parts are mainly deformed by a bending moment except for a twisting moment. The spacer grid of this invention thus stably supports the fuel rods in a fuel assembly. In addition, the shape of the conformal support parts of the grid springs is optimized to a circular or elliptical profile, which accomplishes a desired uniform contact pressure distribution and reduces the peak stress of the conformal support parts, thus reducing a fretting wear of the fuel rods. Due to the conformal surface contact of the grid springs and dimples with the fuel rods, the springs stably support the fuel rods even when a force is exerted to the fuel rods in any direction due to variable operational conditions of the nuclear reactor. The conformal surface contact of the grid springs and dimples with the fuel rods also reduces a fretting wear of the fuel rods regardless of a repeated application of a force to the contact surfaces of the grid springs, dimples and fuel rods. Such enlargement of the elastic range of the grid springs allows the springs stably support the fuel rods within the spacer grid during the effective life of the fuel rods. Moreover, the spacer grid of the present invention may be preferably used for supporting a plurality of tubes or pipes situated lengthwise in an industrial machine using fluid feeding pipelines, boilers or heat exchangers while reducing occurrence of fracture of the tubes or pipes caused by frictional abrasion, fatigue or vibration. 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.