Patent Number: 062367021
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 2, a fuel assembly spacer grid according to an embodiment of the present invention is illustrated. As shown in FIG. 2, the spacer grid, which is denoted by the reference numeral 1, includes a plurality of longitudinally-extending, parallel, spaced vertical straps 2 and a plurality of laterally-extending, parallel, spaced vertical straps 3 perpendicularly interconnecting the longitudinally-extending straps 2, in order to support fuel elements of a nuclear fuel assembly. The spacer grid 1 also includes a plurality of swirl deflectors 20 respectively provided at the upper ends of the interconnections between the straps 2 and 3, a plurality of springs 30 provided at the straps 2 and 3, and a plurality of dimples 40 provided at the straps 2 and 3. As shown in FIG. 3 viewing the spacer grid 1 from above, the springs 30 and dimples 40 have conformal contact portions having the same radius of curvature as fuel elements 11 to be supported by the spacer grid 1. The swirl deflectors 20 have an air vane structure including vanes 23. As shown in FIGS. 2 and 3, the swirl deflectors 20 are configured to have the same vane rotation direction. If desired in terms of an improvement in cooling performance, however, the swirl deflectors 20 may be configured to have reverse vane rotation directions at adjacent cooling water passages, respectively, as shown in FIG. 4. A detailed structure of the swirl deflectors 20 is shown in FIG. 5. As shown in FIG. 5, each swirl deflector 20 has a pair of intersecting triangular base plates 21 extending upwardly from the interconnecting straps 2 and 3 at the interconnection thereof, respectively, and four vanes 23 extending upwardly from respective side surfaces of the base plates 21. In order to generate a swirling flow, the vanes 23 of each swirl deflector 20 are bent in the same direction from the associated base plates 21, respectively. The bent angle of each vane 23 should not excess 90.degree.. Each swirl deflector 20 may be fixed to the straps 2 and 3 by means of welding. In order to obtain a desired strength of the spacer grid 1, the vanes 23 may have a controlled size. Although each swirl deflector 20 has four vanes 23 in the illustrated case, it may have only two vanes attached to a selected one of the straps 2 and 3. Where it is desired to increase the vane area, each vane 23 may be enlarged in such a manner that it has a larger width at the upper portion thereof than that at the lower portion thereof. FIG. 6 shows the shape of the vanes 23 given before the vanes 23 are bent. As shown in FIG. 6, each base plate 21 protrudes upwardly from the upper end of the associated strap 2 or 3. Vanes 23 are disposed on opposite sides of the base plate 21, respectively. FIGS. 7 and 8 illustrate a detailed structure of the springs 30. Each spring 30 protrudes from the associated strap 2 or 3. That is, the spring 30 is attached at one end thereof to the associated strap 2 or 3 and has an elastic free end at the other end thereof. The spring 30 also has a contact portion 31 contacting a fuel element 11. The contact portion 31 is configured to come into surface contact with the circumferential surface of the fuel element 11. In order to adjust the spring force, the spring 30 also has an opening 34. Although the opening 34 has a rectangular shape in the illustrated case, it may have a variety of shapes for desired spring characteristics. The free end of the spring 30 is inclinedly bent, thereby forming a bent end portion 33 having a shape inclined with respect to the axis of the fuel element 11 in such a manner that it has a width increasing gradually as it extends upwardly. The bent end portion 33 of the spring 30 is subjected to a hydraulic drag force when the spring 30 is positioned in a flow of cooling water. Thus, the spring 30 serves as a hydraulic pressure spring. The hydraulic pressure spring 30 varies its spring force in accordance with a variation in the flow rate of cooling water. As the flow rate of cooling water increases, the hydraulic pressure spring 30 increases in spring force, so that it supports the fuel element more firmly. FIG. 9 illustrates a detailed structure of the dimples 40 which serve to support fuel elements 11 at positions opposite to the springs 30. Each dimple 40 protrudes from the associated strap 2 or 3. The dimple 40 has a contact portion 41 contacting a fuel element 11. The contact portion 41 has the same radius of curvature as the fuel element 11 so that the dimple 40 has an increased contact area. By virtue of such an increased contact area, it is possible to reduce abrasion of the fuel element. The dimple 40 also has an opening 42 so as to have an increased height and a reduced strength. FIG. 10 is a partially-broken perspective view showing the interior of the spacer grid 1 shown in FIG. 2 whereas FIG. 11 is a sectional view of FIG. 10. These drawings show the contact relationship between the springs 30 and the fuel element 11 supported by the springs 30, and a method for supporting the fuel element 11. As shown in FIGS. 10 and 11, one spring 30, which is formed on each strap, is positioned at the middle portion (when viewed in a vertical direction) of the strap. Two dimples 40 are positioned above and beneath the spring 30. Accordingly, each fuel element is supported at six points by the surrounding fuel straps. FIG. 12 is a perspective view illustrating one of the longitudinally-extending straps 2 included in the spacer grid 1 of FIG. 2. The longitudinally-extending strap 2 is provided at the upper end thereof with a plurality of uniformly-spaced coupling grooves 2a so that it is interconnected with the laterally-extending straps 3 in a cross fashion. FIG. 13 is a perspective view illustrating one of the laterally-extending straps 3 included in the spacer grid 1 of FIG. 2. The laterally-extending strap 3 is provided at the upper end thereof with a plurality of uniformly-spaced coupling grooves 3a so that it is interconnected with the longitudinally-extending straps 2 in a cross fashion. FIG. 14 is a plan view illustrating the longitudinally-extending strap 2 shown in FIG. 12. As shown in FIG. 14, the vanes 23 of neighboring swirl deflectors are bent in the same direction. The spring 30 and dimples 40, which are provided at each strap, are arranged on the swirl deflector along the same vertical axis. The spring 30 and dimples 40 protrude from the strap in opposite directions in order to support fuel elements disposed at opposite sides of the strap, respectively. FIG. 15 is a cross-sectional view taken along the line A--A of FIG. 14. As shown in FIG. 15, the vanes 23 of each swirl deflector are bent from the base plate 21 of the swirl deflector by a desired angle in order to increase an effect of mixing flows of cooling water while minimizing an interference thereof with the associated fuel element 11. FIG. 15 also shows that each hydraulic pressure spring 30 is bent at its free end by a desired angle with respect to the vertical axis of the associated strap, so that it generates a horizontal pressure when it comes into contact with a flow of cooling water, thereby increasing the spring force supporting the associated fuel element 11. As apparent from the above description, the swirl deflector 20 provided at the spacer grid 1 according to the present invention can produce a strong swirling flow of cooling water, as compared to conventional devices. This is because the swirl deflector 20 includes four vanes 23 formed into an air vane shape at each interconnection between the straps 2 and 3. Where the vanes 23 of the swirl deflector 20 have a streamline shape, it is possible to produce a more efficient swirling flow of cooling water while achieving a reduction in pressure loss. For the production of a strong swirling flow of cooling water, four vanes having the above mentioned structure are provided at both the longitudinally and laterally-extending straps 2 and 3 at each interconnection, respectively. On the other hand, two vanes are provided at a selected one of the straps 2 and 3 at each interconnection for a reduction in the pressure loss caused by the provision of the swirl deflector 20. Since the vanes 23 of each swirl deflector 20 are formed in such a fashion that they are bent from the opposite side surfaces of the associated base plate 21, they swirl a flow of cooling water flowing upwardly from beneath, thereby efficiently guiding the cooling water flow. Accordingly, a reduced pressure loss occurs, as compared to conventional devices. Since each spring 30 is attached at one end thereof to the associated strap 2 or 3 while being provided at the other end thereof with an inclinedly-bent elastic free end, it is subjected to hydraulic pressure when it is positioned in a flow of cooling water. Accordingly, the spring 30 generates not only a mechanical spring force, but also an additional spring force resulting from the hydraulic pressure applied thereto. Thus, it is possible to compensate for a reduction in the initial spring force of the spring. In conventional devices, a flow of cooling water, which is introduced in the spacer grid through a central portion of the spacer grid, strikes a swirling flow of cooling water passing through the spacer grid, thereby offsetting the swirling effect of the swirling flow. In accordance with the present invention, however, the swirl deflector 20 has four integral vanes arranged on quadrant regions defined by the longitudinally and laterally-extending straps. Accordingly, a flow of cooling water, which is introduced in the spacer grid through a central portion of the spacer grid, is forced to be swirled when it passes through the swirl deflector 20 disposed at the downstream of the spacer grid. Thus, the swirling motion of the cooling water flow can be maintained far the downstream of the spacer grid. In accordance with the present invention, the swirling vanes 23 of each swirl deflector 20 are attached to opposite side surfaces of the triangular base plate 21 in such a manner that they extend inclinedly. Accordingly, it is possible to provide a larger vane area at the same projected area, as compared to the vanes of conventional devices. By virtue of such an increased vane area, there is an advantage in terms of the generation of a swirling flow of cooling water. By virtue of the generation of a strong swirling flow of cooling water and a delayed disappearance of the swirling flow, a centrifugal force generated in the cooling water flow causes bubbles of a lower density produced from the cooling water flow on the surfaces of fuel elements 11 to be concentrated on the swirling center of the cooling water flow while causing the liquid portion of the cooling water flow, which has a higher density, to move toward the surfaces of the fuel elements 11. Accordingly, an improvement in the cooling performance of the spacer grid 1 is achieved. In accordance with such an improvement in cooling performance, the spacer grid 1 ultimately suppresses a boiling phenomenon occurring in fuel elements, thereby preventing a leakage of radioactive materials from the fuel elements. This contributes to the safety of the nuclear reactor. Since the vanes 23 of each swirl deflector are attached to the triangular base plate 21 in accordance with the present invention, they have an elongated and inclined base. Accordingly, these vanes 23 are more stable structurally and mechanically, as compared to conventional vanes attached to a base plate having a shape other than the triangular shape. Therefore, it is possible to prevent the vanes from being easily deformed or broken due to an external impact applied thereto during a placement of a fuel assembly in the reactor core or during a transportation of the spacer grid. As apparent from the above description, the present invention provides a fuel assembly spacer grid including springs each configured to generate not only a main spring force caused by a displacement of the spring occurring when the spring comes into contact with a fuel element placed in a reactor core, but also an additional spring force caused by hydraulic pressure applied to the spring. Each spring, which is in a fixed state at one end thereof, has a free bent portion at the other end. When a flow of cooling water flowing upwardly from beneath strikes the bent portion of the spring, it reflects inclinedly from the bent portion of the spring while applying hydraulic pressure to the spring. As a result, the spring applies the pressure to the fuel element supported thereby. That is, the hydraulic pressure of the cooling water flow applied to the spring serves as an additional spring force. Thus, it is possible to compensate for a reduction in the initial spring force of the spring resulting from a change in the property of the spring material. The hydraulic pressure on the spring in the cooling water flow varies in accordance with a variation in the flow rate of the cooling water flow in such a fashion that it increases at a higher flow rate while decreasing at a lower flow rate. Accordingly, there is an advantage in that the spring force adapted to support fuel elements can be adjusted by controlling the flow rate of the cooling water flow. The spring force resulting from the cooling water flow is always generated at a substantially constant level unless the shape of the spring is changed. In an environment where the initial spring force of the spring is gradually reduced due to a repeated irradiation of neutrons, as in the interior of a nuclear reactor, it is possible to sufficiently compensate for the reduced portion of the spring force. Accordingly, the utility of the spring according to the present invention increases, in particular, in the case in which a fuel assembly is placed in a reactor core for an extended period of time. In accordance with the present invention, the spring has a conformal contact portion contacting the circumferential surface of a fuel element, supported thereby, in a larger area. By virtue of such an increased contact area, the spring exhibits a high resistance to a fretting abrasion of the fuel element caused by vibrations and resulting in a damage of the fuel element. In order to solve problems resulting from an excessive increase in the spring force caused by the increased contact area, the spring also has an opening at the conformal contact portion. Accordingly, it is possible to maintain a desired height of the contact between the spring and fuel element without reducing the height of the spring itself. Therefore, the insertion and withdrawal of fuel elements can be achieved without requiring an excessive force. This reduces the possibility of a damage of fuel elements. That is, there is no possibility of a corrosion of fuel elements occurring at damaged areas. Accordingly, it is possible to prevent the life of fuel elements from being reduced. Although the preferred embodiments of the 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.