Patent Number: 052590097
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A portion of a boiling water reactor (BWR) fuel rod assembly employing the spacer system of the invention for retaining fuel rods 13 is generally shown at 10 in FIG. 1. Although the present invention will be described in relation to such a fuel assembly, it will be readily understood by those skilled in the art that the present invention can be used with any type of nuclear fuel assembly, not just BWR fuel assemblies or the BWR fuel assembly shown in FIG. 1. The fuel assembly shown in FIG. 1 has a central water channel which extends along the length of the fuel assembly. A common problem in typical boiling water reactors is that the central region of the fuel assemblies may be undermoderated and over-enriched. In order to increase the flow of moderator, an elongated central water channel is provided which includes a centrally disposed path for the flow of moderator/coolant along the length of the fuel rods in order to improve neutron moderation and economy. The central water channel can be of any cross-sectional area and/or geometry, positioned centrally and symmetrically within the outer channel, or asymmetrically displaced from the central axis within the outer channel, and can be oriented around its central axis so that its walls which extend the length of the assembly are either parallel or non-parallel to the walls of the outer channel. In FIG. 1, central water channel 15 is centrally located within outer channel 11. Outer rectangular channel 11 has retained therein a plurality of rod spacers 12 arranged at different elevations along the length of fuel assembly 10. The bottom-most spacer 12 at the end of the assembly, is shown in FIG. 1. Rod spacers are located at selected spaced intervals, and in a preferred embodiment are typically positioned at 15 to 22-inch intervals along the length of the fuel rod bundle. Another rod spacer 12 positioned along the length of fuel assembly 10 is shown in phantom above the bottom spacer 12. Other spacers, although not shown, are similarly positioned along the length of fuel assembly 10. FIG. 2 is a perspective view looking from the side and down the fuel assembly 10 depicted in FIG. 1 and shows a lower portion of the assembly, including bottom spacer 12, with the outer channel 11 removed. Although most of the fuel rods 13 are not shown in FIG. 2 for clarity of illustration, each of the spacer strips 20 are all shown in their "loaded" position, as explained hereinafter. Each of rod spacers 12 is formed of a lower lattice or grid 12a and a closely adjacent upper lattice or grid 12b. Fuel rods 13 are securely retained by each of the upper and lower lattices of each spacer. Spacers 12 each have four outer sidewalls 9a, b, c, d, which extend along the width and depth of outer channel 11 as shown in FIGS. 1 and 2. Dividing or stiffening plates 18, 19 divide each spacer 12 into nine separate subregions. Eight of such regions (14a through 14h) shown in FIG. 1 contain fuel rods 13 and one region houses central channel member 15. Although any number of plates can be used and form subregions having equal or unequal cross-sectional areas, it will also be readily understood by those skilled in the art that the present invention can be practiced with or without dividing plates which form subregions. FIGS. 3 and 4 are plan views looking up from the bottom of the fuel assembly. The structure of spacers 12 is shown in the perspective view of FIG. 2 and the plan view of FIGS. 3 and 4 showing a cross-sectional view of one subregion, and more particularly region 14a of lower lattice 12a illustrated in FIG. 1. Each of the upper and lower lattices 12b, 12a of spacers 12 is formed of spacer strips. Spacer strips 20a through 20f are provided for each of the upper and lower lattices 12b, 12a of each spacer 12 within each subregion 14a through 14h. Spacer strips 20a and 20f of subregion 14a of lower lattice 12a shown in FIG. 3 are positioned along sidewall 9a and stiffening plate 18, respectively. Ends 22, 23 of each of spacer strips 20a through 20f are respectively received and secured in slots 24 and 25 in stiffening plate 19 and side wall 9b. In lieu of slots for supporting the ends of the spacer strips, welds may be provided. As can be seen in FIG. 1, the orientation of spacer strips 20a through 20f alternate by 90.degree. from subregion to subregion in a lateral direction. Also, as can be seen in FIGS. 1 and 2, for each spacer, spacer strips 20a through 20f in each of the subregions of lower lattice 12a are oriented at 90.degree. relative to spacer strips 20a through 20f in adjacent upper lattice 12b. As shown in FIG. 4, each of spacer strips 20a-f has corrugations 30. Each corrugation 30 is three-sided and extends vertically to form a three-sided groove or channel. Each corrugation has a face 31. With the fuel rods 13 loaded into spacer lattice 12a as shown in FIG. 3, faces 31 of corrugations 30 of spacer strip 20b abut with the oppositely positioned and reversed faces 31 of corrugations 30 of spacer strip 20c. Where adjacent and reversed faces 31 of corrugations 30 of spacer strips 20b and 20c abut one another, faces 31 are welded to one another in every other column, as indicated by the asterisk * at 33. Thus, in each row, the abutting corrugations alternate welded and unwelded. Similarly, the alternate abutting faces 31 of spacer strips 20d and 20e are welded together at 33. In addition, alternate faces 31 of spacer 20a are welded to side wall 9a, and alternate faces 31 of spacer strip 20f are similarly welded at 33 to stiffening plate 18 as shown in FIGS. 3 and 4. Thus, weld points alternate in each row and column within the spacer. The significance of the alternating weld points, is apparent from the FIG. 4 which is of the same spacer shown in FIG. 3, but before loading fuel rods 13 in the spacer. As is shown in FIG. 4, faces 31 of corrugations 30 of adjacent spacer strips which are not welded together are spaced apart due to the resilience and shape of the individual spacer strips. The spaced apart relationship of unloaded spacers are shown as gaps 37b-37d in FIG. 4. The space or gap between face 31 of spacer strip 20a and wall 9a is shown as 37a and the spaces between faces 31 of strip 20f and stiffening plate 18 are shown as 37e and 37f in the unloaded spacer shown in FIG. 4. Thus, in the unloaded condition, corrugations 30 are not in line along a row, but rather are displaced and staggered or tilted by the integral spring force provided by the spacer strip material. The positioned fuel rods 13, once they are loaded in the unloaded spacer strips, are shown in phantom in FIG. 4. Accordingly, with the fuel rods loaded, corrugations 30 are in line such as shown in FIG. 3 and exert a retaining pressure on the fuel rods 13 which are positioned between the opposite facing concave corrugations which form rod supporting cells 38. Thus, supporting cells are created between oppositely facing concave portions of the corrugations. With the exception of the fuel rod in each outer corner of the fuel assembly, each fuel rod 13 in each cell 38 is supported by two spring edges 34 of corrugation 30 forcing rod 13 against two stationary supports 35 of corrugation 30 as shown in FIGS. 3 and 4. As can best be seen in FIGS. 1 and 2, the spacer strips in each subregion of the upper lattice grid 12b are oriented 90.degree. relative to those in lower lattice grid 12a. Thus, the direction of the forces applied by spring edges 34 against the fuel rods and the fuel rods against supports 35 in upper lattice grid 12b are similarly oriented 90.degree. relative to the forces in lower lattice grid 12a. Furthermore, as is evident from FIG. 1, the alternating direction of the spacer strips from subregion to subregion provides improved uniformity for both flow resistance and lateral strength. The arrangement provides a very strong overall support arrangement which opposes a failure due to lateral loading. In the region between four loaded adjacent fuel rods 13, an open space 39 is formed by two adjacent spacer strips (FIG. 3). Open spaces 39 which are formed by spacer strips between any four adjacent fuel rods within a subregion allow steam bubbles to flow unobstructed up the center between the four adjacent fuel rods, while allowing primarily liquid moderator to flow around the fuel rods, thus enhancing heat transfer. Four such open areas 39 are provided in each subassembly in the embodiment shown in FIG. 3. In addition, several other partial open areas 40 are also provided. Spring relaxation as a result of irradiation of spacer strips 20, and more particularly spring edges 34 and stationary support 35 of corrugations 30, is reduced by constructing the stiffening plates 18, 19 of annealed zircaloy and spacer strips 20a, 20b, 20c, 20d, 20e, and 20f of cold worked stress relieved zircaloy. The greater irradiation induced growth rate of the cold worked material restrained by the lower growth rate of the annealed material tends to cause the resilient spacer strips to move toward the fuel rods and provide a spring force against the fuel rods, reducing irradiation induced spring relaxation. As shown in FIGS. 2 and 5, each of the spacers 12 preferably has inwardly bent guiding tabs 16 extending from the top edge of side walls 9a, b, c and d at the upper spacer lattice 12b. Inwardly bent guiding tabs 16 allow ease of placement of the outer channel 11 over the spacers. Inwardly bent guiding tabs 16 and outwardly bent tabs 17 also serve the purpose of directing liquid water which has condensed on the outer and inner channels 11, 15 towards the fuel rods, enhancing local heat transfer. According to another feature of the invention, kick-outs 41 positioned on side walls 9a, 9b, 9c and 9b (FIG. 5) and tabs 42a, 42b (FIG. 2) positioned at the top of stiffening plates 18, 19 may be provided. Kick-outs 41 and tabs 42a, 42b are arranged to impart a circular motion to the two-phase coolant flow. This circular motion tends to coalesce small water droplets into larger water droplets aiding in their transfer to the fuel rod surfaces. Tests have shown that the net effect is an increase in water film thickness on the fuel rod, resulting in an improvement in local heat transfer. While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.