Patent Number: 047818840
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION The numeral 10 generally designates a fuel assembly unit. The fuel assembly 10 includes an upper end fitting 12, a lower end fitting 14, spacer grids 16 supporting fuel rods 17, and a skirt portion 18 shown partially broken away in FIG. 1 to illustrate a debris catching strainer grid constructed accordingly to the principles of the invention, generally designated by the numeral 20. Behind the skirt 18, within the compartments defined by the debris catching strainer grid 20 and at the ends of fuel rods 17, are solid fuel rod end caps 22. Each fuel rod end cap 22 is located in an end cap compartment of the type illustrated in perspective in FIG. 2. The end cap compartment is defined by pairs of first intersecting and slottedly interlocked grid forming strips 24 known as top grid strips because of their lower slots 27. Strips 24 are for assembly with a second pair of intersecting grid forming strips 26, which are known as bottom grid strips because their slots 27 for slottedly interlocking with the grid forming strips 24 are located and along their upper margin. When assembled, the strips 24 in the area of intersection are above the strips 26. The pairs of first and second intersecting and slottedly interlocked grid forming strips 24 and 26 are attached to the perimeter strip 28 shown in detail in FIGS. 10, 11 and 12. The end cap compartments, as can be seen in FIG. 2, include vertical rows of integral leaves 30 intermediate their intersection of the pairs of first and second intersecting and slottedly interlocked strips 24 and 26. Each of the leaves 30 of a vertical row have a distance of projection out of the plane of its respective strip 24 or 26 different from the others of its row with the lower leaf 30 projecting out of the plane of its strip the most, while each successive leaf in the row as flow proceeds upwardly past it, projects a lesser amount. Collectively, these rows act as a strainer to prevent debris from leaving the inactive region of the fuel assembly in the area of skirt 18 and fuel rod end caps 22 and proceeding along the fuel rods 17 into the active region of the fuel assembly 10 along the sides of the cladding of fuel rods 17. The rods 17 are hollow and filled with fuel material in the active region as opposed to being solid in the fuel end cap region. As seen, for example, in FIG. 2, each of the leaves 30 in a vertical row is spaced from its adjacent leaves 30 in the row by an amount equal to the width of a leaf 30a in a row on the opposite side of the strip of which it is integral. For convenience, the designation 30 has been made for leaves which extend outwardly from the compartment illustrated in FIGS. 2 through 4 and the designation 30a is given to leaves which extend inwardly into the compartment illustrated in FIGS. 2, 3 and 4. FIGS. 5 through 9 show the details of the strips utilized in making up the embodiment of the strainer grid 20 of which the compartment shown in perspective in FIG. 2 is a part. The fragmentary strips 24 and 26 illustrated in FIGS. 5 though 9 include weld material tabs 32 to provide material for nugget welds when the strips 24 and 26 are in assembled grid-like condition. FIGS. 10 through 12 show the perimeter member 28 with its equivalent structure to that of strips 24 and 26 to the extent necessary to make complete compartments on the outside margins on the grid 20. Obviously, only leaves 30a which are inwardly directed toward the compartment are required. In the illustrated embodiment of FIGS. 2 through 12, all of the leaves 30 and 30a are constructed by bending them out of the plane at angles which are multiples of 15.degree.. Thus, the lowermost outwardly extending leaf 30 leaves the plane of the strips 24, for example, at 45.degree. and the next leaf 30 up, at 30.degree.. The lowermost leaf 30a leaves the plane of the strip 24 at 60.degree. and the next uppermost leaf 30a of the innervertical row leaves the plane of the strip 24 at 30.degree.. This is the same for the leaves 30a of all of the strips 24, 26 and includes perimeter strip 28. It will be seen that an effective strainer grid structure is shown by the embodiment of FIGS. 2 through 12, in the form of a grid which is attached, for example, by welding to the upper surface of the lower end fitting 14. The grid is oriented such that the solid fuel rod end caps 22 have the leaves 30 and 30a between them such over a flow hole 34 in the lower end fitting 14. Accordingly, the reactor coolant flow is directly presented to the grid device 20 in a manner in which the grid is likely to collect and retain debris in the reactor coolant flowing through flow hole 34 and upward along and between the solid fuel rod end caps 22. In the embodiment shown in FIGS. 13 through 19, parts analogous to the parts of the embodiment of FIGS. 2 through 12 are designated with the numeral 1 preceding the other digits of the designating number. Thus, top grid strips 124 and bottom grid strips 126 are utilized as the pairs of intersecting and slottedly interlocking strips in the embodiment of FIGS. 13 to 19 except the strips 124 and 126 are in reality, blanks which are half of a strip in order that double intersections, as shown in FIG. 19, can be produced. To accommodate these double blank intersecting strips, double slots 127a and 127b are provided in the lower margin of blank strip 124 in the upper margin of blank strip 126. A suitable perimeter strip 128 is provided, as shown in FIGS. 17 and 18 into which welding tabs 132 are inserted through holes 132a. In the case of the embodiment of FIGS. 13 through 19, leaves 130 c and 130d are provided on the blank strips 124 and 126. In this embodiment there are no vertical rows and the leaves 130c and 130d are mirror images of each other designed to coact with the contour of the fuel rod end caps 22. They are of asymmetric shape with the portions of their greatest distance of projection out of the plane of the strips 124 and 126 remote from the midpoint of the strips between their intersections. Since the leaves are in rather close proximity to the contour of the end caps 22, they provide a means for trapping debris against the end caps 22. The end caps 22 are of solid material and are therefore capable of accepting a great deal of wear without penetration of the fuel barrier. While the illustrated embodiments show only a limited number of leaves per strip, more are possible. The grid strip thicknesses relate to the number of leaves but they may be as thin as 12 mils to minimize the overall fuel assembly pressure drop. Accordingly, it will be seen that a novel debris catching strainer grid for capturing and retaining deleterious debris carried by reactor coolant before it enters the active region of the fuel assembly and creates fuel rod cladding damage is provided.