Patent Number: 
Section: claims

1. A spacer grid used in a nuclear fuel assembly such that a plurality of spacer grids are regularly and transversely arranged along the fuel assembly to support a plurality of fuel rods within the fuel assembly while maintaining a desired pitch of the fuel rods and promoting the mixing of a coolant flowing longitudinally upwardly along nuclear fuel rods, comprising: a plurality of first inner straps each having a rectangular-shaped first strap body, with a plurality of main supports and a plurality of upper sub-supports alternately formed along a top edge of the first strap body while being spaced apart from each other at regular intervals, a plurality of lower sub-supports formed along a bottom edge of the first strap body while being spaced apart from each other at regular interval, a pair of flow mixing vanes symmetrically formed along a top edge of each of said main supports, and a plurality of upper vertical slits extending from center of top edge of each said main supports and each upper sub-supports of the first strap body toward a middle of said first strap body; and  a plurality of second inner straps each having a rectangular-shaped second strap body, with a plurality of main supports and a plurality of upper sub-supports alternately formed along a top edge of the second strap body while being spaced apart from each other at regular intervals, a plurality of lower sub-supports formed along a bottom edge of the second strap body while being spaced apart from each other at regular interval, a pair of flow mixing vanes symmetrically formed along a top edge of each of said main supports of the second strap body, and a plurality of lower vertical slits extending from center of bottom edge of each said lower sub-supports of the second strap body toward a middle of said second strap body;  whereby said first and second inner straps are interlaced at right angles at the vertical slits such that the interlaced inner straps form a plurality of square cells for receiving the fuel rods, with the main supports of the first and second inner strap crossing the upper sub-supports of the second and first inner straps while forming a plurality of upper axial junction lines, and the lower sub-supports of the first and second inner straps crossing each other while forming a plurality of lower axial junction lines, said inner straps being seam-welded to each other along the upper and lower junction lines to form a plurality of side weld lines, and said flow mixing vanes of the interlaced inner straps guiding axial flows of coolant to gaps between the fuel rods, thus forming cross flows of coolant. 2. The spacer grid according to  claim 1 , wherein said upper and lower vertical slits of the first and second inner straps, and a plurality of vane gaps each formed between the two flow mixing vanes of each of the main supports of said second inner straps are spaced apart from each other at the same interval as a pitch of the fuel rods. claim 1 3. The spacer grid according to  claim 1 , wherein the two flow mixing vanes, formed on each of the main supports of the first and second inner straps and disposed in a central region of a flow channel, are rotationally symmetrical about crossing line of the first and second inner straps. claim 1 4. The spacer grid according to  claim 1 , wherein the two flow mixing vanes, formed on each of the main supports of the first and second inner straps, are deflected in opposite directions, with peaks of said flow mixing vanes directed toward the gaps between the fuel rods set in the square cells. claim 1 5. The spacer grid according to  claim 1 , wherein each of said main supports of the first and second inner straps has an equilateral trapezoidal shape, with a top edge of said main support being integrated with the two flow mixing vanes and being parallel to a bottom edge integrated with the top edge of an associated strap body. claim 1 6. The spacer grid according to  claim 1 , wherein each of said upper and lower sub-supports of the first and second inner straps is integrated with the bottom edge of an associated strap body and has an isosceles triangular shape. claim 1 7. The spacer grid according to  claim 1 , wherein said interlaced first and second inner straps are seam-welded together along at least one of each of four upper and four lower axial junction lines formed at each intersection. claim 1 8. The spacer grid according to  claim 1 , wherein the two flow mixing vanes, formed on each of the main supports of the first and second inner straps, each have a first edge perpendicular to the top edge of the main support, a second edge extending upward from a top end of said first edge while forming an obtuse angle between the first and second edges, a third edge extending downward from a top end of said second edge while being curved with a predetermined curvature, a fourth edge extending between the top edge of the main support and the third edge while being parallel to the first edge, and a base portion of the vane bending along bottom points of the first and fourth edges while being integrated with the top edge of the main support. claim 1