Patent Number: 
Section: claims

1. A unit spacer grid strap comprising:a plurality of first convexities protruding in one direction so as to be placed in contact with a nuclear fuel rod;a plurality of second convexities alternating with the first convexities, protruding in the other direction so as to be in contact with another nuclear fuel rod; anda plurality of joints, each of which connects the first convexity and the second convexity,wherein each of the first and second convexities includes:an intermediate support continuously connected with the joint between the first convexity and the second convexity;an upper support on a top edge of the unit spacer grip strap spaced apart from the intermediate support, and continuously connected with one of the two neighboring joints, the upper support having a shorter length than the intermediate support so that an end of the upper support is suspended in the space between the two neighboring joints; anda lower support on a bottom edge of the unit spacer grid strap spaced apart from the intermediate support, and continuously connected with the other of the two neighboring joints, the lower support having a shorter length than the intermediate support so that an end of the lower support is suspended in the space between the two neighboring joints. 2. The unit spacer grid strap as set forth in claim 1, wherein the upper support and the lower support have point symmetry with respect to a middle point of the intermediate support. 3. The unit spacer grid strap as set forth in claim 1, wherein at least one of the first and second convexities has a greater diameter than the others. 4. A spacer grid for nuclear fuel rods comprising:a plurality of unit spacer grids in a stacked configuration, each unit spacer grid including:a plurality of unit spacer grid straps disposed in a row at regular intervals, each having convexities alternating with each convexity on opposite sides of each strap at regular intervals, at least one of the convexities having a diameter greater than the others; anda plurality of fixing grid straps connected to respective opposite ends of the unit spacer grid straps so as to fix the unit spacer grid straps,each convexity is partitioned into an intermediate support, an upper support on a top edge of the unit spacer grid strap above the intermediate support, and a lower support on a lower edge of the unit spacer grid strap below the intermediate support;the upper support is configured so that one end thereof adjacent to the intermediate support is suspended in a space;the lower support is configured so that one end thereof adjacent to the intermediate support is suspended in the space,wherein the unit spacer grids are rotated in one direction at an angle of 90 or 180 degrees with respect to adjacent unit spacer grids in the stacked configuration. 5. The spacer grid for nuclear fuel rods as set forth in claim 4, wherein:the upper support and the lower support have point symmetry with respect to a middle point of the intermediate support. 6. The spacer grid for nuclear fuel rods as set forth in claim 4, wherein the stacked unit spacer grids are coupled by fixing rods inserted into respective four corners or middles of four faces thereof.