Patent Number: 
Section: claims

1. A spacer grid comprising:interlocked straps comprising metal sheets or plates welded together to define a spacer grid having a top and bottom, the interlocked straps defining a plurality of cells comprising vertical passages connecting the top and bottom of the spacer grid, the cells including:upper dimples disposed proximate to the top of the spacer grid and distal from a mid-plane of the spacer grid,lower dimples disposed proximate to the bottom of the spacer grid and distal from the mid-plane of the spacer grid,cantilevered upper springs having fuel rod engagement surfaces disposed proximate to the top of the spacer grid and distal from the mid-plane of the spacer grid, andcantilevered lower springs having fuel rod engagement surfaces disposed proximate to the bottom of the spacer grid and distal from the mid-plane of the spacer grid,wherein the upper springs and the lower springs are anchored to the straps by a single base that is in a plane of its corresponding strap at the mid-plane of the spacer grid. 2. The spacer grid of claim 1, wherein the fuel rod engagement surfaces of the cantilevered upper and lower springs comprise flat topped domes. 3. The spacer grid of claim 1, wherein the fuel rod engagement surfaces of the cantilevered upper and lower springs comprise hooks. 4. The spacer grid of claim 1, wherein the outermost straps of the interlocked straps do not include cantilevered upper springs and do not include cantilevered lower springs. 5. The spacer grid of claim 4, wherein the outermost straps of the interlocked straps have the same thickness as the other straps. 6. The spacer grid of claim 1, wherein the upper and lower dimples face inward toward the center of the spacer grid and the upper and lower springs face outward away from the center of the spacer grid. 7. The spacer grid of claim 1, wherein each cell configured to receive a fuel rod includes:a first cell defining wall having upper and lower dimples;a second cell defining wall having upper and lower dimples;a third cell defining wall opposite from and facing the first cell defining wall wherein the third cell defining wall has upper and lower springs; anda fourth cell defining wall opposite from and facing the second cell defining wall wherein the fourth cell defining wall has upper and lower springs. 8. The spacer grid of claim 7, wherein the first and second cell defining walls face toward the center of the spacer grid and the third and fourth cell defining walls face away from the center of the spacer grid. 9. A spacer grid comprising:interlocked straps of metal sheets or plates welded together to form a spacer grid having a top and bottom, the interlocked straps defining a plurality of cells comprising vertical passages connecting the top and bottom of the spacer grid; anda fuel rods retention system comprising a set of dimples protruding from walls of the cells and a set of springs protruding from walls of the cells, the set of dimples not including any dimples configured to contact fuel rods at the mid-plane of the spacer grid, the set of springs not including any springs configured to contact fuel rods at a mid-plane of the spacer grid,wherein the set of springs is anchored to the straps by a single base that is in a plane of its corresponding strap at the mid-plane of the spacer grid. 10. The spacer grid of claim 9, wherein the set of dimples includes:a set of upper dimples configured to contact fuel rods above the mid-plane of the spacer grid; anda set of lower dimples configured to contact fuel rods below the mid-plane of the spacer grid. 11. The spacer grid of claim 10, wherein the set of springs includes:a set of upper springs configured to contact fuel rods above the mid-plane of the spacer grid; anda set of lower springs configured to contact fuel rods below the mid-plane of the spacer grid.