A spacer grid includes interlocked straps comprising metal sheets or plates welded together to define a spacer grid having a top and bottom. The interlocked straps define a plurality of cells comprising vertical passages connecting the top and bottom of the spacer grid. The cells include: upper dimples proximate to the top of the spacer grid and distal from the mid-plane of the spacer grid; lower dimples 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 proximate to the top of the spacer grid and distal from the mid-plane of the spacer grid; and cantilevered lower springs having fuel rod engagement surfaces proximate to the bottom of the spacer grid and distal from the mid-plane of the spacer grid.
1. A fuel assembly comprising:
a bundle of fuel rods comprising fissile material; and
a spacer grid securing the fuel rods of the bundle together, the spacer grid comprising a plurality of interlocked straps, having a top and a bottom and including a plurality of cells comprising passages from the top to the bottom of the spacer grid through which fuel rods pass, the cells including:
a plurality of upper dimples disposed proximate to the top of the spacer grid and distal from a mid-plane of the spacer grid,
a plurality of lower dimples disposed proximate to the bottom of the spacer grid and distal from the mid-plane of the spacer grid,
upper cantilevered springs defined by a plurality of upper cutaways, the upper cantilevered springs being anchored to the spacer grid by a single base in a plane of a corresponding strap and at the mid-plane of the spacer grid and having cantilevered free ends located above the mid-plane of the spacer grid, and
lower cantilevered springs defined by a plurality of lower cutaways, the lower cantilevered springs being anchored to the spacer grid by a single base in a plane of a corresponding strap and at the mid-plane of the spacer grid and having cantilevered free ends located below the mid-plane of the spacer grid,
wherein the upper cantilevered springs of each cell bias the fuel rod passing through the cell against the upper dimples of the cell and the lower cantilevered springs of each cell bias the fuel rod passing through the cell against the lower dimples of the cell, and
wherein one of the plurality of upper dimples and the plurality of lower dimples is contiguous with the plurality of upper cutaways and the plurality of lower cutaways, respectively.
2. The fuel assembly of claim 1 , wherein the spacer grid is a mid-grid and the fuel assembly further comprises:
end spacer grids further securing the fuel rods of the bundle together, each end spacer grid being closer to an end of the bundle of fuel rods than the mid-grid, each end spacer grid having a top and a bottom and including a plurality of open cells through which fuel rods pass from the top to the bottom, the cells including:
upper dimples disposed proximate to the top of the end spacer grid and distal from the mid-plane of the end spacer grid,
lower dimples disposed proximate to the bottom of the end spacer grid and distal from the mid-plane of the end spacer grid,
upper cantilevered springs anchored to the end spacer grid at the mid-plane of the spacer grid and having cantilevered free ends located above the mid-plane of the spacer grid, and
lower cantilevered springs anchored to the end spacer grid at the mid-plane of the spacer grid and having cantilevered free ends located below the mid-plane of the spacer grid.
3. The fuel assembly of claim 2 , wherein:
the cantilevered free ends of the upper and lower cantilevered springs of the end spacer grids comprise flat-topped domes, and
the cantilevered free ends of the upper and lower cantilevered springs of the mid-grid comprise hooks.
4. The fuel assembly of claim 3 , wherein:
end spacer grids comprise interlocked nickel-chromium alloy straps; and
the mid-grid comprises interlocked zirconium alloy straps.
5. The fuel assembly of claim 1 , wherein the upper dimples extend to the top of the spacer grid and the lower dimples extend to the bottom of the spacer grid.
6. The fuel assembly of claim 5 , wherein the upper cantilevered springs do not include any portion of the mid-plane of the spacer grid, and the lower cantilevered springs do not include any portion of the mid-plane of the spacer grid.
7. The fuel assembly of claim 1 , wherein the spacer grid comprises interlocked straps comprising metal sheets or plates, and the outermost straps of the spacer grid do not include upper cantilevered springs and do not include lower cantilevered springs.
8. The fuel assembly of claim 1 , wherein the upper and lower dimples face inward toward the center of the spacer grid and the upper and lower cantilevered springs face outward away from the center of the spacer grid.
9. The fuel assembly of claim 1 , wherein the cantilevered free ends of the upper and lower cantilevered springs are arranged as pairs and the upper and lower dimples are arranged as pairs to collectively define a fuel rod biasing system that is symmetric about the mid-plane of the spacer grid.
10. The fuel assembly of claim 1 , wherein the bundle of fuel rods has one of rectangular, square, and hexagonal symmetry in a plane transverse to the fuel rods.
11. A fuel assembly comprising:
a bundle of fuel rods comprising fissile material; and
a spacer grid securing the fuel rods of the bundle together, the spacer grid comprising a plurality of interlocked straps, having a top and a bottom and including a plurality of cells comprising passages from the top to the bottom of the spacer grid through which fuel rods pass, the cells including:
a plurality of upper dimples disposed proximate to the top of the spacer grid and distal from a mid-plane of the spacer grid,
a plurality of lower dimples disposed proximate to the bottom of the spacer grid and distal from the mid-plane of the spacer grid,
upper cantilevered springs defined by a plurality of upper cutaways, the upper cantilevered springs being wherein each upper cantilevered spring is anchored to the spacer grid at a single cantilever anchor located in a plane of a corresponding strap and at or above the mid-plane of the spacer grid and extends upward toward the top of the spacer grid terminating in a cantilevered free end located above the mid-plane of the spacer grid, and
lower cantilevered springs defined by a plurality of lower cutaways, the lower cantilevered springs being wherein each lower cantilevered spring is anchored to the spacer grid at a single cantilever anchor located in a plane of a corresponding strap and at or below the mid-plane of the spacer grid and extends downward toward the bottom of the spacer grid terminating in a cantilevered free end located below the mid-plane of the spacer grid,
wherein the upper cantilevered springs of each cell bias the fuel rod passing through the cell against the upper dimples of the cell and the lower cantilevered springs of each cell bias the fuel rod passing through the cell against the lower dimples of the cell, and
wherein one of the plurality of upper dimples and the plurality of lower dimples is contiguous with the plurality of upper cutaways and the plurality of lower cutaways, respectively.
12. The fuel assembly of claim 11 wherein:
the upper dimples and the upper cantilevered springs cooperate to define an upper grip plane containing the upper dimples and the upper cantilevered springs that grips the fuel rods; and
the lower dimples and the lower cantilevered springs cooperate to define a lower grip plane containing the lower dimples and the lower cantilevered springs that grips the fuel rods.
13. The fuel assembly of claim 11 wherein the spacer grid comprises interlocked straps comprising metal sheets or plates welded together, the passages from the top to the bottom of the spacer grid being defined between the straps.
14. The fuel assembly of claim 1 wherein the spacer grid comprises interlocked straps comprising metal sheets or plates welded together, the passages from the top to the bottom of the spacer grid being defined between the straps.