Patent Number: 054886341
Section: claims

1. In a nuclear fuel assembly, a unitary one-piece lower tie plate grid comprising: a lower grid portion and an upper grid portion;  means for supporting fuel rods above the lower tie plate grid including said upper and lower grid portions;  said supporting means comprising a plurality of laterally spaced cylindrical bosses having apertures extending therethrough and cylindrical boss portions extending upwardly from said lower grid portion, said bosses being sized for receiving lower ends of the fuel rods in said apertures, said supporting means further including web portions extending upwardly from said lower grid portion and interconnecting said boss portions to define with said boss portions a plurality of flow spaces in said upper grid portion extending from said lower grid portion and opening through an upper surface of said lower tie plate grid;  said boss portions having vertical centerlines arranged at the corners of square matrices with said web portions extending linearly between said boss portions along sides of the square matrices and convex portions of said cylindrical boss portions extending between perpendicularly oriented web portions of each matrix, said web portions and said convex portions defining side walls of the flow spaces;  said lower grid portion of said lower tie plate grid including a plurality of openings extending therethrough and opening into said flow spaces for separating debris from a coolant flowing through said lower grid portion into said flow spaces between said boss and said web portions.  a lower tie plate assembly including an inlet nozzle, a unitary one-piece lower tie plate grid and a transition structure defining a flow volume for receiving coolant entering said nozzle and flowing coolant to said lower tie plate grid;  said unitary one-piece lower tie plate grid having a lower grid portion and an upper grid portion;  said lower tie plate grid comprising a plurality of laterally spaced cylindrical bosses having apertures extending therethrough and cylindrical boss portions extending upwardly from said lower grid portion, said bosses being sized for receiving lower ends of the fuel rods in said apertures, said lower tie plate grid further including web portions extending upwardly from said lower grid portion and interconnecting said boss portions to define with said boss portions a plurality of flow spaces in said upper grid portion extending from said lower grid portion and opening through an upper surface of said lower tie plate grid;  said boss portions having vertical centerlines arranged at the corners of square matrices with said web portions extending linearly between said boss portions along sides of the square matrices and convex portions of said cylindrical boss portions extending between perpendicularly oriented web portions of each matrix, said web portions and said convex portions defining side walls of the flow spaces;  said lower grid portion of said lower tie plate grid including a plurality of openings extending therethrough and opening into said flow spaces for receiving the coolant from said flow volume and flowing the coolant through said lower grid portion into said flow spaces between said boss and said web portions;  said plurality of openings being aligned with one another along a diagonal direction relative to said square matrices.  upper and lower tie plate assemblies;  a nuclear fuel bundle between said upper and lower tie plate assemblies and including a plurality of fuel rods;  said lower tie plate assembly including means for supporting said nuclear fuel bundles, said lower tie plate assembly further including a lower tie plate grid having a lower grid portion and an upper grid portion;  said lower tie plate grid comprising a plurality of laterally spaced cylindrical bosses having apertures extending therethrough and cylindrical boss portions extending upwardly from said lower grid portion, said bosses being sized for receiving lower ends of said fuel rods in said apertures, said supporting means further including web portions extending upwardly from said lower grid portion and interconnecting said boss portions to define with said boss portions a plurality of flow spaces in said upper grid portion extending from said lower grid portion and opening through an upper surface of said lower tie plate grid;  said boss portions having vertical centerlines arranged at the corners of square matrices with said web portions extending linearly between said boss portions along sides of the square matrices and convex portions of said cylindrical boss portions extending between perpendicularly oriented web portions of each matrix, said web portions and said convex portions defining side walls of the flow spaces;  said lower grid portion of said lower tie plate grid including a plurality of openings extending therethrough and opening into said flow spaces for flowing a coolant flowing through said lower grid portion into said flow spaces between said boss and said web portions;  said plurality of openings being aligned with one another along a diagonal section relative to said square matrices. 2. A unitary one-piece lower tie plate grid according to claim 1 wherein said openings have a generally square configuration with linear sides thereof extending about 45.degree. relative to the sides of the square matrices. 3. A unitary one-piece lower tie plate grid according to claim 1 wherein the ratio of the overall thickness of the tie plate grid to the thickness of the lower grid portion thereof is within a range of 5-7 to 1, the ratio of the flow area through said openings to the area of the lower grid portion being about 0.15. 4. A unitary one-piece lower tie plate grid according to claim 1 wherein a first array of said plurality of openings opening into each flow space have a generally square configuration with linear sides, a second array of said plurality of openings opening into each flow space having a plurality of sides in excess of four sides, adjacent sides of each opening of each array thereof having a radius therebetween. 5. A unitary one-piece lower tie plate grid according to claim 4 wherein each opening of said first array thereof being located such that a centerline thereof intersects a diagonal of a square matrix and which diagonal passes through said convex portions, each opening of said second array of said plurality of openings having a side oriented generally parallel to a web portion. 6. A unitary one-piece lower tie plate grid according to claim 1 wherein said openings along a lower surface of said tie plate grid are radiussed, side wall portions of said openings opening into said flow spaces being laterally divergent in a direction toward said upper tie plate surface, and a throat between said radiussed openings and said divergent side wall portions having a minimum cross-sectional area whereby each said opening defines a venturi in a direction toward said flow spaces. 7. A unitary one-piece lower tie plate grid according to claim 6 wherein each said opening is defined by a step adjacent said throat between said radiussed opening and said divergent side wall portions whereby the size of each opening is increased immediately downstream of said step. 8. A unitary one-piece lower tie plate grid according to claim 6 wherein a plurality of said openings extending through said lower grid portion and opening into each flow space between the side walls thereof, said plurality of openings being aligned with one another along a diagonal direction relative to said square matrices. 9. A unitary one-piece lower tie plate grid according to claim 6 wherein the ratio of the overall thickness of the tie plate grid to the thickness of the lower grid portion is within a range of 3-4 to 1, the thickness of the lower grid portion being less than twice the size of the openings. 10. A unitary one-piece lower tie plate grid according to claim 6 wherein a first array of said plurality of openings opening into each flow space have a generally square configuration with linear sides, a second array of said plurality of openings opening into each flow space having a plurality of sides in excess of four sides, adjacent sides of each opening of each array thereof having a radius therebetween, each opening of said first array thereof being located such that a centerline thereof intersects a diagonal of a square matrix and which diagonal passes through said convex portions, each opening of said second array of said plurality of openings having a side oriented generally parallel to a web portion. 11. In a nuclear fuel assembly, a fuel rod support structure, comprising: 12. A fuel rod support structure according to claim 11 wherein said openings along a lower surface of said tie plate grid are radiussed, side wall portions of said openings opening into said flow-spaces being laterally divergent in a direction toward said upper tie plate surface, and a throat between said radiussed openings and said divergent side wall portions having a minimum cross-sectional area whereby each said opening defines a venturi in a direction toward said flow spaces. 13. A fuel rod support structure according to claim 11 wherein a first array of said plurality of openings opening into each flow space having a generally square configuration with linear sides, a second array of said plurality of openings opening into each flow space having a plurality of sides in excess of four sides, adjacent sides of each opening of each array thereof having a radius therebetween, each opening of said first array thereof being located such that a centerline thereof intersects a diagonal of a square matrix and which diagonal passes through said convex portions, each opening of said second array of said plurality of openings having a side oriented generally parallel to a web portion. 14. A nuclear fuel bundle and support therefor comprising: 15. A nuclear fuel bundle and support therefor according to claim 14 wherein said openings along a lower surface of said tie plate are radiussed, side wall portions of said openings opening into said flow spaces being laterally divergent in a direction toward said upper tie plate surface, and a throat between said radiussed openings and said divergent side wall portions having a minimum cross-sectional area whereby each said opening defines a venturi in a direction toward said flow spaces. 16. A nuclear fuel bundle and support therefor according to claim 14 wherein a first array of said plurality of openings opening into each flow space have a generally square configuration with linear sides, a second array of said plurality of openings opening into each flow space having a plurality of sides in excess of four sides, adjacent sides of each opening of each array thereof having a radius therebetween, each opening of said first array thereof being located such that a centerline thereof intersects a diagonal of a square matrix and which diagonal passes through said convex portions, each opening of said second array of said plurality of openings having a side oriented generally parallel to a web portion.