Patent Number: 
Section: claims

1. A nuclear fuel assembly comprising: a fuel bundle having a plurality of fuel rods;  a fuel rod support structure including a lower tie plate having an inlet nozzle, a lower tie plate grid and a transition structure for receiving coolant entering said nozzle and flowing coolant through said transition structure to said lower tie plate grid;  said lower tie plate grid including a plurality of spaced bosses defining holes sized for receiving lower ends of said fuel rods within the holes of said bosses;  said lower tie plate grid further including webs interconnecting said bosses to define with said bosses a plurality of flow openings through the lower tie plate grid for flowing coolant through said tie plate grid, said webs having upper edges recessed below upper edges of said bosses;  a flat filter plate extending in a plane overlying the lower tie plate grid and disposed on upper edges of said bosses and spaced from said upper edges of said webs to define spaces therebetween, said flat filter plate having a plurality of spaced holes therethrough in registration with the holes in said bosses and bounded by a peripheral margin generally corresponding to a peripheral margin of the lower tie plate grid, said filter plate having a plurality of apertures therethrough spaced from said filter plate holes and in registration with the flow openings between said bosses and said webs and with the spaces between said filter plate and the upper edges of said webs for flowing coolant therethrough, the area of each said aperture being smaller than the area of each said hole through said filter plate and the ratio of the total number of apertures to the total number of said holes through said filter plate being in excess of ten; and  a predetermined number of said fuel rods having end plugs received in said registering holes of said filter plate and said bosses and engaging said filter plate about the margins of said holes therethrough to position and maintain the filter plate against the ends of the bosses of the lower tie plate grid. 2. An assembly according to  claim 1  wherein the ratio of the total number of apertures to the total number of said holes through said filter plate is in excess of fifteen. claim 1 3. An assembly according to  claim 1  wherein the cross-sectional area of each said hole through said filter plate is at least fifteen times the cross-sectional area of each said aperture. claim 1 4. An assembly according to  claim 1  wherein the cross-sectional area of each said hole through said filter plate is at least twenty times the cross-sectional area of each said aperture. claim 1 5. An assembly according to  claim 1  wherein said end plugs of said predetermined number of fuel rods are smooth-sided and rest in registering holes holding down the debris filter plate. claim 1 6. An assembly according to  claim 1  wherein said plurality of fuel rods includes tie rods having threaded end plugs for threaded engagement in threaded registering holes of said bosses to secure said fuel bundle to said lower tie plate, said threaded end plugs being spaced from margins of the holes about said filter plate in final securement thereof to said lower tie plate. claim 1 7. An assembly according to  claim 1  wherein said filter plate apertures form an open area through said filter plate of approximately at least 30% of the area of said filter plate. claim 1 8. An assembly according to  claim 1  wherein the ratio of the total number of apertures to the total number of said holes through said filter plate is in excess of fifteen, the cross-sectional area of each said hole through said filter plate being at least fifteen times the cross-sectional area of each said aperture. claim 1 9. An assembly according to  claim 1  wherein said end plugs of said predetermined number of fuel rods are smooth-sided and rest in registering holes holding down the filter plate, said plurality of fuel rods including tie rods having threaded end plugs for threaded engagement in threaded registering holes of said bosses to secure said fuel bundle to said lower tie plate, said threaded end plugs being spaced from margins of the holes about said filter plate in final securement thereof to said lower tie plate. claim 1 10. An assembly according to  claim 9  wherein the cross-sectional area of each said hole through said filter plate is at least fifteen times the cross-sectional area of each said aperture. claim 9 11. An assembly according to  claim 9  wherein said filter plate apertures form an open area through said filter plate of approximately at least 30% of the area of said filter plate. claim 9 12. An assembly according to  claim 9  wherein the ratio of the total number of apertures to the total number of said holes through said filter plate is in excess of fifteen, the cross-sectional area of each said hole through said filter plate being at least fifteen times the cross-sectional area of each said aperture. claim 9 13. A nuclear fuel assembly comprising: a fuel bundle having a plurality of fuel rods;  a fuel rod support structure including a lower tie plate having an inlet nozzle, a lower tie plate grid and a transition structure for receiving coolant entering said nozzle and flowing coolant through said transition structure to said lower tie plate grid;  said lower tie plate grid including a plurality of spaced bosses defining holes sized for receiving lower ends of said fuel rods within the holes of said bosses;  said lower tie plate grid further including webs interconnecting said bosses to define with said bosses a plurality of flow openings through the lower tie plate grid for flowing coolant through said tie plate grid, said webs having upper edges recessed below upper edges of said bosses;  a flat filter plate extending in a plane and disposed on upper edges of said bosses and spaced from said upper edges of said webs to define spaces therebetween, said flat filter plate having a plurality of spaced holes therethrough in registration with the holes in said bosses and bounded by a peripheral margin generally corresponding to a peripheral margin of the lower tie plate grid, said filter plate having a plurality of apertures therethrough in registration with the flow openings between said bosses and said webs and with the spaces between said filter plate and the upper edges of said webs for flowing coolant therethrough, the cross-sectional area of each said filter plate hole being at least fifteen times the cross-sectional area of each said aperture through said filter plate and the ratio of the total number of apertures to the total number of said holes through said filter plate being in excess of ten; and  a predetermined number of said fuel rods having end plugs received in said registering holes of said filter plate and said bosses. 14. An assembly according to  claim 13  wherein the cross-sectional area of each said hole through said filter plate is at least fifteen times the cross-sectional area of each said aperture. claim 13 15. An assembly according to  claim 13  wherein said filter plate apertures form an open area through said filter plate of approximately at least 30% of the area of said filter plate. claim 13 16. An assembly according to  claim 13  wherein the ratio of the total number of apertures to the total number of said holes through said filter plate is in excess of fifteen. claim 13 17. An assembly according to  claim 13  wherein said end plugs of said predetermined number of fuel rods are smooth-sided and rest in registering holes holding down the filter plate. claim 13 18. An assembly according to  claim 13  wherein said plurality of fuel rods includes tie rods having threaded end plugs for threaded engagement in threaded registering holes of said bosses to secure said fuel bundle to said lower tie plate, said threaded end plugs being spaced from margins of the holes about said filter plate in final securement thereof to said lower tie plate. claim 13 19. An assembly according to  claim 13  wherein said filter plate apertures form an open area through said filter plate of approximately at least 30% of the area of said filter plate. claim 13 20. An assembly according to  claim 3  wherein said end plugs of said predetermined number of fuel rods are smooth-sided and rest in registering holes, said plurality of fuel rods including tie rods having threaded end plugs for threaded engagement in threaded registering holes of said bosses to secure said fuel bundle to said lower tie plate, said threaded end plugs being spaced from margins of the holes about said filter plate in final securement thereof to said lower tie plate. claim 3 21. An assembly according to  claim 20  wherein the cross-sectional area of each said hole through said filter plate is at least fifteen times the cross-sectional area of each said aperture. claim 20 22. An assembly according to  claim 20  wherein said filter plate apertures form an open area through said filter plate of approximately at least 30% of the area of said filter plate. claim 20 23. An assembly according to  claim 20  wherein the ratio of the total number of apertures to the total number of said holes through said filter plate is in excess of fifteen. claim 20