Patent Number: 048141372
Section: claims

1. A fuel pellet for a nuclear reactor fuel rod comprising: a substantially cylindrical central section;  a convex first end section smoothly joined to one axial end of said central section at a first junction, said first junction approximating a smooth and continuous curved surface;  a concave second end section joined to said central section at a second junction, said second junction approximating a smooth and continuous curved surface, wherein the curvature of said concave second end section is conformed to the curvature of said convex first end section.  a tubular cladding jacket means for accommodating a plurality of fuel pellets;  a plurality of interior fuel pellets disposed in said cladding jacket, said fuel pellets each comprising:  an end plug means, disposed at a first end of said cladding jacket means, for retaining said interior fuel pellets within said cladding jacket means;  a spring means, disposed at a second end of said cladding jacket means, for positioning said interior fuel pellets; and  a second end plug means, disposed at said second end of said cladding jacket means, for retaining said interior fuel pellets within said cladding jacket means.  a substantially cylindrical first terminal pellet central section;  a first terminal pellet bearing section joined to said first terminal pellet central section at a first junction, said first junction approximating a smooth and continuous curved surface, wherein the curvature of said first terminal pellet bearing section conforms to the curvature of said end plug means bearing surface; and  a first terminal pellet concave end section joined to said first terminal pellet central section at a second junction, said second junction approximating a smooth and continuous curved surface, wherein the curvature of said first terminal pellet concave end section conforms to the curvature of said interior pellet convex end section.  wherein the curvature of said first terminal pellet bearing section conforms to the curvature of said first end plug means bearing surface such that said first terminal pellet bearing section is securely accommodated in said first end plug means.  a substantially cylindrical second terminal pellet central section;  a second terminal pellet bearing section joined to said second terminal pellet central section at a first junction, said first junction approximating a smooth and continuous curved surface, and wherein said second terminal pellet bearing section has a substantially planar surface adapted to accommodate an end portion of said spring means; and  a second terminal pellet convex end section joined to said second terminal pellet central section at a second junction, said second junction approximating a smooth and continuous curved surface, wherein the curvature of said second terminal pellet convex section conforms to the curvature of said interior pellet concave end section.  a substantially cylindrical terminal pellet central section;  a terminal pellet bearing section joined to said terminal pellet central section at a first junction, said first junction approximating a smooth and continuous curved surface, and wherein said terminal pellet bearing section has a substantially planar surface adapted to accommodate an end portion of said spring means; and  a terminal pellet convex end section joined to said terminal pellet central section at a second junction, said second junction approximating a smooth and continuous curved surface, wherein the curvature of said terminal pellet convex end section conforms to the curvature of said interior pellet concave end section.  a bundle comprising a plurality of generally cylindrical fuel rods each comprising a cladding and a plurality of control rods;  a plurality of spacer grid means for retaining said bundle, said spacer grid means each being axially displaced from one another along a longitudinal extent of said bundle;  a plurality of interior fuel pellets stacked end-to-end within each of said fuel rods, each of said interior fuel pellets comprising:  an end plug means, having a bearing surface, for retaining said interior fuel pellets in said fuel rod;  a spring means for positioning said interior fuel pellets in said fuel rod; and  a first terminal fuel pellet disposed contiguously to said end plug including:  a substantially cylindrical second terminal pellet central section;  a second terminal pellet bearing section joined to said second terminal pellet central section at a first junction, said second terminal pellet bearing section having a substantially planar surface adapted to accommodate an end portion of said spring means; and  a second terminal pellet convex end section joined to said second terminal pellet central section at a second junction;  wherein the curvature of said second terminal pellet convex section is formed so as to securely accommodate an interior pellet concave end section. 2. The fuel pellet according to claim 1, further comprising first and second depression means disposed in said first and second end sections for absorbing expansion of said fuel pellet. 3. The fuel pellet according to claim 1, further comprising a plurality of fission gas venting paths disposed radially along each of said first and second end sections. 4. The fuel pellet according to claim 3, wherein said fission gas venting paths comprise shallow fillet-shaped grooves integrally formed in said first and second end sections. 5. A fuel rod for a nuclear reactor comprising: 6. A fuel rod according to claim 5, wherein said first end plug means includes a bearing surface and wherein the fuel rod further comprises a first terminal fuel pellet disposed immediately adjacent said first end plug means bearing surface, said first terminal fuel pellet comprising: 7. The fuel rod according to claim 6, wherein said first end plug means bearing surface and said first terminal pellet bearing section are substantially planar. 8. The fuel rod according to claim 6, wherein said first end plug means bearing surface is concave and said first terminal pellet bearing section is convex; and 9. A fuel rod according to claim 6, further comprising a second terminal fuel pellet disposed immediately adjacent said spring means, said second terminal fuel pellet comprising: 10. A fuel rod according to claim 9, wherein said first terminal pellet concave end section and said second terminal pellet convex end section each further comprise a depression means for absorbing thermal expansion of said first and second terminal pellets, respectively. 11. A fuel rod according to claim 5, further comprising a terminal fuel pellet disposed immediately adjacent said spring means, said terminal fuel pellet comprising: 12. A fuel rod according to claim 5, wherein each of said interior pellets further comprise first and second depression means, disposed in said first and second interior pellet end sections, for absorbing thermal expansion of said interior fuel pellets. 13. The fuel rod according to claim 5, wherein each of said interior fuel pellet further comprise a plurality of fission gas venting paths disposed radially along each of said first and second interior fuel pellet end sections. 14. The fuel pellet according to claim 13, wherein said fission gas venting paths comprise shallow fillet-shaped grooves integrally formed in said interior fuel pellet first and second end sections. 15. A fuel assembly for a nuclear reactor comprising: 16. A fuel assembly for a nuclear reactor according to claim 15, wherein each of said fuel rods further comprise: 17. A fuel assembly for a nuclear reactor according to claim 16, wherein each of said fuel rods further comprise a second terminal pellet disposed contiguously to said spring means and comprising: 18. A fuel assembly for a nuclear reactor according to claim 17, wherein said first and second terminal pellet bearing sections are joined to said first and second terminal pellet central sections, respectively, at junctions which approximate a smooth and continuous curved surface. 19. A fuel assembly for a nuclear reactor according to claim 17, wherein each of said interior pellets further comprise depression means for absorbing expansion of said interior pellets. 20. A fuel assembly for a nuclear reactor according to claim 15, wherein each of said interior pellets further comprise depression means for absorbing expansion of said interior pellets. 21. A fuel assembly for a nuclear reactor according to claim 15, wherein each of said interior pellets further comprise a plurality of fission gas venting paths disposed radially along each of said first and second interior pellet end sections. 22. The fuel assembly according to claim 21, wherein said fission gas venting paths comprise shallow fillet-shaped grooves integrally formed in said first and second interior pellet end sections. 23. A fuel pellet for a nuclear reactor fuel rod assembly, the pellet having a substantially cylindrical longitudinal outer surface terminating at one end in an axially symmetric convex end surface and at the other end in an axially symmetric concave end surface, the end surfaces joining smoothly with the longitudinal surface, and at least a peripheral section of the convex end surface being congruent to a peripheral section of the concave end surface. 24. The fuel pellet according to claim 23, further comprising depression means disposed in each of said convex and concave end surfaces. 25. The fuel pellet according to claim 23, further comprising a plurality of fission gas venting paths disposed radially along each of said convex and concave end surfaces. 26. The fuel pellet according to claim 25, wherein said fission gas venting paths comprise shallow fillet-shaped grooves integrally formed in said convex and concave end surfaces.