Patent Number: 046845046
Section: claims

1. In a fuel assembly for use at non-control rod locations of a nuclear reactor core, said fuel assembly including top and bottom nozzles and a plurality of longitudinal structural members extending between and attached to said nozzles for forming said assembly into an integral unitary structure, at least certain of said structural members including an elongated hollow cladding tube extending between said top and bottom nozzles and means secured to opposite ends of said tube for hermetically sealing said tube and attaching it to said top and bottom nozzles, the improvement which comprises: (a) a quantity of irradiation-induced creep resistant material disposed within said tube; and  (b) pretensioning means positioned within said tube for applying a predetermined axially-directed compressive load to said creep resistant material therein and reacting said load so as to axially preload said tube in a state of pretension having a magnitude sufficient to substantially counteract an axial load typically transmitted through said unitary structure of said fuel assembly and thereby greatly reduce the compressive stress in said tube of said structural member.  (a) an elongated hollow cladding tube extending between said top and bottom nozzles;  (b) means secured to opposite ends of said tube for hermetically sealing said tube and attaching it to said top and bottom nozzles;  (c) a quantity of irradiation-induced creep resistant material disposed within said tube, said creep-resistant material being a ceramic material in pellet form and coated with a burnable absorber material; and  (d) pretensioning means positioned within said tube for applying a predetermined axially-directed compressive load to said creep resistant ceramic pellet stack therein and reacting said load so as to axially preload said tube in a state of pretension having a magnitude sufficient to substantially counteract an axial load typically transmitted through said unitary structure of said fuel assembly and thereby greatly reduce the compressive stress in said tube of said structural member. 2. The longitudinal structural member as recited in claim 1, wherein said creep-resistant material is a ceramic material in pellet form. 3. The longitudinal structural member as recited in claim 2, wherein said ceramic material is zirc oxide. 4. The longitudinal structural member as recited in claim 2, wherein said ceramic material is coated with a burnable absorber material. 5. The longitudinal structural member as recited in claim 1, wherein said pretensioning means is an elongated bellows type device positioned within said tube between said creep resistant material and one of said tube ends, said interior of said bellows type device being pressurized to create a predetermined axial force therein which places said creep resistant material in compression and said tube in said state of pretension. 6. The longitudinal structural member as recited in claim 5, wherein said pretensioning means further includes pressurization of the remainder of said tube. 7. The longitudinal structural member as recited in claim 1, wherein said pretensioning means is an arrangement of belleville springs positioned within said tube between said creep resistant material and one of said tube ends so as to create a predetermined axial force therein which places said creep resistant material in compression and said tube in said state of pretension. 8. The longitudinal structural member as recited in claim 7, wherein said belleville springs in said arrangement thereof are both stacked in parallel and in series. 9. In a fuel assembly for use at non-control rod locations of a nuclear reactor core, said fuel assembly including top and bottom nozzles and a plurality of longitudinal structural members extending between and attached to said nozzles for forming said assembly into an integral unitary structure, at least certain of said structural members comprising: 10. The longitudinal structural member as recited in claim 9, wherein said pretensioning means is an elongated bellows type device positioned within said tube between said creep resistant ceramic pellet stack and one of said tube ends, said interior of said bellows type device being pressurized to create a predetermined axial force therein which places said creep resistant ceramic pellet stack in compression and said tube in said state of pretension. 11. The longitudinal structural member as recited in claim 10, wherein said pretensioning means further includes pressurization of the remainder of said tube. 12. The longitudinal structural member as recited in claim 9, wherein said pretensioning means is an arrangement of belleville springs positioned within said tube between said creep resistant ceramic pellet stack and one of said tube ends so as to create a predetermined axial force therein which places said creep resistant ceramic pellet stack in compression and said tube in said state of pretension. 13. The longitudinal structural member as recited in claim 12, wherein said belleville springs in said arrangement thereof are both stacked in parallel and in series.