Patent Number: 046559957
Section: claims

1. A nuclear fuel assembly comprising: (a) a flow channel;  (b) a lower nozzle assembly structurally attached to said flow channel to form therewith an external envelope;  (c) an invertible fuel bundle adapted to be inserted into said envelope, said fuel bundle comprising a plurality of elongated fuel rods held in a spaced lateral array between top and bottom tie plates, each of said top and bottom tie plates being substantially identical and having means for supporting said fuel bundle within said envelope in either of two mututally inverted vertical orientations whereby the orientation of the fuel bundle in a flow channel may be reversed during burn-up operation.  (a) inserting a nuclear fuel bundle having substantially identical top and bottom tie plates into said external envelope with one of said top and bottom tie plates engaging said bottom nozzle assembly to support said fuel bundle thereon;  (b) operating said reactor for a period of time;  (c) withdrawing said fuel bundle from said envelope;  (d) inverting said fuel bundle;  (e) reinserting said inverted fuel bundle into said envelope with the other of said top and bottom tie plates engaging said bottom nozzle to support said fuel assembly thereon; and  (f) continuing to operate said reactor with said inverted fuel bundle.  (a) providing fission gas plenums in each fuel rod of said fuel bundle;  (b) disposing some of said fission gas plenums at the top of said fuel bundle and a remainer of said fission gas plenums at the bottom of said fuel bundle.  (a) orienting alternate fission gas plenums at the top of the fuel bundle; and  (b) orienting intervening alternate fission gas plenums at the bottom of the fuel bundle.  (a) inserting the self-supporting nuclear fuel bundle having substantially identical top and bottom tie plates into said fuel bundle envelope with one of said top and bottom tie plates engaging said bottom nozzle assembly to support said fuel bundle thereron;  (b) operating said reactor for a period of time to partially burn said fuel whereby fuel positioned at the lower portion of said fuel bundle is burned to a larger extent than fuel positioned at an upper portion of said fuel bundle;  (c) withdrawing said fuel bundle from said envelope;  (d) inverting the top and bottom of said fuel bundle;  (e) reinserting said inverted fuel bundle into said envelope whereby the fuel burned to a larger extent is positioned at the upper portion of the fuel bundle;  (f) continuing to operate said reactor with said inverted fuel bundle whereby the fuel in said fuel bundle is more evenly burned. 2. The fuel assembly of claim 1 wherein said fuel rods comprise a tubular cladding containing nuclear fuel, each of said fuel rods having a fission gas plenum disposed within said cladding to accommodate fission gases released during operation of said fuel assembly, wherein some of said fission gas plenums are disposed adjacent said top tie plate and the remainder of said fission gas plenums are disposed adjacent said bottom tie plate. 3. The fuel assembly of claim 2 wherein fission gas plenums of alternate fuel rods are disposed adjacent said top tie plate and intervening alternate fuel rod fission gas plenums are disposed adjacent said bottom tie plate. 4. The fuel assembly of claim 1 wherein said means for supporting said fuel bundle comprise a plurality of tie plate legs extending from said top and bottom tie plates, said tie plate legs having means for engaging said lower nozzle assembly and for supporting said fuel bundle thereon. 5. The fuel assembly of claim 2 wherein said means for supporting said fuel bundle comprises a plurality of tie plate legs extending from said top and bottom tie plates, said tie plate legs having means for engaging said lower nozzle assembly and for supporting said fuel bundle. 6. The fuel assembly of claim 4 wherein said tie plate legs have lifting slots formed integrally therein. 7. The fuel assembly of claim 5 wherein said tie plate legs have lifting slots formed integrally therein. 8. The fuel assembly of claim 4 further comprising an orifice plate assembly disposed in said lower nozzle assembly and having means for engaging said tie plate legs and means for engaging said lower nozzle assembly. 9. The fuel assembly of claim 5 further comprising an orifice plate assembly disposed in said lower nozzle assembly and having means for engaging said tie plate legs and means for engaging said lower nozzle assembly. 10. The fuel assembly of claim 2 wherein each of said fuel rods further has an end plug at each end thereof, each of said end plugs having an extension extending from said cladding, said top and bottom tie plates each having a plurality of apertures therein for slidably accepting said extensions. 11. The fuel assembly of claim 2 wherein said cladding is provided with a crimp in the region of said fission gas plenums to prevent said nuclear fuel from occupying said fission gas plenums. 12. A method of operating a nuclear reactor having a core containing a fuel bundle external envelope comprising a flow channel and a bottom nozzle assembly, said method comprising the steps of: 13. The method of claim 12 further comprising the steps of: 14. The method of claim 13 wherein the step of disposing further comprises: 15. A method of operating a BWR nuclear reactor having a core containing a fuel bundle envelope comprising a flow channel and bottom nozzle assembly, said method being operable to enhance burn up of nuclear fuel and to reduce fuel cycle operating costs and comprising the steps of: