Patent Number: 051851202
Section: claims

1. In a nuclear reactor core having fuel bundles and hollow control blades, a method for affecting neutron spectral shift during a core fuel cycle including beginning and end intervals comprising: providing a liquid poison in said blades for reducing nuclear reactivity in said core;  displacing said liquid poison in at least a first one of said blades with a first fluid having a first moderating ratio for obtaining a hard neutron spectra adjacent to said first blade during said cycle beginning interval for converting fertile fuel in said fuel bundles to fissile fuel; and  increasing moderating ratio of said first blade using a second fluid for obtaining a soft neutron spectra adjacent to said first blade during said cycle end interval for burning said fissile fuel.  a nuclear reactor core including fuel bundles and hollow control blades, and operable over a core fuel cycle including beginning and end intervals for generating power;  means for providing a liquid nuclear poison in said blades for reducing nuclear reactivity in said core;  means for displacing said liquid poison from at least a first one of said blades with a first fluid having a first moderating ratio for obtaining a hard neutron spectra adjacent to said first blade during said cycle beginning interval for converting fertile fuel in said fuel bundles to fissile fuel; and  means for increasing moderating ratio effected by said first blade using a second fluid for shifting said hard neutron spectra to a soft neutron spectra adjacent to said first blade during said cycle end interval for burning said fissile fuel.  a poison reservoir including said liquid poison;  a first conduit joining said reservoir in flow communication with said first blade; and  said poison reservoir being elevated at least in part higher than said first blade so that gravity acting on said liquid poison in said poison reservoir is effective for moving said liquid poison through said first conduit for completely filling said first blade with said poison upon cessation of said liquid poison displacing means.  a pressure source;  a second conduit joining said pressure source in flow communication with said first blade and containing said first fluid; and  said pressure source being effective for selectively pressurizing said first fluid to selectively displace said poison from said first blade and into said poison reservoir.  said second conduit containing both said first fluid and said second fluid having a second moderating ratio greater than said first moderating ratio, said first fluid being disposed between said poison and said second fluid; and  said pressure source being effective for selectively pressurizing said second fluid for displacing in turn said first fluid and said poison so that only said first fluid is in said first blade during said beginning interval for obtaining said hard neutron spectra, and only said second fluid is in said first blade during said end interval for obtaining said soft neutron spectra.  said first control blade is in the form of an inverted U-shaped trap;  said first conduit includes an inverted U-shaped first trap;  said second conduit includes an inverted U-shaped second trap;  said first fluid is a gas and said second fluid is a liquid; and  said first, second, and blade traps are effective for preventing mixing together of said liquid poison, said first fluid, and said second fluid.  an admission valve disposed in flow communication in said first conduit;  a purge valve disposed in flow communication in said second conduit;  means for supplying said second fluid; and  said admission valve being positionable to block flow of said poison into said first blade while allowing said supplying means to provide said second fluid into said first blade to displace said first fluid which in turn is vented through said second conduit and out said second valve.  said first control blade includes first and second chambers, said first chamber being joined in flow communication with said first and second conduits for receiving said first fluid to displace said liquid poison;  means for providing alternatively in said second chamber said second fluid and a third fluid having respective second and third moderating ratios; and  said first chamber containing either said liquid poison or said first fluid when said second chamber contains said second fluid, and said first fluid when said second chamber contains said third fluid, so that said first blade is effective for reducing reactivity when containing said liquid poison, obtaining said hard neutron spectra when containing both said first and second fluids, and obtaining said soft neutron spectra when containing both said first and third fluids. 2. A method according to claim 1 further including displacing said liquid poison in said first blade with said first fluid against the force of gravity acting on said liquid poison so that said liquid poison is automatically returned into said first blade by gravity upon cessation of said displacing step. 3. A method according to claim 2 further including displacing said first fluid in said first blade with said second fluid having a second moderating ratio greater than said first moderating ratio for obtaining said soft neutron spectra. 4. A method according to claim 3 wherein said first and second fluids are gases. 5. A method according to claim 4 wherein said first fluid is helium and said second fluid is nitrogen. 6. A method according to claim 3 wherein said first fluid is a gas and said second fluid is a liquid. 7. A method according to claim 6 wherein said first fluid is nitrogen and said second fluid is light or heavy water. 8. A method according to claim 2 wherein said first control blade includes two chambers, said first fluid is disposed in one of said chambers, and said moderating ratio increasing step is obtained by providing said second fluid into said other chamber, said second fluid having a second moderating ratio for collectively affecting with said first fluid said soft neutron spectra. 9. A system for affecting neutron spectral shift comprising: 10. A system according to claim 9 wherein said liquid poison providing means include: 11. A system according to claim 10 wherein said displacing means include: 12. A system according to claim 11 wherein said moderating ratio increasing means include: 13. A system according to claim 12 wherein: 14. A system according to claim 13 wherein said first, second, and blade traps have respective first, second, and third heights selected so that said liquid second fluid is disposed fully in said first control blade and between said first and second traps for obtaining said soft neutron spectra during said end interval. 15. A system according to claim 14 wherein said pressure source provides a pressurized third fluid in the form of a gas for displacing said second fluid and in turn said first fluid and said liquid poison, said third fluid being disposed in said second trap during said end interval. 16. A system according to claim 15 wherein said second conduit further includes a second reservoir disposed in flow communication between said second trap and said pressure source for receiving at least part of said second fluid from said second conduit upon depressurization of said second fluid. 17. A system according to claim 16 further including a plurality of said first control blades disposed in parallel flow with said first and second conduits. 18. A system according to claim 11 further including means for replacing said first fluid from said first blade with said second fluid having a second moderating ratio greater than said first moderating ratio for obtaining said soft neutron spectra. 19. A system according to claim 18 wherein said first fluid replacing means include: 20. A system according to claim 11 wherein: 21. A system according to claim 20 wherein said first fluid is helium gas, said second fluid is nitrogen gas, and said third fluid is water.