Patent Number: 054065976
Section: claims

1. A boiling water reactor comprising: a pressure vessel having an upper head and a lower head;  a reactor core containing a plurality of laterally spaced apart fuel bundles disposed in said vessel above said lower head to define therebetween a lower plenum;  a chimney extending upwardly from said core in flow communication therewith;  said core and said chimney being spaced radially inwardly from an inner surface of said vessel to define an annular downcomer in flow communication with said lower plenum;  a steam separator assembly extending upwardly from said chimney in flow communication therewith;  a steam dryer assembly spaced upwardly above said steam separator assembly in flow communication therewith and below said upper head;  said vessel being fillable with reactor water to a level through said steam separator assembly, with said core receiving said reactor water recirculated downwardly through said downcomer, through said lower plenum, and upwardly into said core, said core being effective for boiling said reactor water to generate a steam-water mixture which flows upwardly through said chimney and into said steam separator assembly and in turn through said steam dryer assembly for removing water therefrom for discharging steam from said vessel;  a plurality of control rods selectively positionable in said core;  a plurality of control rod drives (CRDs) extending downwardly from said lower head and operatively joined to respective ones of said control rods for selectively translating said control rods upwardly out of said core and downwardly into said core; and  a plurality of channels disposed in said chimney above said core and laterally spaced apart from each other to define therebetween guide slots for receiving said control rods translated upwardly out of said core by said CRDs, said channels being aligned with said fuel bundles for receiving and channeling upwardly therefrom to said steam separator assembly said steam-water mixture.  each of said control rods has a cruciform transverse configuration; and  each of said chimney channels has a square configuration, and four adjacent ones of said chimney channels are disposed together so that said guide slots defined therebetween collectively have a cruciform configuration for receiving and guiding a respective one of said control rods.  a tubular CRD housing extending downwardly through said lower head;  a drive rod extending upwardly from said CRD housing and coupled to a respective one of said control rods;  means for selectively translating said drive rod upwardly for withdrawing said control rod upwardly from said core and into said chimney, and for selectively translating said drive rod downwardly for inserting said control rod into said core from said chimney; and  means for selectively releasing said drive rod for allowing gravity to insert said control rod into said core.  said drive rod is a drive screw; and  said translating means comprise:  a motor for selectively rotating said drive tube and in turn said drive nut in a first direction for translating downwardly said drive screw, and in a second, opposite, direction for translating upwardly said drive screw.  a bottom endplate having a drive shaft extending downwardly through a lower manifold of said CRD housing and operatively coupled to said motor for rotating said drive tube;  a central core tube extending upwardly from said drive tube bottom endplate around said drive screw, and spaced radially inwardly from an inner surface of said drive tube to define an annular core channel extending upwardly to said drive nut;  a piston slidably disposed in said core channel and operatively coupled to said nut for selectively engaging and releasing said drive nut from said drive screw; and  said drive tube bottom end including a plurality of fluid ports disposed therethrough in flow communication between said lower manifold and said core channel for channeling a pressurized fluid into said core channel to generate a pressure force against said piston for engaging said nut and said drive screw, and for venting said pressurized fluid from said core channel to release said pressure force from said piston for releasing said drive nut from said drive screw.  said drive nut comprises a first shank extending radially outwardly through said core tube and having an inclined first cam surface, and a first spring biasing said drive nut away from said drive screw in a retracted position;  said piston comprises a second shank extending upwardly toward said first cam surface and having an inclined second cam surface in abutting sliding contact with said first cam surface, and a second spring biasing said piston downwardly away from said drive nut for allowing said first spring to release said drive nut from said drive screw; and  said first and second springs being sized so that said pressure force generated against said piston by said pressurized fluid is effective for lifting said piston upwardly against said second spring to allow said second cam surface to slide against said first cam surface to urge said drive nut radially inward toward said drive screw against said first spring to engage said drive nut with said drive screw. 2. A reactor according to claim 1, further comprising a gravity-driven cooling system including a pool of makeup water disposed vertically above said reactor water level of said vessel and in flow communication with said vessel for selectively draining by gravity said makeup water into said vessel; said core having a core height; and said lower plenum having a height less than said core height. 3. A reactor according to claim 2 wherein: 4. A reactor according to claim 3 wherein said chimney includes an upper grid, and said chimney channels are supported by hanging downwardly from said upper grid. 5. A reactor according to claim 3 wherein each of said fuel bundles has a square configuration, and each of said chimney channels is sized for covering four of said fuel bundles for channeling said steam-water mixture upwardly therefrom. 6. A reactor according to claim 1 wherein each of said CRDs comprises: 7. A reactor according to claim 6 wherein: 8. A reactor according to claim 7 wherein said drive tube comprises: 9. A reactor according to claim 8, further comprising means for selectively providing said pressurized fluid to said lower manifold for engaging said drive nut with said drive screw, and for venting said pressurized fluid from said lower manifold for releasing said drive nut from said drive screw to allow gravity to insert said control rod into said core. 10. A reactor according to claim 9 wherein: 11. A reactor according to claim 10 wherein said core tube includes a plurality of vent ports adjacent to said fluid ports in said drive tube bottom endplate for allowing fluid in said core tube to be vented in turn through said vent port, said core channel, and said fluid ports into said lower manifold as said drive screw is translated downwardly inside said core tube. 12. A reactor according to claim 7 wherein said CRD housing includes an open top end facing toward said core; and said drive tube is predeterminedly sized smaller than said CRD housing top end for being upwardly removable from said CRD housing without obstruction.