Patent Number: 047939664
Section: claims

1. The method of operating a nuclear reactor having a vessel, a nuclear core within said vessel, at least an inlet nozzle and an outlet nozzle, and control-rod assemblies within a plenum within said vessel; the said method including: conducting a coolant in sequence generally vertically through said core and generally vertically and generally transversely through said control-rod assemblies in said plenum, said coolant flowing at a high velocity through said core, reducing the velocity of said coolant as it flows through said plenum by the step of expanding the volume through which said coolant flows through said plenum over substantially the whole volume of said plenum, said volume-expanding step including the step of compensating for the pressure variation of said coolant along said plenum between the generally lower regions of said plenum and the generally upper regions of said plenum, and thereafter conducting said coolant through said outlet nozzle. 2. The method of claim 1 wherein the volume-expanding step also includes the step of suppressing the direct flow of coolant from the plenum through the outlet nozzle. 3. The method of claim 1 wherein the reactor includes a shroud interposed between the control-rod assemblies and the outlet nozzle and wherein the volume-expanding step includes the step of suppressing the flow of the coolant into and generally vertically through the channel between the outer boundary of the control assemblies and the shroud under the pressure laterally of the coolant in the generally vertically lower regions of the plenum. 4. A nuclear reactor including a vessel, a nuclear core within said vessel, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies within said vessel, said control-rod assemblies including control rods movable between a plenum above said core and receptors within said core, drive rods for said control rods and guide means for said control rods within said plenum, the in-flowing coolant, after passing through said core, flowing generally upwardly and generally transversely through said plenum to said outlet nozzle in the operation of said reactor, the pressure of said coolant decreasing progressively in said plenum as said coolant flows generally upwardly therethrough, and a shroud interposed between said guide means and said outlet nozzle in the path of said transversely-flowing coolant, said shroud encircling said guide means and having holes along substantially the whole extent thereof providing passage to said outlet nozzle for said transversely-flowing coolant, the areas of said holes in said shroud increasing progressively from its generally vertically lower region to its generally vertically upper region in accordance with said variation in pressure of said coolant between said generally lower region and said generally upper region of said plenum to preclude concentration of said coolant by the higher pressure in the generally lower regions of said shroud, and said holes being so distributed over said shroud that the transversely flowing coolant is distributed over the whole surface of said shroud, whereby the velocity of said transversely-flowing coolant is reduced to a magnitude substantially below the magnitude at which said transversely-flowing coolant would cause said control rods, said drive rods or said guide means to be subject to metal fatigue. 5. A nuclear reactor including a vessel, a nuclear core within said vessel, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies within said vessel, said control-rod assemblies including control rods movable between a plenum above said core and receptors within said core, drive rods for said control rods and guide means for said control rods within said plenum, the in-flowing coolant, after passing through said core, flowing generally transversely through said plenum to said outlet nozzle in the operation of said reactor, a shroud interposed between said guide means and said outlet nozzle in the path of said transversely-flowing coolant, said shroud encircling said guide means and having holes along substantially the whole extent thereof providing passage to said outlet nozzle for said transversely-flowing coolant, said holes being so distributed over said shroud and/or so dimensioned that the transversely-flowing coolant is distributed over the whole surface of said shroud, whereby the velocity of said transversely-flowing coolant is reduced to a magnitude substantially below the magnitude at which said transversely-flowing coolant would cause said control rods, said drive rods or said guide means to be subject to metal fatigue, and barrier plates extending annularly between said shroud and said guide means along the vertical length of said shroud and guide means to suppress axial flow of coolant in the annular region between said shroud and said guide means under pressure of the transversely-flowing coolant. 6. The nuclear reactor of claim 5 wherein the guide means includes support plates extending vertically along the guide means, and the barrier plates are located substantially at the same level as said support plates. 7. A nuclear reactor including a vessel, a nuclear core within said vessel, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies within said vessel, said control-rod assemblies including control rods movable between a plenum above said core and receptors within said core, drive rods for said control rods and guide means for said control rods within said plenum, the in-flowing coolant, after passing through said core, flowing generally transversely through said plenum to said outlet nozzle in the operation of said reactor, and a shroud interposed between said guide means and said outlet nozzle in the path of said transversely-flowing coolant, said shroud encircling said guide means and having holes along substantially the whole extent thereof except along a limited region thereof generally opposite said outlet nozzle thereby producing indirect passage of said transversely-flowing coolant but suppressing direct flow of said transversely-flowing coolant from said control-rod assemblies through said outlet nozzle, said holes being otherwise so distributed over said shroud and/or so dimensioned that the transversely-flowing coolant is distributed over the whole surface of said shroud, whereby the velocity of said transversely-flowing coolant is reduced to/magnitude substantially below the magnitude at which said transversely-flowing coolant would cause said control rods, said drive rods or said guide means to be subject to metal fatigue. 8. A nuclear reactor including a vessel, a nuclear core within said vessel, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies within said vessel, said control-rod assemblies including control rods movable between a plenum above said core and receptors within said core, drive rods for said control rods and guide means for said control rods within said plenum, the in-flowing coolant, after passing through said core, flowing generally upwardly and generally transversely through said plenum to said outlet nozzle in the operation of said reactor, the pressure of said coolant in said plenum decreasing progressively as said coolant flows generally upwardly therethrough, and a shroud interposed between said guide means and said outlet nozzle in the path of said transversely-flowing coolant, said shroud encircling said guide means and having holes along substantially the whole extent thereof providing passage to said outlet nozzle for said transversely-flowing coolant, the numbers of said holes in said shroud increasing progressively from its generally vertically lower region to its generally vertically upper region in accordance with said variation of said pressure of the coolant between said generally lower region and said generally upper region of said plenum to preclude concentration of the coolant by the higher pressure in the lower areas of said shroud, said holes being otherwise so distributed over said shroud that the transversely-flowing coolant is distributed over the whole surface of said shroud, whereby the velocity of said transversely-flowing coolant is reduced to a magnitude substantially below the magnitude at which said transversely-flowing coolant would cause said control rods, said drive rods or said guide means to be subject to metal fatigue. 9. A nuclear reactor including a vessel, a nuclear core within said vessel, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies within said vessel, said control-rod assemblies including control rods movable between a plenum above said core and receptors within said core, drive rods for said control rods and guide means for said control rods within said plenum, the in-flowing coolant, after passing through said core, flowing generally upwardly and generally transversely through said plenum to said outlet nozzle in the operation of said reactor, the pressure of said coolant decreasing progressively in said plenum as said coolant flows generally upwardly through said plenum, and a shroud interposed between said guide means and said outlet nozzle in the path of said transversely-flowing coolant, said shroud encircling said guide means and having holes along substantially the whole extent thereof providing passage to said outlet nozzle for said transversely-flowing coolant, the areas of the holes in said shroud increasing progressively from its generally vertically lower region to its generally vertically upper region in accordance with said variations of said pressure of said coolant between said generally lower region and said generally upper region of said plenum to preclude concentration of said coolant by the higher pressure in the generally lower region of said shroud. 10. A nuclear reactor including a vessel, a nuclear core within said vessel, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies within said vessel, said control-rod assemblies including control rods movable between a plenum above said core and receptors within said core, drive rods for said control rods and guide means for said control rods within said plenum, the in-flowing coolant, after passing through said core, flowing generally upwardly and generally transversely through said plenum to said outlet nozzle in the operation of said reactor, the pressure of said coolant in said plenum decreasing progressively as said coolant flows generally upwardly therethrough, and a shroud interposed between said guide means and said outlet nozzle in the path of said transversely-flowing coolant, said shroud encircling said guide means and having holes along substantially the whole extent thereof providing passage to said outlet nozzle for said transversely-flowing coolant, the number of holes in said shroud increasing progressively from its generally vertically lower region to its generally vertically upper region in accordance with the variation in the pressure of said coolant between said generally lower region and said generally upper region of said plenum to preclude concentration of said coolant by the higher pressure in the lower region of said shroud. 11. A nuclear reactor including a vessel, a nuclear core within said vessel, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies within said vessel, said control-rod assemblies including control rods movable between a plenum above said core and receptors within said core, drive rods for said control rods and guide means for said control rods within said plenum, the in-flowing coolant, after passing through said core, flowing generally transversely through said plenum to said outlet nozzle in the operation of said reactor, and a shroud interposed between said guide means and said outlet nozzle in the path of said transversely-flowing coolant, said shroud encircling said guide means and having holes along substantially the whole extent thereof providing passage to said outlet nozzle for said transversely-flowing coolant, said holes being so distributed over said shroud and/or so dimensioned that direct flow of said transversely-flowing coolant from said plenum through said shroud and through said outlet nozzle is suppressed and the transversely-flowing coolant is distributed over the whole surface of said shroud flowing into said outlet nozzle indirectly, whereby the velocity of said transversely-flowing coolant is reduced to a magnitude substantially below the magnitude at which said transversely-flowing coolant would cause said control rods, said drive rods or said guide means to be subject to metal fatigue. 12. A nuclear reactor including a vessel, a nuclear core within said vessel, upper internals within said vessel above said core, said vessel having at least an inlet nozzle and an outlet nozzle for circulating coolant through said core, control-rod assemblies including control rods movable between said upper internals above said core and receptors within said core, said upper internals including drive rods for said control rods and guide means for said control rods, the in-flowing coolant, after passing through said core, flowing generally transversely through said upper internals to said outlet nozzle in the operation of said reactor, and means, interposed between said upper internals and said outlet nozzle, for suppressing direct flow of said coolant from said upper internals through said outlet nozzle and for otherwise distributing said transversely-flowing coolant over the volume extending substantially throughout the length of said guide means and conducting said transversely-flowing coolant indirectly through said nozzle, the said distributing means being so structured that, the velocity of said transversely-flowing coolant is reduced to a magnitude substantially below the magnitude at which said transversely-flowing coolant would cause said control rods, said drive rods or said guide means to be subject to metal fatigue.