Patent Number: 
Section: claims

1. In a nuclear reactor vessel having a core plate and a generally hemispherically shaped bottom head with a plurality of penetrations enabling control rod drives to pass through said plurality of penetrations to support a respective plurality of control rods, a method of repairing or mitigating crack formation at said hemispherically shaped bottom head of the vessel comprising the steps of:passing plural discrete segments of a first caisson through holes in the core plate to a location about a tube secured to said hemispherically shaped bottom head and surrounding a one of said plurality of penetrations;assembling said plural discrete segments about said tube;sealing said plural discrete segments to one another and to said generally hemispherically shaped bottom head;passing a second caisson through one of said holes in the a core plate and causing a lower edge of said second caisson to engage an upper edge of said first caisson;removing water from said first and second caissons to provide a water-free environment for welding; andlowering a welding head through said second caisson and into the first caisson to apply a weld at a weld site about the tube or along said generally hemispherically shaped bottom head. 2. A method according to claim 1, wherein said first caisson has a diameter greater than a diameter of said one of the holes in said core plate. 3. A method according to claim 1, including forming at least a pair of segments of the first caisson uniquely and different in configuration relative to one another to accommodate said generally hemispherically shaped bottom head. 4. A method according to claim 1, wherein said generally hemispherically shaped bottom head has a vertically extending axis through its lowest point, including forming at least a pair of said plural segments of said first caisson having different heights relative to one another such that upper edges thereof lie in a horizontal plane normal to said vertically extending axis. 5. A method according to claim 4, wherein bottom edges of said plural segments are tapered to accommodate inclines along said generally hemispherically shaped bottom head. 6. A method according to claim 1, including passing a holddown through a hole in the core plate, securing said holddown to said generally hemispherically shaped bottom head adjacent the weld site and engaging said holddown and at least one of said plural segments to maintain said at least one of said segments adjacent an interior surface of said generally hemispherically shaped bottom head prior to sealing said plural segments to one another and to said generally hemispherically shaped bottom head. 7. A method according to claim 6, including forming the holddown in at least two distinct parts, separately passing said at least two distinct parts through one or more of the holes in the core plate, assembling said at least two distinct parts to one another below the core plate and in a water environment of the reactor vessel to form assembled holddown, and engaging the assembled holddown and said at least one segment to maintain said at least one segment adjacent the interior surface of the generally hemispherically shaped bottom head. 8. A method according to claim 7, wherein the generally hemispherically shaped bottom head includes a plurality of control rod drive housings projecting upwardly therefrom at locations about said tube, separately passing a plurality of distinct holddown parts through one or more of the holes in the core plate for assembly into a plurality of holddowns, securing said plurality of holddowns to control rod drive housings about the tube, and engaging said plurality of holddowns and said plurality of segments to one another to maintain said plurality of segments adjacent the generally hemispherically shaped bottom head about the tube. 9. In a nuclear reactor vessel having a core plate and a generally hemispherically shaped bottom head, a method of repairing or mitigating crack formation at the bottom head of the vessel comprising the steps of:passing segments of a first caisson through holes in the core plate to a location adjacent the bottom head;assembling the segments to form an enclosure;sealing the segments to one another and to the bottom head;passing a second caisson through a core plate hole and sealing a lower end thereof to an upper end of the first caisson;removing water from the first and second caissons to provide a water-free environment for welding; andlowering a welding head through the second caisson and into the first caisson to apply a weld. 10. A method according to claim 9, including providing arcuate segments, assembling the arcuate segments to form an assembled first caisson having a diameter for greater than a diameter of the core plate hole.