Patent ID: 7754993

Claim:
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; and lowering 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.