Patent Number: 
Section: claims

1. A nuclear reactor vessel comprising:an elongated cylindrical body defining an internal cavity containing primary coolant water;a nuclear fuel core disposed in the internal cavity;an elongated cylindrical shroud disposed in the internal cavity, the shroud comprising an inner shell, an outer shell, and a plurality of intermediate shells disposed between the inner and outer shells;a plurality of annular cavities formed between the inner and outer shells by the intermediate shells, the annular cavities each being filled with the primary coolant water;the shroud being comprised of a plurality of vertically-stacked shroud segments each including an annular top closure plate and an annular bottom closure plate, wherein the inner, outer, and intermediate shells of each shroud segment is seal welded to their respective top and bottom closure plates collectively forming a self-supporting shroud segment structure; andeach shroud segment detachably coupled to an adjoining shroud segment. 2. The reactor vessel of claim 1, wherein the annular cavities are fluidly interconnected by a plurality of drain holes formed through the outer shell and intermediate shells, the drain holes in the outer shell spaced longitudinally apart along a length of the outer shell, and the drain holes in the intermediate shells being spaced longitudinally apart along a respective length of each intermediate shell. 3. The reactor vessel of claim 1, wherein the intermediate shells each have a length that is coextensive with respective lengths of the inner and outer shells. 4. The reactor vessel of claim 1, wherein the shroud forms a riser region inside the shroud and a downcomer region between the shroud and the body of the reactor vessel. 5. The reactor vessel of claim 4, wherein the fuel core is disposed inside the shroud. 6. The reactor vessel of claim 4, wherein the shroud has a bottom which is spaced vertically apart from a bottom of the reactor vessel to form a fluid flow path between the downcomer and riser regions. 7. The reactor vessel of claim 1, further comprising a plurality of seismic springs disposed between the shroud and the body of the reactor vessel for lateral restraint. 8. The reactor vessel of claim 7, wherein the seismic springs are arcuately shaped concave leaf springs each comprising a plurality of individual leaves joined together and opposite ends engaging an interior surface of the cylinder body of the reactor vessel. 9. The reactor vessel of claim 1, further comprising a clamp coupling the top closure plate of one shroud segment to the bottom closure plate of an adjoining shroud segment. 10. The reactor vessel of claim 9, wherein the clamp is pivotably mounted to a shroud segment. 11. The reactor vessel of claim 9, further comprising a seismic restraint fixedly attached to the clamp, the seismic restraint configured to engage the body of the reactor vessel. 12. A nuclear reactor vessel comprising:an elongated cylindrical body defining an internal cavity containing primary coolant water;a nuclear fuel core disposed in the internal cavity;an elongated cylindrical shroud disposed in the internal cavity, the shroud comprising a plurality of vertically-stacked shroud segments each including an annular top closure plate, an annular bottom closure plate, an inner shell, an outer shell, and a plurality of radially spaced apart intermediate shells disposed between the inner and outer shells;an outer annular cavity formed between the intermediate shells and the outer shell;an inner annular cavity formed between the intermediate shells and the inner shell;a plurality of intermediate annular cavities formed between the intermediate shells; anda plurality of drain holes formed through the outer shell and each intermediate shell, the drain holes in the outer shell spaced longitudinally apart along a length of the outer shell, and the drain holes in the intermediate shells being spaced longitudinally apart along a respective length of each intermediate shell, the drain holes operable to fluidly interconnect the outer, inner, and intermediate annular cavities;wherein the inner, outer, and intermediate shells of each shroud segment is seal welded to their respective top and bottom closure plates collectively forming a self-supporting shroud segment structure, each shroud segment detachably coupled to an adjoining shroud segment;wherein the outer, inner, and intermediate annular cavities are each filled with the primary coolant water.