Patent Number: 
Section: claims

1. An apparatus comprising:a nuclear reactor core comprising fissile material;a core basket; anda core former surrounding the nuclear reactor core, the core former comprising a stack of two or more single-piece annular rings wherein each single-piece annular ring comprises neutron-reflecting material,wherein adjacent single-piece annular rings of the stack of single-piece annular rings include a key feature disposed on a first one of the adjacent single-piece annular rings and a keyway feature disposed on a second one of the adjacent single-piece annular rings, wherein the key feature is received in the keyway feature to prevent relative rotation of the first one and the second one of the adjacent single-piece annular rings, and wherein the stack of single-piece annular rings does not include welds or fasteners that axially restrain adjacent single-piece annular rings together or that secure the rings to the core basket. 2. The apparatus of claim 1, wherein the single-piece annular rings are forged or cast annular rings. 3. The apparatus of claim 2, wherein the single-piece annular rings are stainless steel annular rings. 4. The apparatus of claim 1, wherein the single-piece annular rings are stainless steel annular rings. 5. The apparatus of claim 1, wherein adjacent single-piece annular rings of the stack have mating surfaces shaped to define an annular joint between the adjacent single-piece annular rings that provides a tortuous path for fluid flow into or out of the core former via the joints. 6. The apparatus of claim 1, wherein the core basket contains the nuclear reactor core and the core former. 7. The apparatus of claim 6, wherein an annular gap is defined between the core former and the core basket. 8. The apparatus of claim 6, wherein the outer surface of the core former includes axially extending channels. 9. The apparatus as set forth in claim 6, further comprising:a pressure vessel containing the nuclear reactor core, the core former, and the core basket. 10. The apparatus as set forth in claim 9, wherein the core basket is suspended from a mid-flange of the pressure vessel. 11. The apparatus of claim 1, wherein each single-piece annular ring is a monolithic element without joints or seams. 12. The apparatus of claim 1, wherein the core former does not include welds and does not include fasteners. 13. An apparatus comprising:a core basket; anda core former sized for a nuclear reactor core, the core former comprising a stack of two or more single-piece annular rings wherein each single-piece annular ring comprises neutron-reflecting material and has an inner surface conforming with a periphery of the nuclear reactor core,wherein a first one of the single-piece annular rings includes a circumferential recess in one of its top surface and its bottom surface, and a second one of the single-piece annular rings includes a circumferential protrusion in one of its top surface and its bottom surface, and the circumferential protrusion is received within the circumferential recess,wherein the stack of single-piece annular rings does not include welds or fasteners axially restraining adjacent single-piece annular rings together or that secure the rings to the core basket, andwherein the first one of the single-piece annular rings includes a key feature disposed on one of its top surface and its bottom surface and the second one of the adjacent single-piece annular rings includes a keyway feature disposed on one of its top surface and its bottom surface, and the key feature is received in the keyway feature to prevent relative rotation of the first one and the second one of the adjacent single-piece annular rings. 14. The apparatus of claim 13, wherein the core basket contains the nuclear reactor core and the core former. 15. The apparatus of claim 13, further comprising:a pressure vessel containing the nuclear reactor core, the core former, and the core basket. 16. The apparatus of claim 13, wherein the outer surface of the core former includes axially extending channels.