Patent Number: 
Section: claims

1. A nuclear reactor comprising:an elongated reactor vessel having a lower portion sealed at a lower end and having an open upper end on which an annular flange is formed and a central axis extending along an elongated dimension;a reactor vessel head having an annular portion on an underside of the head that is machined to form a sealing surface;a removable annular seal ring, sized to seat on the reactor vessel flange between the flange and the sealing surface on the underside of the reactor vessel head, the seal ring being interposed between the sealing surface on the underside of the vessel head and the flange on the reactor vessel lower portion and having a thickness sized to sealably accommodate radial passages through which utility conduits pass from outside of the reactor vessel to an interior thereof to transport one or more utilities comprising hydraulic fluid for hydraulic mechanisms, instrumentation signals or power for electrical mechanisms, the removable annular seal ring including one or more of such radial passages;a reactor internals assembly comprising a lower internals which includes a reactive core and an upper internals situated above the core, the internals assembly being seated within the reactor vessel, wherein the removable annular seal ring is attached to the reactor internals assembly; anda substantially annular passage between an interior wall of the reactor vessel lower portion and the internals assembly for a downward flow of relatively cool reactor coolant to access an underside of the reactive core, wherein at least a portion of the removable annular seal ring extends over the annular passage where it is attached to the reactor internals assembly, wherein the portion of the removable annular seal ring that extends over the annular passage includes axially extending openings for the passage of reactor coolant. 2. The nuclear reactor of claim wherein the axially extending openings are circumferentially spaced from the radial passages. 3. The nuclear reactor of claim wherein the removable annular seal ring is attached to the upper internals. 4. The nuclear reactor of claim 3 wherein the removable annular seal ring is removable from the reactor vessel with removal of the upper internals. 5. The nuclear reactor of claim 4 wherein at least one or more of the utility conduits are an integral part of the upper internals and include a utility disconnect outside of the reactor vessel. 6. The nuclear reactor of claim 1 wherein the annular seal ring has an upper and a lower double o-ring seals on opposite sides that mate with the reactor vessel flange on one of the opposite sides and the annular portion of the reactor vessel head on another of the opposite sides, the annular seal ring having a hole extending between the upper and lower double o-ring seals allowing leakage to be detected through both sets of seals via one reactor vessel flange leak-off line. 7. The nuclear reactor of claim 6 including one reactor vessel flange leak-off line extending from the lower double o-ring seal. 8. The nuclear reactor of claim 1 wherein the removable annular seal ring is forged from a metal having substantially the same thermal expansion properties as the reactor vessel. 9. The nuclear reactor of claim 8 wherein the removable annular seal ring is forged from either (i) carbon steel in which the surfaces in contact with reactor coolant are clad with stainless steel or (ii) Alloy 690. 10. The nuclear reactor of claim 1 including a plurality of holes axially through the annular seal ring in line with openings in the reactor vessel head and the reactor vessel flange through which studs pass that anchor the head to the flange with the seal ring captured therebetween. 11. The nuclear reactor of claim 10 wherein one or more of the radial passages extend in between two adjacent ones of the holes. 12. The nuclear reactor of claim 1 wherein the utility conduits are sealed to the radial passages on the inner diameter of the seal ring.