Patent Number: 
Section: claims

1. Apparatus for thermally insulating a vessel during normal operating conditions and cooling the vessel in the event that an accident condition has a potential for raising the vessel temperature above a predefined limit, comprising:a vessel having a bottom head and a sidewall extending upwardly therefrom;thermal insulation surrounding the vessel and spaced from the bottom head and the sidewall; andat least one inlet door hingedly attached to the thermal insulation, the inlet door being sealed by gravity against a stop on the thermal insulation during normal operation to close an inlet to a space between the vessel and the thermal insulation and floatable with a given buoyancy when immersed in a liquid fluid to move to an open position to permit the liquid fluid to gain access to the space between the vessel and the thermal insulation under predetermined abnormal operating conditions to cool the vessel without the inlet door providing an obstruction to flow of the fluid in the open position. 2. The apparatus of claim 1 wherein the vessel has an axial centerline and the bottom head has an outside diameter and wherein the vessel bottom head and the thermal insulation define an annulus that increases in volume from the axial centerline to the outside diameter of the bottom head. 3. The apparatus of claim 1 wherein the inlet door is designed so that when the fluid enters the space between the thermal insulation and the vessel the inlet door will fail in an open position if it loses its buoyancy. 4. The apparatus of claim 1 wherein the vessel is a nuclear reactor pressure vessel that is supported in a reactor cavity suspended above a floor of the reactor cavity, the reactor cavity having upwardly extending side walls surrounding at least a lower portion of the vessel and spaced from the vessel, the thermal insulation being interposed between the side wall of the reactor cavity and the vessel and spaced from the side wall of the reactor cavity to define a cooling air path therebetween and the space between the vessel and the thermal insulation defining a liquid fluid flow path therebetween. 5. The apparatus of claim 4 wherein the fluid flow path and the cooling air path have separate exits from the reactor cavity. 6. The apparatus of claim 4 further including:a neutron shield for shielding at least some of a stream of neutrons from exiting the reactor cavity, the neutron shield surrounding and spaced from the vessel and spaced from the side wall of the reactor cavity so that the fluid flow path and the cooling air path respectively pass by the inner and outer surface of the neutron shield; anda fluid exit door hingedly attached above the neutron shield, the fluid exit door having a closed position and an open position, the closed position sealing the fluid flow path from an exit flow path from the reactor cavity and the open position providing the fluid flow path access to the exit flow path from the reactor cavity without forming an obstruction to air flow in the cooling air path. 7. The apparatus of claim 6 wherein the fluid exit door when open to permit the fluid cooling the vessel to exit remains in the open position until reset. 8. The apparatus of claim 6 wherein the vessel has nozzles disposed within a nozzle gallery that surrounds the vessel at a nozzle elevation wherein the fluid exit door permits the fluid to exit to the nozzle gallery. 9. The apparatus of claim 1 wherein the inlet door has vent holes to prevent pressure differentials between the interior and the outside of the door during vessel heat-up and cool-down. 10. The apparatus of claim 1 wherein the door is a metal shell filled with a hydrophobic material. 11. The apparatus of claim 4 further including:a neutron shield for shielding at least some of a stream of neutrons from exiting the reactor cavity, the neutron shield surrounding and spaced from the vessel to provide an opening for the fluid flow path;a second set of insulation surrounding an upper portion of the vessel above the fluid flow path and the neutron shield; anda fluid exit door hingedly attached to the second set of insulation at an upper end of the exit door, the fluid exit door sealing an exit to the fluid flow path when in a closed position. 12. The apparatus of claim 11 wherein the fluid exit door is a metal enclosure filled with a buoyant hydrophobic material. 13. The apparatus of claim 8 including a plurality of fluid exit doors hingedly attached to the neutron shield at a plurality of circumferential locations both at and between positions of the nozzles. 14. The apparatus of claim 13 wherein the fluid flow path and the cooling air path have separate exits from an upper portion of the reactor cavity and the cooling air path only exits the upper portion of the reactor cavity approximately at the nozzle positions. 15. The apparatus of claim 14 wherein the fluid flow path exits the reactor cavity approximately at and between the nozzle positions. 16. The apparatus of claim 13 wherein the plurality of fluid exit doors extend substantially continuously, circumferentially around the vessel. 17. The apparatus of claim 1 wherein the thermal insulation surrounding a lower portion of the vessel, has a rounded transition extending from below the bottom head to around the sidewall of the vessel, substantially following a contour of the vessel.