Patent Number: 
Section: claims

1. A modular nuclear reactor system comprising;a reactor pressure vessel having a removable head;a primary coolant loop of the nuclear reactor enclosed within the reactor pressure vessel for circulating a primary coolant within the reactor pressure vessel;a containment pressure vessel enclosing the reactor pressure vessel, the containment pressure vessel being substantially submerged in a liquid pool;an in-containment pool system located within the containment pressure vessel, outside of the reactor pressure vessel and at least in part occupying a lower portion of the containment pressure vessel below an upper elevation of a reactor core housed within the reactor pressure vessel, the in-containment pool system having a reserve reservoir of the primary coolant that is isolated from the primary coolant loop during normal reactor operation, with the reserve reservoir connected to an inlet to the reactor pressure vessel;a sump, normally isolated from the in-containment pool system, at least in part extending into a lower portion of the containment pressure vessel, for collecting the primary coolant escaping out of the primary coolant loop and into the containment pressure vessel; anda circulation system configured to passively circulate, by natural circulation by convection, the primary coolant from the sump through at least a portion of the in-containment pool system and into the reactor pressure vessel upon a preselected operating condition of the modular reactor system. 2. The modular nuclear reactor system of claim 1 wherein the in-containment pool system is connected to a cold leg of the primary coolant loop through a check valve. 3. The modular nuclear reactor system of claim 1 wherein a portion of the in-containment pool system reservoir of the primary coolant is connected to the sump through a sump check valve and a sump injection nozzle that opens a normally closed fluid communication path from the sump to the in-containment pool system reservoir when the reactor pressure vessel is in a depressurized condition and a level of the primary coolant within the sump reaches a given level. 4. The modular nuclear reactor system of claim 3 including a depressurization system for equalizing the pressure within the reactor pressure vessel and the containment pressure vessel. 5. The modular nuclear reactor system of claim 4 including one or more in-containment pool tanks containing reactor coolant wherein the in-containment pool tanks are supported within the containment pressure vessel, outside of the reactor pressure vessel, at an elevation above the reactor core within the reactor pressure vessel, wherein the in-containment pool tanks are connected to and in fluid communication with the reserve reservoir. 6. The modular nuclear reactor system of claim 5 including one or more core makeup tanks containing primary coolant at a pressure substantially equal to a pressure within the reactor pressure vessel during normal reactor operation, the core makeup tanks being supported within the containment pressure vessel outside of the reactor pressure vessel, at an elevation above the reactor core and being connected at an upper portion of the core makeup tanks to a hot leg of the primary coolant loop and at a lower portion of the core makeup tanks to a cold leg of the primary coolant loop, and an isolation valve for isolating the lower portion of the core makeup tanks from the cold leg of the primary coolant loop. 7. The modular nuclear reactor system of claim 6 including a passive residual heat removal system for cooling the reactor coolant within the core makeup tanks when the isolation valve is in an open condition. 8. The modular nuclear reactor system of claim 7 wherein the passive residual heat removal system has a first heat exchanger with a primary side and a secondary side, the primary side of the first heat exchanger is in fluid communication with the reactor coolant in the core makeup tank and the secondary side of the first heat exchanger is in fluid communication with a primary side of a second heat exchanger having a secondary side in fluid communication with an ultimate heat sink pool, the ultimate heat sink pool extending to an elevation above the containment pressure vessel. 9. The modular nuclear reactor system of claim 8 wherein the ultimate heat sink pool is in fluid communication with means for replenishing the liquid pool when the liquid pool drops below a preset level. 10. The modular nuclear reactor system of claim 6 wherein the depressurization system is connected to the hot leg of the primary coolant loop. 11. The modular nuclear reactor system of claim 10 including a second depressurization system connected to the core makeup tank. 12. The modular nuclear reactor system of claim 6 wherein the depressurization system is connected to the core makeup tank. 13. The modular nuclear reactor system of claim 6 including a protection and safety monitoring system which is configured to monitor the occurrence of a loss of coolant accident or a steam line break and upon such occurrence issue a control signal to open the isolation valve. 14. The modular nuclear reactor system of claim 13 wherein the protection and safety monitoring system is configured to monitor the reactor coolant level within the core makeup tank and when the reactor coolant within the core makeup tank goes below a preselected level the protection and safety monitoring system issues a control signal to activate the depressurization system. 15. The modular nuclear reactor system of claim 14 wherein activation of the depressurization system also activate vent valves on the in-containment pool tanks that vent an interior of the in-containment pool tanks to the containment pressure vessel. 16. The modular nuclear reactor system of claim 15 wherein the in-containment pool tanks are configured to drain into the core through the in-containment pool system reservoir when the pressure in the reactor pressure vessel substantially equals the pressure in the containment pressure vessel and the level of the primary coolant within the reactor pressure vessel drops below a selected level. 17. The modular nuclear reactor system of claim 16 wherein an in-vessel retention valve in the in-containment pool system is configured to be responsive to an electrical signal which is issued under certain adverse operating conditions to open the in-vessel retention valve to allow at least some of the primary coolant within the in-containment pool system to flow around an outside of a lower portion of the reactor pressure vessel when the primary coolant in the in-containment pool tanks drop below a predetermined level and the primary coolant in the sump is above the given level. 18. The modular nuclear reactor system of claim 16 wherein the reactor pressure vessel includes a steam generator heat exchanger having a primary side as part of the primary coolant loop and a secondary side connected in a closed loop with a steam drum located outside of the containment pressure vessel, wherein the secondary side of the steam generator heat exchanger has steam generator isolation valves for isolating the steam generator heat exchanger from the steam drum and wherein the protection and safety monitoring system is configured to monitor for a primary or secondary side break and when a primary or secondary side break is detected the protection and safety monitoring system issues a safeguards signal to the steam generator isolation valves that isolate the steam generator heat exchanger from the steam drum.