Patent Number: 
Section: claims

1. An apparatus comprising:a nuclear reactor including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a subterranean containment structure containing the nuclear reactor; andan ultimate heat sink pool disposed at grade level wherein an upper portion of the subterranean containment structure defines at least a portion of the bottom of the ultimate heat sink pool;wherein the upper portion of the subterranean containment structure comprises an upper dome that defines at least a portion of the bottom of the ultimate heat sink pool; andwherein an uppermost extremity of the upper dome of the subterranean containment structure extends above the surface of the ultimate heat sink pool to define an island surrounded by the ultimate heat sink pool. 2. The apparatus of claim 1, wherein the upper dome of the subterranean containment structure includes grooves or undulations. 3. The apparatus of claim 1, wherein the subterranean containment structure comprises steel. 4. The apparatus of claim 1, wherein the nuclear reactor comprises a pressurized water reactor (PWR) and the subterranean containment structure is large enough to simultaneously accommodate both the PWR and at least one steam generator designed to operate in or with the PWR. 5. The apparatus of claim 4, wherein the PWR is an integral PWR and the subterranean containment structure is large enough to simultaneously accommodate both the PWR and an internal steam generator disposed outside of the PWR but designed to operate in the integral PWR. 6. An apparatus comprising:a nuclear reactor including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a subterranean containment structure containing the nuclear reactor; andan ultimate heat sink pool disposed at grade level wherein an upper portion of the subterranean containment structure defines at least a portion of the bottom of the ultimate heat sink pool;a secondary containment structure containing the subterranean containment structure and the ultimate heat sink pool, the secondary containment structure having vents arranged to allow water evaporated or boiled off of the ultimate heat sink pool to escape from the secondary containment structure; andgutters disposed in the secondary containment structure to admit surface water from outside the secondary containment structure into the ultimate heat sink pool. 7. An apparatus comprising:a nuclear reactor including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a subterranean containment structure containing the nuclear reactor; andan ultimate heat sink pool disposed at grade level wherein an upper portion of the subterranean containment structure defines at least a portion of the bottom of the ultimate heat sink pool; andwherein the portion of the bottom of the ultimate heat sink pool defined by the upper portion of the subterranean containment structure has an area of at least      Q          decay      ⁢                          ⁢      heat            U    ·          (                        T          max                -                  T          UHS                    )      where Tmax denotes the maximum allowable temperature inside the subterranean containment structure, TUHS denotes the maximum allowable temperature of the ultimate heat sink pool, Qdecay heat denotes the highest postulated value for heat generated by fission product decay following reactor shutdown, and U denotes the overall heat transfer coefficient for heat transfer from the a subterranean containment structure to the ultimate heat sink pool. 8. The apparatus of claim 7, wherein the ultimate heat sink pool has a capacity of at least 300,000 gallons. 9. An apparatus comprising:a nuclear reactor including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a subterranean containment structure containing the nuclear reactor; andan ultimate heat sink pool disposed at grade level wherein an upper portion of the subterranean containment structure defines at least a portion of the bottom of the ultimate heat sink pool;a condenser comprising a heat exchanger including hot and cold flow paths disposed inside the subterranean containment structure; andcooling water lines operatively connecting the condenser with the ultimate heat sink pool. 10. The apparatus of claim 9, wherein ends of the cooling lines disposed in the ultimate heat sink pool terminate in one of (i) open ends and (ii) connections with a heat exchanger disposed in the ultimate heat sink pool. 11. An apparatus comprising:a pressurized water reactor (PWR) including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a subterranean containment structure containing the nuclear reactor; andan ultimate heat sink pool having a bottom defined at least in part by an upper portion of the subterranean containment structure, wherein the upper portion of the subterranean containment structure comprises an upper dome and the upper dome of the subterranean containment structure protrudes above the surface of the ultimate heat sink pool to define an island surrounded by the ultimate heat sink pool. 12. An apparatus comprising:a pressurized water reactor (PWR) including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a subterranean containment structure containing the nuclear reactor; andan ultimate heat sink pool having a bottom defined at least in part by an upper portion of the subterranean containment structure, wherein a contact area Awet between the ultimate heat sink pool and the upper portion of the subterranean containment structure satisfies a criterion U·Awet·ΔTmin≧Qdecay heat where Qdecay heat denotes the highest postulated value for heat generated by fission product decay following reactor shutdown, and U denotes the overall heat transfer coefficient for heat transfer from the a subterranean containment structure to the ultimate heat sink pool, and ΔTmin denotes the minimum temperature difference between the subterranean containment structure and the ultimate heat sink pool postulated to occur during any accident scenario under consideration. 13. An apparatus comprising:a pressurized water reactor (PWR) including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a subterranean containment structure containing the nuclear reactor;an ultimate heat sink pool having a bottom defined at least in part by an upper portion of the subterranean containment structure;a condenser comprising a heat exchanger including hot and cold flow paths disposed inside the subterranean containment structure; andcooling water lines operatively connecting the condenser with the ultimate heat sink pool. 14. An apparatus comprising:a nuclear reactor including a pressure vessel and a nuclear reactor core disposed in the pressure vessel;a containment structure containing the nuclear reactor;an ultimate heat sink pool disposed on top of the containment structure wherein the containment structure defines a bottom of the ultimate heat sink pool;a condenser comprising a heat exchanger including hot and cold flow paths disposed inside the containment structure; andcooling water lines operatively connecting the condenser with the ultimate heat sink pool.