Patent Number: 
Section: claims

1. A nuclear power plant comprising:a nuclear reactor containment structure defining an interior space in which a nuclear reactor is located;a cooling vessel comprising a water tank located outside the nuclear reactor containment structure, wherein the water tank comprises a portion that is lower than the bottom of the interior space of the nuclear reactor containment structure;water contained in the water tank:a vapor release pipe connecting between the interior space of the nuclear reactor containment structure and the water tank, the vapor release pipe being configured to transfer vapor from the interior space to the water tank based on a pressure difference between the interior space and the water tank; anda water return pipe connecting between the water tank and the interior space of the nuclear reactor containment structure, the water return pipe being configured to transfer water from the water tank to the interior space of the nuclear reactor containment structure based on a water head difference between water contained in the water tank and water in the interior space of the nuclear reactor containment structure,wherein the water is contained in the water tank only in the portion such that the water head in the water tank is lower than the bottom of the interior space of the nuclear reactor containment structure such that the water would not automatically flow from the water tank to the nuclear reactor containment structure during normal operation of the nuclear power plant,wherein in the event of an accident, the vapor release pipe is configured to transfer vapor from the interior space to the water tank where it condenses, which causes the water head in the water tank to rise to a level higher than the bottom of the interior space of the nuclear reactor containment structure such that is automatically returned to the interior space of the nuclear reactor containment structure from the water tank even in the absence of pumping. 2. The nuclear power plant of claim 1, wherein the water tank has a bottom surface lower than the bottom of the interior space such that the water tank has a volume configured to contain water under the level of the bottom surface of the interior space sufficient to maintain the water head in the water tank lower than the bottom surface of the interior space for about 3 hours after vapor begins to be transferred from the nuclear reactor containment structure to the water tank. 3. A cooling system of a nuclear power plant, comprising:a pressure vessel located outside a nuclear reactor containment structure, the pressure vessel having a water tank, the nuclear reactor containment structure defining an interior space for housing a nuclear reactor, wherein the water tank comprises a portion that is lower than the bottom of the interior space of the nuclear reactor containment structure;water contained in the water tank;a release pipe connecting between the interior space of the nuclear reactor containment structure and the water tank, the release pipe being configured to transfer water vapor and fission products generated in the nuclear reactor containment structure in the event of an accident to the water tank;a water return pipe connecting between the water tank and the interior space of the nuclear reactor containment structure, the water return pipe being configured to transfer water from the water tank to the interior space based on a water head difference between water contained in the water tank and water in the interior space of the containment structure,wherein the water is contained in the water tank only in the portion such that the water head in the water tank is lower than the bottom of the interior space of the nuclear reactor containment structure such that the water would not automatically flow from the water tank to the nuclear reactor containment structure during normal operation of the nuclear power plant,wherein in the event of an accident, the release pipe is configured to transfer vapor from the interior space to the water tank where it condenses, which causes the water head in the water tank to rise to a level higher than the bottom of the interior space of the nuclear reactor containment structure such that water is automatically transferred to the interior space of the nuclear reactor containment structure from the water tank even in the absence of pumping. 4. The cooling system of claim 3, wherein the pressure vessel is provided with one or more heat-radiation fins. 5. The cooling system of claim 3, wherein a cooling pool containing water for cooling the pressure vessel is formed around a lower portion of the pressure vessel. 6. The cooling system of claim 3, further comprising an exhaust pipe connected to the pressure vessel such that gas generated within the pressure vessel is discharged out thereof through the exhaust pipe. 7. The cooling system of claim 6, wherein the pressure vessel comprises a filter for removal of fission products. 8. The cooling system of claim 7, wherein the pressure vessel comprises a moisture separator located beneath the filter. 9. The cooling system of claim 3, wherein the release pipe is provided with a release isolation valve located inside the containment structure and a release valve located outside the containment structure. 10. The cooling system of claim 3, wherein the water return pipe is provided with a recovery isolation valve located inside the containment structure, and a recovery valve located outside the containment structure. 11. The cooling system of claim 3, further comprising a supply tank connected to the pressure vessel by a supply pipe, the supply pipe configured to supply water to the water tank of the pressure vessel. 12. The cooling system of claim 11, wherein:the supply tank comprises a temperature sensor configured for sensing a temperature in the water tank; andthe supply pipe is provided with a supply valve which is configured to open the supply pipe when the temperature in the water tank is equal to or greater than a set temperature. 13. The cooling system of claim 3, further comprising a gas recovery pipe connected to the pressure vessel and the containment structure such that gas generated within the pressure vessel flows into the containment structure. 14. The cooling system of claim 3, further comprising:a gas tank containing non-condensable gas and connected to the release pipe, wherein in the event of an accident, the gas tank is configured to release the non-condensable gas into a flow of water vapor and fission products from the containment structure such that the non-condensable gas is released into water contained in the water tank together with the water vapor and fission products for preventing undesirable water hammer from occurring; anda bypass pipe connecting between the containment structure and the pressure vessel while bypassing the gas tank. 15. The cooling system of claim 14, wherein the bypass pipe further comprises a bypass valve to control opening and closing of the bypass pipe. 16. The cooling system of claim 14, wherein the bypass pipe is connected to a position higher than the level of the condensed water in the pressure vessel. 17. The cooling system of claim 14, wherein the release pipe comprises a second release valve located between a portion from which the bypass pipe diverges and a portion to which the pressure vessel is connected. 18. The nuclear power plant of claim 14, wherein the non-condensable gas is nitrogen gas.