Patent Number: 
Section: claims

1. A reactor containment vessel comprising:a primary reactor containment vessel containing a reactor pressure vessel and having a certain pressure resistance higher than atmospheric pressure and a certain leak-tightness;a secondary reactor containment vessel installed outside of the primary reactor containment vessel and having a pressure resistance and a leak-tightness equivalent to the pressure resistance and the leak-tightness of the primary reactor containment vessel;an air bag installed in a folded state in the secondary reactor containment vessel, and upon an occurrence of an accident in the primary reactor containment vessel, the air bag is configured toreceive high-pressure gas released from the primary reactor containment vessel, andexpand from the folded state to an expanded state while confining the gas therein; anda gas phase vent pipe connecting the primary reactor containment vessel and the air bag,wherein a pressure in the air bag is equalized with a pressure in the secondary containment vessel at a time when expansion of the air bag is finished, thereby passively preventing leakage of the high-pressure gas from the primary containment vessel and the air bag. 2. The reactor containment vessel according to claim 1, whereinan isolation-communication switching unit is attached to the gas phase vent pipe. 3. The reactor containment vessel according to claim 1, whereinthe primary reactor containment vessel includes a dry well containing the reactor pressure vessel and a wet well containing a suppression pool at its lower part and a gas phase at its upper part,the secondary reactor containment vessel is disposed above the primary reactor containment vessel, andthe gas phase vent pipe connects the air bag and a gas phase of the wet well. 4. The reactor containment vessel according to claim 3, whereinthe secondary reactor containment vessel includes an operation floor and is separated from the primary reactor containment vessel by a primary reactor containment vessel upper lid. 5. The reactor containment vessel according to claim 3, whereina pool storing cooling water is installed inside the secondary reactor containment vessel, and a water surface of the cooling water in the pool is exposed to an atmosphere in the secondary reactor containment vessel,a strainer is installed inside the pool, and a drain pipe is guided to inside of the dry well by the strainer,a leading end of the drain pipe is opened in a lower dry well disposed below a vessel support for supporting the reactor pressure vessel in the dry well through a check valve, a U-shape seal, and an injection valve, and,when the air bag is expanded to cause pressure in the secondary reactor containment vessel to rise, the atmosphere in the secondary reactor containment vessel pressurizes the water surface of the cooling water in the pool to allow the cooling water to be guided to the lower dry well. 6. The reactor containment vessel according to claim 5, whereina core catcher is installed on a floor of the lower dry well, and,when the air bag is expanded to cause the pressure in the secondary reactor containment vessel to rise, the atmosphere in the secondary reactor containment vessel pressurizes the water surface of the cooling water in the pool to allow the cooling water to be guided to the lower dry well so as to flood core debris dropped in the core catcher for cooling. 7. A nuclear power plant comprising:a reactor pressure vessel incorporating core fuel;a primary reactor containment vessel containing a reactor pressure vessel and having a certain pressure resistance higher than atmospheric pressure and a certain leak-tightness;a secondary reactor containment vessel installed outside of the primary reactor containment vessel and having a pressure resistance and a leak- tightness equivalent to the pressure resistance and the leak-tightness of the primary reactor containment vessel;an air bag installed in a folded state in the secondary reactor containment vessel, and upon an occurrence of an accident in the primary reactor containment vessel, the air bag is configured toreceive high-pressure gas released from the primary reactor containment vessel, andexpand from the folded state to an expanded state while confining the gas therein; anda gas phase vent pipe connecting the primary reactor containment vessel and the air bag;wherein a pressure in the air bag is equalized with a pressure in the secondary containment vessel at a time when expansion of the air bag is finished, thereby passively preventing leakage of the high-pressure gas from the primary containment vessel and the air bag. 8. The reactor containment vessel according to claim 1, wherein the air bag is comprised of a plastic material. 9. The nuclear power plant according to claim 7, wherein the air bag is comprised of a plastic material.