Patent Number: 
Section: claims

1. A containment vessel comprising:a horizontally-extending base mat supporting a load of a boiling water reactor pressure vessel accommodating a core;an inner shell disposed on the base mat so as to gas-tightly cover the boiling water reactor pressure vessel; andan outer shell disposed on the base mat so as to horizontally cover only an outer periphery of the inner shell in a gas-tight manner, the outer shell not covering the whole inner shell,the inner shell including:a first cylindrical side wall having a lower end connected to the base mat and an upper end located higher than at least an upper end of the core and horizontally surrounding a periphery of the boiling water reactor pressure vessel;a containment vessel head covering an upper portion of the boiling water reactor pressure vessel and being formed of a steel so as to be capable of being removed upon refueling;a first top slab gas-tightly connecting a periphery of the containment vessel head and an upper end portion of the first cylindrical side wall;a dry well constituting a part of the first cylindrical side wall and accommodating the boiling water reactor pressure vessel;a wet well constituting a part of the first cylindrical side wall;a suppression pool accommodated in the wet well; anda vent pipe connecting the dry well to the suppression pool, the outer shell including:a second cylindrical side wall having a lower end connected to the base mat and surrounding an outer periphery of the first cylindrical side wall in a gas-tight manner, a top of the second cylindrical side wall being not higher than a top of the first top slab;a second top slab gas-tightly connecting an upper end of the second cylindrical side wall and the inner shell, the second cylindrical side wall not extending above the second top slab; andan outer well which is a space gas-tightly surrounded by the second cylindrical side wall, the second top slab, and the base mat, an atmosphere in the outer well being replaced by nitrogen to make an oxygen concentration lower than ordinary air, whereinthe second cylindrical side wall and the second top slab form a pressure boundary. 2. The containment vessel according to claim 1, further comprising:a gas-phase vent pipe connecting a gas-phase portion of the wet well and the outer well; andan isolation and connection switching system mounted to the gas-phase vent pipe and configured to be closed during reactor normal operation and be opened upon occurrence of a reactor accident. 3. The containment vessel according to claim 1, whereinatmosphere in the dry well and the wet well and atmosphere in at least some space in the outer well are replaced by nitrogen to make an oxygen concentration lower than air during reactor normal operation. 4. The containment vessel according to claim 3, whereina part of the outer well is partitioned to form an equipment room with air atmosphere, andatmosphere in the outer well excluding the equipment room during reactor normal operation is replaced by nitrogen to make an oxygen concentration lower than normal air. 5. The containment vessel according to claim 1, whereinan outer pool in which pool water is pooled is provided in lower part of the outer well. 6. The containment vessel according to claim 5, further comprising a gas-phase vent pipe connecting a gas-phase portion of the wet well and the outer well, wherein a leading end of the gas-phase vent pipe is disposed in the pool water of the outer pool. 7. The containment vessel according to claim 6, whereina scrubbing nozzle is attached to a leading end of the gas-phase vent pipe. 8. The containment vessel according to claim 5, whereina medication that increases a dissolving property of radioactive iodine is mixed in the pool water of the outer pool. 9. The containment vessel according to claim 5, whereinnon-radioactive iodine is mixed in the pool water of the outer pool. 10. A nuclear power plant comprising:a core;a boiling water reactor pressure vessel accommodating the core; anda containment vessel including:a horizontally-extending base mat supporting a load of the boiling water reactor pressure vessel accommodating the core;an inner shell disposed on the base mat so as to gas-tightly cover the boiling water reactor pressure vessel; andan outer shell disposed on the base mat so as to horizontally cover only an outer periphery of the inner shell in a gas-tight manner, the outer shell not covering the whole inner shell,the inner shell including:a first cylindrical side wall having a lower end connected to the base mat and an upper end located higher than at least an upper end of the core and horizontally surrounding a periphery of the boiling water reactor pressure vessel;a containment vessel head covering an upper portion of the boiling water reactor pressure vessel and being formed of a steel so as to be capable of being removed upon refueling;a first top slab gas-tightly connecting a periphery of the containment vessel head and an upper end portion of the first cylindrical side wall;a dry well constituting a part of the first cylindrical side wall and accommodating the boiling water reactor pressure vessel;a wet well constituting a part of the first cylindrical side wall;a suppression pool accommodated in the wet well; anda vent pipe connecting the dry well to the suppression pool, the outer shell including:a second cylindrical side wall having a lower end connected to the base mat and surrounding an outer periphery of the first cylindrical side wall in a gas-tight manner, a top of the second cylindrical side wall being not higher than a top of the first top slab;a second top slab gas-tightly connecting an upper end of the second cylindrical side wall and the inner shell, the second cylindrical side wall not extending above the second top slab; andan outer well which is a space gas-tightly surrounded by the second cylindrical side wall, the second top slab, and the base mat, an atmosphere in the outer well being replaced by nitrogen to make an oxygen concentration lower than ordinary air, whereinthe second cylindrical side wall and the second top slab form a pressure boundary. 11. The nuclear power plant according to claim 10, whereina fuel pool is arranged above the first and second top slabs. 12. The nuclear power plant according to claim 10, whereinan upper protective barrier covering an upper portion of the containment vessel is provided.