Patent Number: 
Section: claims

1. A boiling water nuclear reactor comprising:a core;a reactor pressure vessel containing the core;a reactor containment vessel including: a dry well containing the reactor pressure vessel, and a wet well containing a pressure suppression pool;a vent pipe connecting the dry well and the pressure suppression pool;a gravity-driven water injection pool arranged at a position higher than the core to hold boric acid aqueous solution therein;an emergency core water-injection piping system for causing the boric acid aqueous solution in the gravity-driven water injection pool to fall by gravity so as to be injected into the reactor pressure vessel in case of a reactor accident;a static containment vessel cooling system pool arranged outside the reactor containment vessel;a static containment vessel cooling system heat exchanger arranged in the static containment vessel cooling system pool;a dry well connection pipe connecting an upper part of the static containment vessel cooling system heat exchanger and the dry well;a gas vent pipe for discharging noncondensible gas in the static containment vessel cooling system heat exchanger into an inside of the pressure suppression pool;an intra dry well heat exchanger arranged in the dry well;an external heat exchanger arranged outside the reactor containment vessel;a dry well cooling water pipe connecting the intra dry well heat exchanger and the external heat exchanger in order to discharge heat obtained by the intra dry well heat exchanger at the external heat exchanger; anda boric acid aqueous solution heating device configured to heat the boric acid aqueous solution in the gravity-driven water injection pool by using part of the heat obtained by the intra dry well heat exchanger. 2. The reactor according to claim 1, wherein the gravity-driven water injection pool is arranged in the dry well. 3. The reactor according to claim 2, further comprising a drain pipe for discharging condensed water in the static containment vessel cooling system heat exchanger into the gravity-driven water injection pool. 4. The reactor according to claim 1, wherein:the gravity-driven water injection pool is arranged in the reactor containment vessel and partitioned from the dry well and the wet well; andthe nuclear reactor further comprises a first communication pipe holding a gas phase section in an upper part of the gravity-driven water injection pool and a gas phase section above the pressure suppression pool in the wet well in communication with each other. 5. The reactor according to claim 4, further comprising a drain pipe for discharging condensed water in the static containment vessel cooling system heat exchanger into the gravity-driven water injection pool. 6. The reactor according to claim 1, wherein:the gravity-driven water injection pool is held gas-tight and arranged above the reactor containment vessel; andthe nuclear reactor further comprises a second communication pipe holding a gas phase section above the gravity-driven water injection pool and the dry well in communication with each other. 7. The reactor according to claim 6, further comprising a drain pipe for discharging condensed water in the static containment vessel cooling system heat exchanger into a lower part of the dry well. 8. The reactor according to claim 1, wherein the gravity-driven water injection pool is held gas-tight and arranged above the reactor containment vessel; andthe nuclear reactor further comprises a third communication pipe holding a gas phase section in an upper part of the gravity-driven water injection pool and a gas phase section above the pressure suppression pool in the wet well in communication with each other. 9. The reactor according to claim 8, further comprising a drain pipe for discharging condensed water in the static containment vessel cooling system heat exchanger into a lower part of the dry well. 10. The reactor according to claim 1, further comprising an emergency dry well water-injection piping system for injecting the boric acid aqueous solution in the gravity-driven water injection pool into a lower part of the dry well in case of a reactor accident. 11. The reactor according to claim 1, wherein the pressure suppression pool can be used to fill a fuel exchange pool with water when replacing fuel loaded in the core. 12. The reactor according to claim 1, further comprising: a boric acid aqueous solution injection system including a boric acid aqueous solution injection tank arranged outside the reactor containment vessel to store pressurized boric acid aqueous solution of a quantity less than a quantity of water in the gravity-driven water injection pool and adapted to inject boric acid aqueous solution into the core from the boric acid aqueous solution injection tank. 13. The reactor according to claim 1, further comprising: a pressure-reducing valve arranged at the reactor pressure vessel at a position above the core so as to be capable of discharging steam from an inside of the reactor pressure vessel into the dry well. 14. An emergency core cooling system of a boiling water nuclear reactor, the emergency core cooling system comprising:a gravity-driven water injection pool to hold boric acid aqueous solution therein, the gravity-driven water injection pool being arranged at a position higher than the core;an emergency core water-injection piping system for causing the boric acid aqueous solution in the gravity-driven water injection pool to fall by gravity so as to be injected into a reactor pressure vessel containing the core, in case of a reactor accident;a static containment vessel cooling system pool arranged outside a reactor containment vessel that contains the reactor pressure vessel at a position above a pressure suppression pool;a static containment vessel cooling system heat exchanger arranged in the static containment vessel cooling system pool;a dry well connection pipe connecting an upper part of the static containment vessel cooling system heat exchanger and a dry well that contains the reactor pressure vessel and that is part of the reactor containment vessel;a gas vent pipe for discharging noncondensible gas in the static containment vessel cooling system heat exchanger into an inside of the pressure suppression pool;an intra dry well heat exchanger arranged in the dry well;an external heat exchanger arranged outside the reactor containment vessel;a dry well cooling water pipe connecting the intra dry well heat exchanger and the external heat exchanger in order to discharge heat obtained by the intra dry well heat exchanger at the external heat exchanger; anda boric acid aqueous solution heating device configured to heat the boric acid aqueous solution in the gravity-driven water injection pool by using part of the heat obtained by the intra dry well heat exchanger.