Patent Number: 
Section: claims

1. An emergency core cooling system of a boiling water nuclear plant, comprising:four active safety divisions exclusively used for an active emergency core cooling system, each of the active safety divisions including only one motor-driven low pressure core cooling system so as to reduce a volume of a building housing the low pressure core cooling system; andone or more passive safety divisions including a passive safety system that does not require motor drive, whereina number of the active safety divisions is larger by two or more than a number required upon occurrence of a design basis accident,each of the active safety divisions includes an emergency power source for supplying electric power to the motor-driven low pressure core cooling system, andthe passive safety system is configured to cool a core of the boiling water nuclear plant for at least 8 hours during an accident without being replenished with cooling water from outside. 2. The emergency core cooling system according to claim 1, whereina number of the passive safety divisions is one,the only one motor-driven low pressure core cooling system of each of the active safety divisions exclusively used for the active emergency core cooling system is a low pressure core cooling system that is also used as a residual heat removal system,the low pressure core cooling system has at least 100% of an injection capacity required for cooling the core upon occurrence of at least the design basis accident in a state where a reactor pressure is low,the residual heat removal system has at least 50% of a heat removal capacity required for cooling the core and a containment vessel upon occurrence of the design basis accident, andthe passive safety division includes at least an isolation condenser. 3. The emergency core cooling system according to claim 1, whereinthe emergency power source of each of the active safety divisions is an emergency diesel generator. 4. The emergency core cooling system according to claim 1, further comprisingat least one auxiliary power source configured to selectively supply power to any one of the active safety divisions. 5. The emergency core cooling system according to claim 4, whereinthe auxiliary power source is a gas turbine generator. 6. The emergency core cooling system according to claim 1, further comprising:an auxiliary feed water system including one or more turbine-driven reactor core isolation cooling systems driven by main steam supplied from the boiling water reactor. 7. The emergency core cooling system according to claim 1, whereinthe passive safety division includes a passive containment cooling system. 8. The emergency core cooling system according to claim 7, whereinthe passive safety division includes a gravity-driven cooling system. 9. The emergency core cooling system according to claim 7, whereinat least one of the active safety divisions includes a direct current power source and an equalized pressure flooder system driven by the direct current power source. 10. The emergency core cooling system according to claim 1, whereinthe passive safety system includes at least one isolation condenser that holds a predetermined volume of cooling water, the predetermined volume configured to cool the core of the boiling water nuclear plant for at least 8 hours during the accident without being replenished with cooling water from outside. 11. A boiling water nuclear plant provided with an emergency core cooling system, the emergency core cooling system including:four active safety divisions exclusively used for an active emergency core cooling system, each of the active safety divisions including only one motor-driven low pressure core cooling system so as to reduce a volume of a building housing the low pressure core cooling system; andone or more passive safety divisions including a passive safety system that does not require motor drive, whereina number of the active safety divisions is larger by two or more than a number required upon occurrence of a design basis accident,each of the active safety divisions includes an emergency power source for supplying electric power to the motor-driven low pressure active safety system, andthe passive safety system is configured to cool a core of the boiling water nuclear plant for at least 8 hours during an accident without being replenished with cooling water from outside.