Patent Number: 
Section: claims

1. A passive cooling and depressurization system for a pressurized water nuclear plant having a reactor pressure vessel for containing a reactor core cooled by primary coolant, a steam generator connected to the reactor pressure vessel by a hot leg pipe and a cold leg pipe, and a containment vessel containing the reactor pressure vessel, the steam generator, the hot leg pipe and the cold leg pipe, the passive cooling and depressurization system comprising:a pressurizer connected to the hot leg pipe by a riser pipe for pressurizing an inside of a reactor pressure boundary where the primary coolant flows;a cooling water pool;a heat exchanger installed in the cooling water pool including an upper header, a lower header and a heat exchanger tube;a steam supply piping extending from a gas phase of the pressurizer to the upper header of the heat exchanger;a steam supply valve equipped on the steam supply piping;a coolant return pipe extending from the heat exchanger to a liquid phase of the reactor pressure boundary; andan outlet valve equipped on the coolant return pipe,wherein the heat exchanger exchanges heat between water stored in the cooling water pool and steam supplied through the steam supply piping. 2. The passive cooling and depressurization system of claim 1, wherein the steam supply valve is a steam regulator valve that is configured to regulate a flow rate at steam through the steam supply valve. 3. The passive cooling and depressurization system of claim 1 further comprising a depressurizing valve equipped on the steam supply piping in parallel with the steam supply valve, wherein the steam supply valve allows steam of a quantity equivalent to a quantity of steam generated by decay heat of the reactor core to pass through the steam supply valve. 4. The passive cooling and depressurization system of claim 1, wherein at least part of the heat exchanger is arranged above a liquid level in the pressurizer. 5. The passive cooling and depressurization system of claim 1, wherein cooling water pool is an in-containment refueling water storage tank. 6. The passive cooling and depressurization system of claim 1, wherein the cooling water pool is arranged outside the containment vessel and a gas phase of the cooling water pool communicates with air external to the containment vessel. 7. The passive cooling and depressurization system of claim 6, wherein the cooling water pool communicates with a passive containment cooling system pool. 8. The passive cooling and depressurization system of claim 1, wherein the containment vessel is partitioned into a first space and a second space containing the reactor pressure boundary by a diaphragm, the cooling water pool is arranged in the first space, and a gas phase of the cooling water pool communicates with an external air. 9. The passive cooling and depressurization system of claim 8, wherein the cooling water pool communicates with a passive containment, cooling system pool. 10. The passive cooling and depressurization system of claim 1, further comprising a vent pipe connected to the heat exchanger for venting noncondensable gases into the containment vessel and a vent valve being equipped on the vent pipe. 11. The passive cooling and depressurization system of claim 10, wherein an end of the vent pipe opposite to an end connected to the heat exchanger is open in a pressure suppression pool water stored in a pressure suppression chamber. 12. A pressurized water nuclear plant comprising:a reactor core cooled by primary coolant;a reactor pressure vessel containing the reactor core;a steam generator connected to the reactor pressure vessel by a hot leg pipe and a cold leg pipe;a pressurizer of the pressurized water nuclear plant connected to the hot leg pipe by a riser pipe for pressurizing inside of a reactor pressure boundary where the primary coolant flows;a containment vessel containing a reactor pressure vessel, the steam generator, the hot leg pipe, the cold leg pipe, the riser pipe and the pressurizer;a cooling water pool;a heat exchanger installed in the cooling water pool, the heat exchanger including an upper header, a lower header and a heat exchanger tube;a steam supply piping extending from a gas phase of the pressurizer to the upper header of the heat exchanger;a steam supply valve equipped on the steam supply piping;a coolant return pipe extending from the heat exchanger to a liquid phase of the reactor pressure boundary; andan outlet valve equipped on the coolant return pipe,wherein the heat exchanger exchanges heat between water stored in the cooling water pool and steam supplied through the steam supply piping.