Patent Number: 
Section: claims

1. A pressurized water reactor plant comprising:a single reactor vessel housing a reactor core cooled by a high-pressure primary coolant;a single steam generator that generates steam of a secondary coolant; the steam generator having:a plurality of tubes in which the primary coolant heated in the reactor core and discharged outside the reactor vessel is circulated, andone barrel portion housing the plurality of tubes and configured so as to guide the secondary coolant, which is lower in pressure than the primary coolant and higher in pressure than atmospheric pressure around the plurality of tubes;a hot leg pipe that guides the primary coolant heated in the reactor core from the reactor vessel to the plurality of tubes of the steam generator;at least two cold leg pipes configured to return the primary coolant that has been passed through the plurality of tubes of the steam generator to the reactor vessel;at least two reactor coolant pumps that feed the primary coolant in the at least two cold leg pipes to the reactor vessel;a pressurizer provided so as to communicate with a reactor pressure boundary in which the primary coolant flows, having a liquid surface therein, and pressurizing the reactor pressure boundary;a containment vessel containing the reactor vessel, the steam generator, the hot leg pipe, the at least two cold leg pipes, the at least two reactor coolant pumps, and the pressurizer; anda primary system depressurization device that equalizes a pressure of a primary system where the primary coolant flows to a pressure of a secondary system where the secondary coolant flows, when an accident has occurred in which a part of the plurality of tubes in the steam generator is ruptured. 2. The pressurized water reactor plant according to claim 1, wherein the primary system depressurization device has:a cooling water pool for storing cooling water;a heat exchanger submerged in the cooling water pool;a steam supply piping that connects a gas phase of the pressurizer above a liquid surface of the primary coolant to the heat exchanger and has parallel branching pipings;a steam supply valve disposed on one of the parallel branching pipings of the steam supply piping;a depressurization valve disposed on the other one of the parallel branching pipings of the steam supply piping;a condensate return piping that communicates with the heat exchanger and extends to a liquid phase of the reactor pressure boundary; andan injection valve disposed on the condensate return piping. 3. The pressurized water reactor plant according to claim 2, further comprising:a steam generator tube rupture accident detector configured to detect at least one of pressure, water level and radioactivity level of an area of the steam generator where the secondary coolant flows to generate a steam generator tube rupture accident occurrence signal; anda passive cooling and depressurization system automatic activation device that automatically activates the passive cooling and depressurization system in response to the steam generator tube rupture accident occurrence signal. 4. The pressurized water reactor plant according to claim 2, wherein the cooling water pool is disposed inside the containment vessel. 5. The pressurized water reactor plant according to claim 2, wherein the cooling water pool is disposed outside the containment vessel. 6. The pressurized water reactor plant according to claim 1, wherein the containment vessel is made of steel, and a wall thereof is configured so as to be cooled by natural convection of external air. 7. The pressurized water reactor plant according to claim 1, wherein the containment vessel is made of steel, and the pressurized water reactor plant further comprises:an upper pool disposed in an upper portion of the containment vessel, the upper pool storing cooling water; anda water discharge device that makes the cooling water in the upper pool flow down along the wall of the containment vessel by gravity.