- 81 - The primary system comprises cooling loops (three loops for a 900 MWe reactor, four loops for a 1,300 MWe, 1,450 MWe, or EPR reactor), the role of which is to extract the heat released in the core by circulating pressurised water, known as the primary water. Each loop, connected to the reactor vessel containing the core, comprises a circulating, or primary pump, and a steam generator (SG). The primary water, heated to more than 300 °C, is kept at a pressure of 155 bar by the pressuriser, to prevent it boiling. The entire primary system is located inside the containment. The primary system water transfers the heat to the water in the secondary systems, via the steam generators. The steam generators are heat exchangers containing thousands of tubes through which the primary water circulates. These tubes are immersed in the water of the secondary system and heat it to boiling point without ever coming into contact with the primary water. Each secondary system consists essentially of a closed loop through which water runs in liquid form in one part and as steam in the other part. The steam produced in the steam generators is partly expanded in a high-pressure turbine and then passes through moisture separator-reheaters before final expansion in the low-pressure turbines, from which it is then routed to the condenser. The condensed water is then heated and sent back to the steam generators by the extraction pumps relayed by feed pumps through reheaters. The cooling systems The purpose of the cooling systems is to condense the steam coming from the secondary system turbine. To do this the condenser is comprised a heat exchanger containing thousands of tubes in which cold water pumped from an outside source (river, sea) circulates. When the steam comes into contact with the tubes it condenses and can be returned in liquid form to the steam generators. The cooling system water heated in the condenser is then discharged to the natural environment (open circuit) or, when the river flow is too low or the heating too great in relation to the sensitivity of the environment, cooled in a cooling tower (closed or semi-closed circuit). The reactor containment building The PWR reactor containment building fulfils two functions: protection of the reactor against external hazards; containment, thereby protecting the public and the environment against radioactive products likely to be dispersed outside the primary system in the event of an accident. The containments are therefore designed to withstand the pressures and temperatures that could be reached in an accident situation, and offer sufficient leaktightness in such conditions. The schematic diagram below shows the containment building of a 1300 MWe reactor:
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