Horizontal steam generators are used in the VVER type reactors. In commercial power plants, there are 2 to 6 steam generators per reactor each steam generator (vertical design) can measure up to 70 feet (~21m) in height and weigh as much as 800 tons. Design with tube bundle arranged vertically and design with tube bundle arranged horizontally. There are two designes for U-tubes steam generators. The tubes carry the pressurized primary coolant and are surrounded by the secondary water, which is turned into steam. This alternative gives acope for thermal expansion by using U-tubes welded to a single tubesheet. To eliminate the loads exerted on the tubesheets by differential thermal expansion between outside shell and the tubes, a second solutions is often employed. This is the “once-through” type of steam generator. In the first of these, the secondary water flows through straight tubes welded to tubesheets at both ends. Where the coolant is pressurized water, two solutions have been adopted. Each steam generator can contain anywhere from 3,000 to 16,000 tubes, each about 19mm diameter.While the secondary fluid is always water, the reactor coolant (carbon dioxide, sodium, helium) depends on the reactor type. To increase the amount of heat transferred and the power generated, the heat exchange surface must be maximalized. Liquid metal cooled reactors such as the Russian BN-600 reactor also use heat exchangers between a secondary sodium circuit and a tertiary water circuit. The boiling water reactor does not require steam generators since the water boils directly in the reactor core. In other types of reactors, such as the pressurised heavy water reactors of the CANDU design, the primary fluid is heavy water. They are used in the most nuclear power plants, but there are many types according to the reactor type. Steam generators are heat exchangers used to convert feedwater into steam from heat produced in a nuclear reactor core.
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