In a PWR the steam generator is the interface between the reactor coolant, which will be active, and the steam, which drives the main turbines. Its duty is to transfer heat from the reactor coolant water to feedwater from the turbine condensate system and to convert it to nearly dry steam. Each loop of the reactor coolant system contains a vertically mounted U-tube steam generator, as shown in Fig. 10.7. The steam generator consists of three sections, a hemispherical bottom head carrying

the primary coolant inlet and outlet nozzles, an evaporator section enclosing the U-tube bundle, and an upper section enclosing the moisture separators.

The reactor coolant flows through the inverted U-tubes, entering and leaving through the nozzles located in the hemispherical bottom head. The head is divided into inlet and outlet chambers by a vertical partition plate. The Inconel heat transfer tubes are hydraulically expanded into the forged tube sheet and the ends are seal welded to the tube-sheet cladding. The tubes are supported at intervals by stainless steel horizontal support plates, which have clearance holes to permit flow of the steam/water mixture.

Feedwater flows directly from the feedwater distributor ring down through the annular space between the shell and the cylinder surrounding the tube bundle. It then passes in to the bottom of the tube bundle where boiling commences. The resultant water/steam mixture flows upward through the tube bundle and into the upper section. A set of centrifugal moisture separators located above the tube bundle removes most of the entrained water, which passes to the annular space to mix with the incoming feedwater. Steam driers above the moisture separators increase the steam quality to a minimum of 99.75% (i. e. the steam contains not more than 0.25% water). The dried steam passes through a nozzle in the upper dome to one of the four main steam pipes.