13.14. Most Westinghouse PWR systems have either three or four cool­ant loops, each with its own circulation pump and steam generator (Fig. 13.4). These pumps are normally of the single-stage, vertical centrifugal type, each driven by a 6-MW (8000-hp) motor. They are rated at about 6.2 m3/s (98,000 gpm), with a pressure developed of 7.4 x 105 Pa (108 psi). Not shown in the simplified coolant loop figure are connections to numerous auxiliary systems, such as those for safety injection, residual heat removal, and chemical and volume control.

13.15. A typical 1000-MW(el) plant steam generator design, with an overall height of about 20.7 m (68 ft) and an upper shell diameter of 4.4 m (14.5 ft), is shown in Fig. 13.5. Heated primary system coolant from the reactor vessel passes through the inverted U-shaped tubes, and satu­rated steam at 7.6 x 106 Pa(a), i. e., 1100 psia, is formed in the outer “shell” side. The upper part of the steam generator consists of a steam­drum section where centrifugal moisture separators and a steam dryer remove entrained water from the steam. The steam temperature is 291°C (556°F).

13.16. An important component of the steam-generation system is the system pressurizer which is connected to one of the primary coolant loops (see Fig. 13.5). A typical pressurizer is a cylindrical tank, about 16.2 m (53 ft) high and 2.44 m (8 ft) in diameter. During normal steady-state

operation it contains roughly 60 percent of liquid water and 40 percent steam (by volume). Electric immersion heaters in the lower part of the vessel can provide heating, whereas cold water sprayed into the steam space can provide cooling, as required. Both heaters and cooling sprays are operated by pressure signals. If, for any reason, the system pressure should drop, a low-pressure signal would actuate the heaters. The steam generated would then increase the system pressure. On the other hand, if the pressure should rise, a high-pressure signal would operate the cooling water spray. Some of the steam in the pressurizer would be condensed and the system pressure would decrease, if the pressure should increase beyond the control capability of the sprays, one or more motor-operated relief valves would open automatically and steam would be vented to a quench tank partially filled with water. Should the pressure in the quench tank exceed the design value, a rupture disc would allow the tank to vent to the containment sump. An important contribution to the Three Mile Island accident (§12.179) was the failure of the pressurizer relief valve to close

when the pressure was restored to normal. Therefore, assurances that valves would operate reliably have received a great deal of attention since that time.

13.17. Steam from the generators passes to the turbine system which consists of high-pressure and low-pressure stages on the same shaft. Par­tially expanded steam leaving the high-pressure turbine goes through moisture-separator and reheater units before further expansion in the low — pressure turbines. Steam drawn from the high-pressure section and from the steam generator serve as the heat sources for reheating. The exhaust steam from the low-pressure stage passes to the condensers, normally lo­cated under the turbines. The condensate provides the feedwater for the steam generators. Before entering the generators, however, the condensate temperature is increased in a series of feedwater heaters which use steam drawn from the turbines to heat the water.