By far the most common civilian reactor is the pressurized-water reactor (P^^). Reactors of this type were originally developed to drive nuclear submarines. The P^^ circuit is illustrated schematically in Figure 2.7. Water at typically 150 bars (2200 psia) is pumped into a pressure vessel, which contains the reactor core. The water passes downward through an annulus between the reactor core and the pressure vessel and then flows up over the fuel elements. It then leaves through a series of pipes, which pass to the stream generator. The light-water coolant also acts as the moderator for this reactor. The absorption of neutrons by the light water (as described in Chapter 1) necessitates a significant enrichment of the fuel to 3 2% S5U (-4.5 times the concentration in natural uranium).

In the steam generator, the hot water from the reactor passes through verti­cal U-tubes (Figure 2.9), and water at lower pressure is fed into the steam gen­erator shell and contacts the outside of the U-tubes. Steam is generated at approximately 70 bars (1000 psia) and passes from the steam generator into the

turbine and from there to the condenser, the condensate being returned to the steam generator via feed preheaters. Figure 2.7 illustrates one complete coolant loop; PWRs typically have two, three, or four such loops per reactor vessel. A typical four-loop PWR is illustrated in Figure 2.8. The fuel elements in a PWR are illustrated in Figure 2.9; the fuel is in the form of uranium oxide pellets mounted in a 12-ft-long tube made of a zirconium alloy (Zircaloy). The tubes are usually mounted in separate bundles of 17 rows of 17 tubes, with some pins omitted to allow passage of control rods into the core.

In 1993 there were 243 operating civilian PWR power reactors in the world and 33 under construction. Although the steam cycle efficiency of a PWR is rel­atively low (32%), its capital cost may be considerably less than that of an AGR. The main reason for this is the great reduction in core size made possible by the enormous increase in volumetric power density and core rating, as shown in Table 2.3. Another factor contributing to the low capital cost is the fact that much of the P^^ can be constructed off-site under factory conditions.

Because of the high rate of heat generated per unit mass of fuel (fuel ratiniJ, the response of a PWR to changes in operating conditions is much more rapid than that of an AGR. It has been argued that this is a negative safety factor. Even when the reactor is shut down, the level of decay heat is such that the fuel must always be kept covered with water. We shall discuss these safety features in Chapters 5 and 6. Pressurized-water reactors have experienced problems with steam generators, which have failed due to corrosion on the secondary (steam­generating) side. Reactors are often more susceptible to problems outside the core than in it. Although it is now believed that design improvements can pre­vent these corrosion problems, most existing reactors are still prone to them. This is not a major safety issue, but it does limit their performance.