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A B
FIGURE 13.17 (A) These are uranium oxide fuel pellets that are stacked inside fuel rods, which are then locked together in a fuel rod
assembly. (B) A fuel rod assembly. See also Figure 13.20, which shows a fuel rod assembly being loaded into a reactor.

within the fuel rod assemblies slows or increases the chain reac- explosion. In a pressurized water reactor, the energy released is
tion by varying the amount of neutrons absorbed. When they carried away from the reactor by pressurized water in a closed
are lowered completely into the assembly, enough neutrons are pipe called the primary loop (Figure 13.18). The water is pres-
2
absorbed to stop the chain reaction. surized at about 150 atmospheres (about 2,200 lb/in ) to keep it
It is physically impossible for the low-concentration fuel pel- from boiling, since its temperature may be 350°C (about 660°F).
lets to form a supercritical mass. A nuclear reactor in a power In the pressurized light-water (ordinary water) reactor, the
plant can only release energy at a comparatively slow rate, and circulating pressurized water acts as a coolant, carrying heat
it is impossible for a nuclear power plant to produce a nuclear away from the reactor. The water also acts as a moderator, a

Containment building

Containment spray

Safety injection Turbine building
tank
Steam
generator
Pressurizer

Low-pressure safety Control Generator
injection pump rods
Auxiliary building Reactor Turbine Transformer
Refueling Condenser Cooling
water tower
tank

Reactor Condensate
core pump

Main Condensate
feedwater storage tank
High-pressure Containment Containment Reactor pump
safety injection spray pump sump coolant pump
pump Primary loop
FIGURE 13.18 A schematic general system diagram of a pressurized water nuclear power plant, not to scale. The containment building
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is designed to withstand an internal temperature of 149°C (300°F) at a pressure of 4 atmospheres (60 lb/in ) and still maintain its leak-tight
integrity.

340 CHAPTER 13 Nuclear Reactions 13-18