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235 U
92

Neutrons

Fission
products

FIGURE 13.15 A schematic representation of a chain reaction. Each fissioned nucleus releases neutrons, which move out to fission
other nuclei. The number of neutrons can increase quickly with each series.

mass too small to sustain a chain reaction is called a sub critical Control-rod
drives
mass. A mass of sufficiently pure uranium-235 (or plutonium-239)
that is large enough to produce a rapidly accelerating chain Control
rods
reaction is called a supercritical mass. An atomic bomb is simply
a device that uses a small, conventional explosive to push sub- Control-rod
critical masses of fissionable material into a supercritical mass. guide tubes
Fission occurs almost instantaneously in the supercritical mass,
and tremendous energy is released in a violent explosion. Coolant Coolant
inlet outlet

NUCLEAR POWER PLANTS Fuel assembly
Reactor
The nuclear part of a nuclear power plant is the nuclear reactor, core
a steel vessel in which a controlled chain reaction of fissionable Water
material releases energy (Figure 13.16). In the most popular
design, called a pressurized light-water reactor, the fissionable Pressure
vessel
material is enriched 3 percent uranium-235 and 97 percent In-core instrumentation
uranium-238 that has been fabricated in the form of small nozzles
ceramic pellets (Figure 13.17A). The pellets are encased in a long
zirconium alloy tube called a fuel rod. The fuel rods are locked FIGURE 13.16 A schematic representation of the basic
into a fuel rod assembly by locking collars, arranged to permit parts of a nuclear reactor. The largest commercial nuclear power
plant reactors are 23 to 28 cm (9 to 11 in) thick steel vessels
pressurized water to flow around each fuel rod (Figure 13.17B)
with a stainless steel liner, standing about 12 m (40 ft) high with
and to allow the insertion of control rods between the fuel rods. a diameter of about 5 m (16 ft). Such a reactor has four pumps,
Control rods are constructed of materials, such as cadmium, which move 1.67 million L (440,000 gal) of water per minute
that absorb neutrons. The lowering or raising of control rods through the primary loop.

13-17 CHAPTER 13 Nuclear Reactions 339