Botkin Environmental Science Earth as Living Planet 8th txtbk

17.2 What Is Nuclear Energy? 349
Chain reaction
Neutron
U 235 nucleus
Fission
fragment
Energy
released
(heat)
FIGURE 17.4 Fission of uranium-235. A neutron strikes the
U-235 nucleus, producing fission fragments and free neutrons and
releasing heat. The released neutrons may then strike other U-235
atoms, releasing more neutrons, fission fragments, and energy. As
the process continues, a chain reaction develops.
coolant through the reactor). The heat is used to boil water,
releasing steam that runs conventional steam-turbine
electrical generators (Figure 17.5). In most common
reactors, ordinary water is used as the coolant as well as
the moderator. Reactors that use ordinary water are called
“light water reactors” because there is also “heavy water,”
which combines deuterium with oxygen. 6
Most reactors now in use consume more fissionable
material than they produce and are known as burner
reactors. Figure 17.6 shows the main components of
a reactor: the core (consisting of fuel and moderator),
control rods, coolant, and reactor vessel. The core is enclosed
in the heavy, stainless-steel reactor vessel; then, for
safety and security, the entire reactor is contained in a
reinforced-concrete building.
In the reactor core, fuel pins—enriched uranium
pellets in hollow tubes (3–4 m long and less than 1 cm,
or 0.4 in., in diameter)—are packed together (40,000
or more in a reactor) in fuel subassemblies. A minimum
fuel concentration is necessary to keep the reactor critical—that
is, to achieve a self-sustaining chain reaction.
Electricity
Turbine
Generator
Air
Boiler
Heat
Pump
Condenser
Condenser
cooling water
(a)
Primary
concrete
shield
Steel lining
Steam
generator
Steam to
generator
Turbine
Generator
Electricity
Reactor
Coolant
Pump
Pump
Condenser
Cooling water
Condenser
(b)
Concrete containment
structure (shield)
FIGURE 17.5 Comparison of (a) a fossil-fuel power plant and (b) a nuclear power plant with a boiling-water
reactor. Notice that the nuclear reactor has exactly the same function as the boiler in the fossil-fuel power plant.
The coal-burning plant (a) is Ratcliffe-on-Saw, in Nottinghamshire, England, and the nuclear power station (b) is
in Leibstadt, Switzerland. (Source: American Nuclear Society, Nuclear Power and the Environment, 1973.)