plants provide about 17 percent of the world's electricity.
Some countries depend more on nuclear power for electricity
than others. In France, for instance, about 75 percent of the
electricity is generated from nuclear power, according to the
International Atomic Energy Agency. In the United States,
nuclear power supplies about 15 percent of the electricity
overall, but some states get more power from nuclear plants
than others. There are more than 400 nuclear power plants
around the world, with more than 100 in the United States.
Have you ever wondered about how a nuclear power plant
works or how safe nuclear power is? In this edition of How Stuff
Works, we will examine how a nuclear reactor and a
plant work. We'll explain nuclear fission and give you a view
inside a nuclear reactor.
The dome-shaped containment building at the
Shearon Harris Nuclear Power Plant near Raleigh, N.C.,
dwarfs a large truck parked nearby.
Nuclear Fission Uranium is a fairly common
element on Earth and was incorporated into the planet during
the planet's formation. Uranium is originally formed in stars.
Old stars explode, and the dust from these shattered stars
aggregated together to form our planet. Uranium-238 has an
extremely long half-life (4.5 billion years) and, therefore,
is still present in fairly large quantities. U-238 makes up 99
percent of the uranium on the planet. Uranium-235 makes up
about 0.7 percent of the remaining uranium found naturally,
while uranium-234 is even more rare and is formed by the decay
of uranium-238. (Uranium-238 goes through many stages or alpha
and beta decay to form a stable isotope of lead, and U-234 is
one link in that chain.)
Uranium-235 has an interesting property that makes it
useful for both nuclear power production and for nuclear
bomb production. U-235 decays naturally, just as U-238
does, by alpha radiation. U-235 also undergoes spontaneous
fission a small percentage of the time. However, U-235 is one
of the few materials that can undergo induced fission.
If a free neutron runs into a U-235 nucleus, the nucleus will
absorb the neutron without hesitation, become unstable and
split immediately. See How
Nuclear Radiation Works for complete details.
Figure 1 shows a uranium-235 nucleus with a neutron
approaching from the top. As soon as the nucleus captures the
neutron, it splits into two lighter atoms and throws off two
or three new neutrons (the number of ejected neutrons depends
on how the U-235 atom happens to split). The two new atoms
then emit gamma radiation as they settle into their new
states. There are three things about this induced fission
process that make it interesting:
The probability of a U-235 atom capturing a neutron as
it passes by is fairly high. In a reactor working properly
(known as the critical state), one neutron ejected
from each fission causes another fission to occur.
The process of capturing the neutron and splitting
happens very quickly, on the order of picoseconds (1 x
An incredible amount of energy is released, in the form
of heat and gamma radiation, when a single atom splits. The
two atoms that result from the fission later release beta
radiation and gamma radiation of their own as well. The
energy released by a single fission comes from the fact that
the fission products and the neutrons, together, weigh less
than the original U-235 atom. The difference in weight is
converted directly to energy at a rate governed by the
equation E = mc2. Something
on the order of 200 MeV (million electron volts) is released
by the decay of one U-235 atom (if you would like to convert
that into something useful, consider that 1 eV is equal to
1.602 x 10-12 ergs, 1 x
107 ergs is equal to 1
joule, 1 joule equals 1 watt-second, and 1 BTU equals 1,055
joules). That may not seem like much, but there are a lot of
uranium atoms in a pound of uranium. So many, in fact, that
a pound of highly enriched uranium as used to power a
nuclear submarine or nuclear aircraft carrier is equal to
something on the order of a million gallons of gasoline.
When you consider that a pound of uranium is smaller than a
baseball and a million gallons of gasoline would fill a cube
50 feet per side (50 feet is as tall as a five-story
building), you can get an idea of the amount of energy
available in just a little bit of U-235!
for these properties of U-235 to work, a sample of uranium
must be enriched , so that it contains two to three
percent or more of uranium-235. Three percent enrichment is
sufficient for use in a civilian nuclear reactor used for
power generation. Weapons-grade uranium is composed of 90
percent or more U-235.
Inside a Nuclear Power Plant To build a
nuclear reactor, what you need is some mildly enriched
uranium. Typically, the uranium is formed into pellets with
approximately the same diameter as a dime and a length of an
inch or so. The pellets are arranged into long rods, and the
rods are collected together into bundles. The bundles are then
typically submerged in water inside a pressure vessel. The
water acts as a coolant. In order for the reactor to work, the
bundle, submerged in water, must be slightly supercritical.
That would mean that, left to its own devices, the uranium
would eventually overheat and melt.
To prevent this, control rods made of a material
that absorbs neutrons are inserted into the bundle using a
mechanism that can raise or lower the control rods. Raising
and lowering the control rods allow operators to control the
rate of the nuclear reaction. When an operator wants the
uranium core to produce more heat, the rods are raised out of
the uranium bundle. To create less heat, the rods are lowered
into the uranium bundle. The rods can also be lowered
completely into the uranium bundle to shut the reactor down in
the case of an accident or to change the fuel.
The uranium bundle acts as an extremely high-energy source
of heat. It heats the water and turns it to steam. The steam
drives a steam turbine, which spins a generator to produce
power. In some reactors, the steam from the reactor goes
through a secondary, intermediate heat exchanger to convert
another loop of water to steam, which drives the turbine. The
advantage to this design is that the radioactive water/steam
never contacts the turbine. Also, in some reactors, the
coolant fluid in contact with the reactor core is gas (carbon
dioxide) or liquid metal (sodium, potassium); these types of
reactors allow the core to be operated at higher temperatures.
Once you get past the reactor itself, there is very little
difference between a nuclear power plant and a coal-fired or
oil-fired power plant except for the source of the heat used
to create steam.
Electricity for homes and businesses comes
from this generator at the Shearon Harris plant. It
produces 870 megawatts.
Pipes carry steam to power the generator at
the power plant.
The reactor's pressure vessel is typically housed inside a
concrete liner that acts as a radiation shield. That liner is
housed within a much larger steel containment vessel. This
vessel contains the reactor core as well the hardware (cranes,
etc.) that allows workers at the plant to refuel and maintain
the reactor. The steel containment vessel is intended to
prevent leakage of any radioactive gases or fluids from the
plant. Finally, the containment vessel is protected by an
outer concrete building that is strong enough to survive such
things as crashing jet airliners. These secondary containment
structures are necessary to prevent the escape of
radiation/radioactive steam in the event of an accident such
as that at Three Mile Island. The absence of secondary
containment structures in Russian nuclear power plants allowed
radioactive material to escape in an accident at Chernobyl.
Steam rises from the cooling tower at the
Workers in the control room at the nuclear
power plant can keep an eye on the nuclear reactor and
take action if something goes wrong.
Uranium-235 is not the only possible fuel for a power
plant. Another fissionable material is plutonium-239.
Plutonium-239 can be created easily by bombarding U-238 with
neutrons -- something that happens all the time in a nuclear
When a U-235 atom splits, it gives off two or three
neutrons (depending on the way the atom splits). If
there are no other U-235 atoms in the area, then those
free neutrons fly off into space as neutron rays. If the
U-235 atom is part of a mass of uranium so that there
are other U-235 atoms nearby, then one of three things
If, on average, exactly one of the free neutrons
from each fission hits another U-235 nucleus and
causes it to split, then the mass of uranium is said
to be critical. The mass will exist at a stable
temperature. A nuclear reactor must be maintained in a
If, on average, less than one of the free neutrons
hits another U-235 atom, then the mass is
subcritical. Eventually, induced fission will
end in the mass.
If, on average, more than one of the free neutrons
hits another U-235 atom, then the mass is
supercritical. It will heat up. For a nuclear
bomb, the bomb's designer wants the mass of uranium to
be very supercritical so that all of the U-235 atoms
in the mass split in a microsecond. In a nuclear
reactor, the reactor core needs to be slightly
supercritical so that plant operators can raise and
lower the temperature of the reactor. The control rods
give the operators a way to absorb free neutrons so
that the reactor can be maintained at a critical
The amount of uranium-235 in the mass (the level of
enrichment) and the shape of the mass control the
criticality of the sample. You can imagine that if the
shape of the mass is a very thin sheet, most of the free
neutrons will fly off into space rather than hitting
other U-235 atoms. A sphere is the optimal shape. The
amount of uranium-235 that you must collect together in
a sphere to get a critical reaction is about two pounds.
This amount is therefore referred to as the critical
mass. For plutonium-239 the critical mass is about
What Can Go
Wrong Well-constructed nuclear power plants have an
important advantage when it comes to electrical power
generation -- they are extremely clean. Compared with a
coal-fired power plant, nuclear power plants are a dream come
true from an environmental standpoint. A coal-fired power
plant actually releases more radioactivity into the atmosphere
than a properly-functioning nuclear power plant. Coal-fired
plants also release tons of carbon, sulfur and other elements
into the atmosphere (see Question
481 for details).
Unfortunately, there are significant problems with nuclear
Mining and purifying uranium has not, historically, been
a very clean process
Improperly-functioning nuclear power plants can create
big problems. The Chernobyl disaster is the best recent
example. Chernobyl was poorly designed and improperly
operated, but it dramatically shows the worst-case scenario.
Chernobyl scattered tons of radioactive dust into the
Spent fuel from nuclear power plants is toxic for
centuries and, as yet, there is no safe permanent storage
facility for it.
Transporting nuclear fuel to and from plants poses some
risk, although to date, the safety record in the U.S. has
These problems, at least in the U.S.,
have largely derailed the creation of new nulear power plants.
Society seems to have decided that the risks outweigh the