What
is Nuclear Energy?
Nuclear energy is energy that comes form the nucleus
(core) of an atom. Atoms are the particles that make up
all objects in the universe. Atoms consist of neutrons,
protons, and electrons.
Nuclear energy is released from an atom through one of
two processes: nuclear fission. In nuclear fusion, energy
is released when the nuclei of atoms are combined or
fused together. This is how the sun produces energy. In
nuclear fission, energy is released when the nuclei of
atoms are split apart. Nuclear fission is the only method
currently used by nuclear plants to generate electricity.
What is Uranium?
The fuel by nuclear power plants for fissioning is
uranium. Uranium is the heaviest of the 92 naturally
occurring elements and is classified as a metal. It is
also one of the few elements that are easily fissioned.
Uranium was formed when the earth was created and is
found in rocks all over the world. Rocks that contain a
lot of uranium are called uranium ore, or pitchblende.
Uranium, although abundant, is a nonrenewable energy
source.
Two forms (isotopes) or uranium are found in nature,
uranium-235 and uranium-238. These numbers refer to the
number of neutrons and protons in each atom.
Uranium-235 is the form commonly used for energy
production because, unlike uranium-238, its nucleus
splits easily when bombarded by a neutron, causing its
nucleus to split apart into two atoms of lighter weight.
At the same time, the fission reaction releases energy as
heat and radiation, as well as releasing more neutrons.
The newly released neutrons go on to bombard other
uranium atoms, and the process repeats itself over and
over. This is called a chain reaction.
The Nuclear Fuel Cycle
The steps-from mining the uranium ore, through its use in
a nuclear reactor, to its disposal-is called the nuclear
fuel cycle.
Mining-The first step in the cycle is mining the uranium
ore. Workers mine uranium ore much like coal miner mine
coal-in deep underground mines or in open-pit surface
mines. A tone of uranium ore in the United States
typically contains three to ten pounds of uranium.
Milling-After it has been mined, uranium or is crushed.
The crushed ore is usually mixed with an acid, which
dissolves the uranium, but not the rest of the crushed
rock. The acid solution is drained off and dried, leaving
a yellow powder called yellowcake, consisting mostly of
uranium. This process of removing uranium form the ore is
called uranium milling.
Conversion-The next step in the cycle is the conversion
of the yellowcake into a gas called uranium hexafluoride,
or UF6. The uranium hexafluoride is then shipped to a
gaseous diffusion plant for enrichment.
Enrichment-Because less then one percent of uranium ore
contains uranium-235, the form used for energy
production, uranium must be processed to increase the
concentration of uranium-235. This process-called
enrichment-increases the percentage of uranium-235 from
one to five percent. It typically takes place at a
gaseous diffusion plant where the uranium hexafluoride is
pumped through filters that contain very tiny holes.
Because uranium-235 has three fewer neutrons and it one
percent lighter than uranium-238, it moves through the
holes more easily then uranium-238. This method increases
the percentage of uranium-235 as the gas passes through
thousands of filters.
Fuel Fabrication-The enriched uranium is taken to a fuel
fabrication plant where it is prepared fro the nuclear
reactor. Here, the uranium is made into a solid ceramic
material and formed into small barrel-shaped pellets.
These ceramic fuel pellets can withstand very high
temperatures, just like the ceramic tiles on the space
shuttle. Fuel pellets are about the size of your
fingertip, yet each one can produce as much energy as 120
gallons of oil. The pellets are sealed in 12-foot metal
tubes called fuel rods. Finally, the fuel rods are
bundled into groups called fuel assemblies.
Nuclear Reactor-The uranium fuel is now ready for use in
a nuclear reactor. Fissioning takes place in the reactor
core. Surrounding the core of the reactor is a shell
called the reactor pressure vessel. To prevent heat or
radiation leaks, the reactor core and the vessel are
housed in an airtight containment building made of steel
and concrete several feet thick. The reactor core houses
about 200 fuel assemblies. Spaced between the fuel
assemblies are movable control rods. Control rods absorb
neutrons and slow down the nuclear reaction. Water also
flows through the fuel assemblies and control rods to
remove some of the heat from the chain reaction. The
nuclear reaction generates heat energy just as burning
coal or oil generates heat energy. Likewise, the heat is
used to boil water into steam, which turns a turbine
generator to produce electricity. Afterward, the steam is
condensed back into water and cooled in a separate
structure called a cooling tower. This way, the water can
be used again and again.
Spent Fuel Storage-Like most industries, nuclear power
plants produce waste. One of the main concerns about
nuclear power plants is not the amount of waste created,
which is quite small compared to other industries, but
the radioactivity of some of that waste. The fission
process creates radioactive waste products. After about
three cycles, these waste products build up in the fuel
rods, making the chain reaction more difficult. Utility
companies generally replace one-third of the fuel rods
every 12 to 18 months to keep power plants in continuous
operation. The spent fuel is usually stored near the
reactor I a deep pool of water called the spent fuel
pool. During storage, the spent fuel cools down and
begins to lose most of its radioactivity through
radioactive decay. In three months, the spent fuel will
lose 50 percent of its radiation; in one year, 80
percent; in 10 years, 90 percent. The spent fuel pool is
intended as a temporary method for storing nuclear waste.
Eventually, the spent fuel will be reprocessed and/or
transported to a permanent federal disposal site.
Reprocessing-Spent fuel contains both radioactive waste
products and unused nuclear fuel. In fact, about
one-third on the nuclear fuel remains unused when the
fuel rod must be replaced. Reprocessing separates the
unused nuclear fuel from the waste products so that it
can be used in a reactor again. Currently, American
nuclear power plants store the spent fuel in spent fuel
pools-without reprocessing. Why? Mainly because
reprocessing is more expensive than making new fuel from
uranium ore. If uranium prices rise significantly or
storage becomes a bigger problem, reprocessing may gain
favor in the industry.
Waste Repository
Most scientists believe the safest way to store nuclear
waste is in rock formations deep underground-called
geological repositories. In 1982, the U.S. Congress
agreed and passed the Nuclear Waste Policy Act. This law
directed the U.S. Department of Energy to site, design,
construct, and operate America's first repository by
1998. The repository stores radioactive waste from
nuclear power plants and from defense weapons plants. The
same law also established the Nuclear Waste Fund to pay
for the repository. People who use electricity from
nuclear power plants pay 1/10 of a cent for each
kilowatt-hour of nuclear-generated electricity they use.
An average household, which uses about 7,500
kilowatt-hour a year, would contribute $7.50 a year to
the fund if it got all its electricity from nuclear
power. The nation has collected $14 billion into the fund
since 1982. In 1987, Congress passed the Nuclear Waste
Policy Amendments Act. Among others things, this act
proposed Yucca Mountain, Nevada as the nation's first
repository site. If the current plan is approved, nuclear
waste will be sealed in steel canisters and stored in
vaults located 1,000 feet below the surface. Current
projections are that it may open about 2010. Yucca
Mountain is being studied as a repository site because it
is dry and geologically stable (the chance of erupting
volcanoes or earth-quakes is very slim). Yucca Mountain
site is also isolated. Few people live in the area.
Although utility companies currently sore their nuclear
waste in pools of water at the power plant, some
companies will run out of storage space in the next year
or two. Utility companies are asking the Department of
Energy to accept responsibility for the waste. The
Department of Energy would need to store the waste in a
temporary facility prior to its final burial at the
repository.
Nuclear Energy Use
Nuclear energy is an important source of
electricity-second only to coal-providing almost 18
percent of the electricity in the U.S. At the end of
1997m there were 107 nuclear power plants operating in
the U.S. No new plants are planned for the future.
Worldwide, however, nuclear energy is a growing source of
electrical power. Nuclear energy now provides about 17
percent of the world's electricity. The U.S., France,
Japan, and Germany are the world leaders. France gets 75
percent of its electricity from nuclear power.
Nuclear Energy and the Environment
Nuclear power plants have very little impact on the
environment. Generating electricity from nuclear power
produces no air pollution because no fuel is burned. Most
of the water used in the cooling processes is recycles.
In the future, using nuclear energy may become an
important way to reduce the amount of carbon dioxide
produced by burning fossil fuels and biomass. Carbon
dioxide is considered the major greenhouse gas. People
are using more and more electricity. Some experts predict
that we will have to use nuclear energy to produce the
amount of electricity people need at a cost they can
afford. Whether or no we should use nuclear energy to
produce electricity has become a controversial and
sometimes highly emotional issue.
Nuclear Safety
The greatest potential risk from nuclear power plants is
the release of high-level radiation. In the United
States, plants are carefully designed to contain
radiation, and emergency plans are in place to alert and
advise nearby residents if there is an accident. Two
serious accidents have occurred since the industry began
over 30 years ago-Three Mile Island in the Unites States
(1979) and Chernobyl in the Soviet Union (1986). At Three
Mile Island, about half the uranium fuel melted when
water to the reactor core was inadvertently cut off. A
small amount of radioactive material escaped into the
surrounding area before the mistake was discovered. But
due to the safety design features of the plant-multiple
barriers contained most of the radiation-no one was
injured or died as a result of this accident. However,
the accident at Chernobyl was far more serious. It
happened when two explosions blew the top off the reactor
building. A lack of containment structures and other
design flaws caused the release of a large amount of
radioactive material into the surrounding area. More than
100,000 people were evacuated from their homes and about
200 workers were treated for radiation sickness and
burns; 31 or them died. Could a Chernobyl-type accident
occur at an American nuclear plant? Many experts say no.
Old soviet nuclear plants do not have the safety systems
and containment chambers that are standard on all
American plants. American operators are also better
trained than their Eastern European counterparts to
respond to any problems that may occur.
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An atom is the basic element of nuclear energy. Most of
the reactions that occur within the nuclear energy family
happen at the core of the atom.
Here a miner is mining a huge mountain of ore. Mining ore
is the first step in the nuclear energy feul cycle.
Here is what a nuclear power plant would look like while
cooling of the hot uranium.
The only problem with working with nuclear energy is the
fact that it is so dangerous. Shown here is a test
explosion for the very first nuclear bomb. Read the
section titled 'nuclear safety' for more information. |
