انرژی ها نو

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Some of the emissions from fossil fuels, such as the gas
carbon dioxide, help make the atmosphere (the air above
Earth’s surface) act somewhat like a greenhouse. It is warm
inside a greenhouse because the glass roof and walls let light in
and out, but tend to keep heat in. Scientists say that a similar
thing happens when so-called greenhouse gases such as
carbon dioxide collect in the atmosphere. These gases make
Earth’s surface get warmer. The whole planet could be affected.
Ice near the North and South Poles, for example, is beginning to

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melt, which would cause the oceans to rise. Islands and coastal
areas may become flooded. Rainfall in other places could
decrease significantly. In addition, plants and animals would
have to get used to higher temperatures. Some may die.
These drawbacks have made people start to think about
alternative energy sources that could replace fossil fuels.
Nuclear energy is one such source. Nuclear energy is not
perfect. Nuclear fuel, for example, is dangerous and has to
be handled very carefully. Despite problems associated with
nuclear power, though, nuclear energy has good points that
make it a useful energy source. Nuclear power plants have
almost no emissions. Nuclear fuel is not renewable since
it is used up to produce power, but plenty of nuclear fuel
is available, and more can be made in special reactors. In
addition, nuclear power plants can be built almost anywhere,
and they can run round-the-clock.
Other alternative energy sources have advantages of their
own, but they also have drawbacks. Water power can be
used only where there is the right type and amount of water.
Geothermal power relies on heat deep within the ground,
which means that it also can be used only in certain places.
Solar power relies on the Sun’s energy and does not work
when the Sun does not shine. Wind power works only when
the wind blows. Biofuels, such as ethanol made from plants,
need to be burned, which releases emissions. In addition, a
great deal of land is needed to grow the plants used to make
such biofuels. In the future, it is likely that the world will
continue to use a combination of different energy sources—
and some people think nuclear energy may play a bigger role
than it does today

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Splitting Atoms
for Power

Radioactive substances found in
nature give off energy in the form of
radioactivity. In some cases, they may be
a danger to health, but on the whole, this is
not a matter of concern. Take, for instance, uranium
ore. This is rock that contains uranium, so it is radioactive, but it
does not release enough energy to run a generator. Also, if you
pile up the ore, it will not become a bomb. Something more is
needed to produce electric power or cause an explosion.
Actually, at least three things are needed for what is called
“useful” fi ssion—fi ssion that can be used to make electricity
or create an explosion. One thing is the right material. Useful
fi ssion can occur only with certain materials, one of which is
uranium-235. Scientists describe such materials as fi ssionable.
The right amount of fi ssionable material is also needed. There
has to be enough to get the job done. These two things—the
right material and right amount of it—make possible the
third key requirement for useful fi ssion. If the right amount of
material is brought together under suitable conditions, a chain
reaction can take place. The possibility of a chain reaction is
what makes fi ssionable materials such useful sources of energy.
Chain Reaction
To see how a chain reaction works, let’s look at uranium-235
(U-235). Suppose, as often happens, a free neutron—one that

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is not bound to any
nucleus—is flying
through the area.
Suppose as well
that this neutron
happens to hit a
U-235 nucleus. If the neutron is not moving too fast, the nucleus
may capture it. This causes the nucleus to split into two smaller
nuclei. In the process, some energy and two or three neutrons
are released. The chain reaction can continue if one or more of
these newly released neutrons happens to be captured by other
U-235 nuclei. Those nuclei will then split, releasing more energy
and more neutrons. As long as there are enough neutrons and
enough U-235 nuclei, the process can keep going. In this way, a
huge amount of energy is released.
In a bomb, the chain reaction needs to be very fast. This
produces a sudden release of a great deal of energy, making the
bomb explode. The situation is different in a nuclear reactor. In
a reactor, the chain reaction has to occur over a long period of
time. Power plants, or power stations, need a continual supply
of heat to make electricity. In order to provide the right amount
of heat, a reactor has ways of controlling the chain reaction.
These keep the process from going too fast or too slow.
The chain reaction takes place in a part of the reactor called
the core. The core is specially designed for controlling the chain

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reaction. The nuclear fuel, such
as U-235, is typically placed in
long rods. The rods are grouped
together in bundles called fuel
assemblies. There are also other
long rods, known as control rods,
made of a material that absorbs
neutrons. The control rods can
be inserted into the core and
removed as needed. If a chain
reaction starts to go too fast,
some of the control rods can be
inserted. They absorb some of
the neutrons fl ying around in
the core, so that the neutrons
are not available to cause atoms
to undergo fi ssion. If the reactor
power needs to be increased, the control rods can be pulled out.
This makes more neutrons available to cause fi ssion.
Many reactor cores also contain a substance called a
moderator. This slows down the fl ying neutrons to speeds low
enough that U-235 nuclei can capture them. In some reactors,
the moderator also serves as a coolant, keeping the core
from getting too hot and melting. Many reactors use water as
a moderator. Graphite is another substance that is sometimes
used as a moderator.
Obtaining Nuclear Fuel
U-235 is the only fi ssionable form of uranium found in nature.
Natural uranium, however, has very little U-235. Instead

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MARTIN HEINRICH KLAPROTH
German chemist Martin Heinrich Klaproth was born in 1743 in the
town of Wernigerode. He learned chemistry while he was working
for several years as an apothecary—a person who makes and sells
medicines. In 1789, he discovered uranium in a mineral called
pitchblende. He named the new substance for the planet Uranus,
which had been discovered a few years earlier. Scientists did not yet
know about radioactivity. The discovery of radioactivity came more
than a century later.
In addition to uranium, Klaproth discovered the elements
zirconium and cerium. When the University of Berlin was created in
1810, he became its fi rst professor of chemistry. He died on New
Year’s Day 1817 in Berlin.

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natural uranium is usually more than
99 percent U-238. Only about 0.7
percent is U-235. (There also may be
a tiny bit of another isotope, U-234.)
This level of U-235 is far too low for
use in most types of reactors.
As a result, a great deal of work
must be done in order to get the
right type of uranium ready for use.
First, uranium ore is mined. The
ore contains different materials, so
most of the non-uranium rock in the
ore has to be removed. The result is
a material often called yellowcake

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which is usually more
than four-fifths uranium.
Then, more work is
needed. A process known
as enrichment increases
the amount of U-235,
raising the usual 0.7
percent to whatever level
is needed. For reactors,
the enriched uranium
material—in a form called
uranium oxide—is usually
shaped into little ceramic
pellets. These are then put
into fuel rods.
U-235 for use in
nuclear reactors can be
obtained in other ways
as well. Some countries
reprocess used, or spent,
fuel rods. These rods no
longer have enough U-235 to support a chain reaction, but they
still contain some, which can be extracted and processed to
make new fuel pellets.
Plutonium—more specifically, the isotope plutonium-239,
or Pu-239—can also be used for nuclear fuel. (It is also used
to make nuclear bombs.) Plutonium decays more rapidly than
uranium. As a result, almost none can be found in nature.
Instead, it is specially made from other elements. Special
reactors that are used to make a great deal of nuclear fuel such

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as plutonium are known as
breeder reactors.
Other types of reactor
fuel also exist, and two of
them will probably get more
and more attention in the
future. One is called MOX,
which stands for Mixed
OXide. MOX is a mixture
of the substances uranium
oxide and plutonium oxide.
To make MOX, plutonium
from spent (used) reactor fuel
and from weapons is mixed
with uranium oxide. Some
countries have used MOX fuel
for years. The second fuel is thorium-232, or Th-232. Thorium
is abundant on Earth. Th-232 does not undergo fi ssion, but it
does something else that is important. When a Th-232 nucleus
captures a neutron, it produces an isotope of uranium known
as U-233. U-233 is fi ssionable, so Th-232 could be used in a
breeder reactor to make U-233.
Different Types of Reactors
There are a number of different kinds of nuclear reactors. U.S.
power stations today use just two. One, called the pressurized
water reactor, is the most common type of nuclear reactor in
the world. The other type of power reactor found in the United
States is the boiling water reactor. Both types use uranium
oxide fuel. Both usually surround the core with several walls

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?How a Generator
Makes Electricity
A generator changes one type of energy—the energy of
movement—into another type: electrical energy. A generator
takes advantage of a few basic facts about magnets and
magnetism. If you put a piece of iron (or certain other
substances) near a magnet, a force will pull the iron to
the magnet. The area around the magnet in which this
force acts is called a magnetic fi eld. Something interesting
happens when you move a conductor (a material that can
carry, or conduct, electricity) through a magnetic fi eld. The
fi eld causes electricity—an electric current—to fl ow in the
conductor. A current will also fl ow if the conductor is held
steady and the magnetic fi eld moves past it. Either of these
methods may be used in a generator to produce a current.
Nuclear power plant generators usually get the
movement they need from the turning movement produced
by a turbine. The electric current that comes from a nuclear
power plant generator switches direction many times a
second. The current is known as alternating current. This is
the kind of current used in the public power system, or grid.