Download Fusion or Fission

C.12ABC: Nuclear Chemistry
Atomic Structure and Nuclear Chemistry
Fusion or Fission
(Lexile 1020L)
1 Nuclear reactions can
generate a lot of energy.
However, not all nuclear
reactions are the same.
There are two very
distinct types of nuclear
reactions—fusion
reactions and fission
reactions. What are the differences and similarities between nuclear fission and nuclear fusion?
As you have learned, all atoms are made of subatomic particles called protons, neutrons, and
electrons. In the simplest terms, nuclear fission occurs when large, unstable atoms split into
smaller atoms to achieve a more stable state. Nuclear fusion is the opposite of fission. Fusion
occurs when smaller atoms bind together to form a larger, more stable atom. In both cases, the
reaction occurs to bring the atom to a more stable state by a reduction of potential energy.
2 Nuclear fission reactions occur when the nucleus of an atom is split into fragments. When this
occurs, smaller fragments are created. Most often, the result is two fragments of relatively
equal mass. How is this energy created? You can compare the mass of the starting atom and
the masses of the final products. You will find that the mass of the final products is less than
the mass of the starting atom. Yes, in this type of nuclear reaction, mass is lost! This means
that matter is lost. This loss of matter is known as the mass defect. Remember, the law of
conservation of mass is for non-nuclear changes. Nuclear reactions are described by
conservation of mass-energy. The small amount of lost mass is converted directly into large
amounts of energy. The energy created from this reaction can be used to generate electricity in
power plants or for the huge explosions of atomic bombs.
3 One type of atom used for fission reactions is an unstable isotope of uranium known as
uranium-235. The stable isotope is uranium-238. In order for this reaction to occur, a neutron
must collide with the uranium-235 nucleus, creating uranium-236. This collision creates enough
energy to split the newly-formed nucleus into smaller nuclei. A huge amount of energy is
released in the process. As the nucleus splits and releases energy, more neutrons are
produced. These may collide with another uranium-235 nuclei, causing it to undergo fission.
This will create more energy and produce even more neutrons. The reaction will become
something called a chain reaction if enough neutrons are produced during collisions
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C.12ABC: Nuclear Chemistry
Atomic Structure and Nuclear Chemistry
to keep the reaction going. The minimum amount of material that is needed for the reaction to
keep going is called the critical mass. The trouble with fission reactions is that radiation and
nuclear waste products are created in the process. This is a problem, as many nuclear power
plants use fission to produce energy, producing a lot of radioactive byproducts as a result.
4 Conversely, in fusion reactions, the nuclei from atoms with low atomic weights combine to
create heavier nuclei. This reaction does not require neutrons in order to occur. Two light nuclei
must collide with a great deal of energy. Fusion reactions require extremely high heat. In fact,
the heat required can exceed several million degrees Celsius (yes, several million!) To put
these types of temperatures in perspective, the temperature at the surface of the sun is around
5,600 ºC. Meanwhile, the temperature near the core of the sun is calculated to be around 15
million ºC. The temperature of the sun stated here should give you a hint of a nuclear reaction
that you can observe. The sun is a product of an ongoing fusion reaction that has been
occurring for billions of years.
5 Why is such high heat required for these reactions? Remember, the protons of atoms are all
positively charged. In order to combine atoms, the repulsion of the positive protons of the
combining atoms must first be overcome. Extremely high heat gives the nuclei the kinetic
energy needed to overcome the repulsion. And as shown with the sun, fusion reactions are
also very exothermic. Therefore, fusion reactions have the capability of producing more energy
than is required to initiate the reaction. This creates a self-sustaining reaction that holds great
promise for energy production. A fusion reaction will sometimes begin with a fission reaction to
get it started. In the right conditions, it can sustain itself, once started. One of the benefits of
fusion reactions is that there are no radioactive byproducts. On earth, fusion reactions result in
the element helium. Heavier elements, like beryllium or carbon, are produced by fusion in stars.
Hydrogen bombs are produced by fusion reactions in which several isotopes of the element
hydrogen are combined to produce helium.
6 How can you remember the difference between these two types of nuclear reactions? Think in
terms of vocabulary that you may already know. A fissure is a crack or opening that can appear
on the surface of earth. It is a separation. A fission reaction will therefore "separate" an
unstable atom into parts. A fusion reaction will "fuse" two atoms together.
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C.12ABC: Nuclear Chemistry
Atomic Structure and Nuclear Chemistry
1
2
3
There are two distinct types of nuclear reactions. One is a fusion
reaction, and one is a fission reaction. In which reaction is the
nucleus of an atom split into two or more atoms?
A
Fusion reactions
B
Fission reactions
C
Both nuclear reactions
D
Neither nuclear reaction
In order for fission reactions to be successful, they must be selfperpetuating, meaning they must be able to keep themselves
going. What do you call the minimum amount of material that is
needed for the reaction to keep going?
A
The isotope
B
The mass defect
C
The critical mass
D
The neutron
Our sun is a star that is fueled by a specific type of nuclear reaction.
Which type of nuclear reaction is this?
A
Fusion
B
Solar
C
Fission
D
Radioactive
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C.12ABC: Nuclear Chemistry
Atomic Structure and Nuclear Chemistry
4
5
Why is the fusion reaction not currently used to produce energy,
especially as it is a "clean" form of energy that does not create
radioactive or nuclear byproducts?
A
There are currently not enough fuels for fusion reactions.
B
Fusion reactions are highly endothermic, making them
dangerous.
C
Fusion reactions do not yet produce enough energy.
D
The heat it takes to start a fusion reaction is too high to
contain.
Fission reactions are currently used to provide energy for many
different sources. Where does the energy produced during the
reaction come from?
A
From the mass lost during the reaction, or the mass defect
B
From the mass created during the reaction, or the mass
increase
C
From the critical mass of the reaction
D
From the protons created during the reaction
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