Name: ________________________________
ChemCom
Nuclear Fission and Nuclear Fusion
Purpose:
-
To compare and contrast nuclear fission and nuclear fusion.
To understand the Laws of Conservation of Mass as applied to nuclear
reactions.
7.1 The Strong Force
Because each proton in the nucleus carries a +1 charge, there are tremendous repelling
forces constantly trying to force the protons apart—it is like a highly compressed spring
just waiting for a chance to uncoil. So what holds the protons together?
The strong force keeps the nucleus together. It is one of the four Fundamental Forces of
Nature. (The other three are gravity, electromagnetism, and the weak force.) We
experience two of these fundamental forces, gravity and electromagnetism, in our daily
lives; we feel gravity pull us toward the earth and know that magnets stick to the
refrigerator. But the strong force and weak force are completely outside of our experience
because they are only capable of acting over very short distances. In fact, the strong force
only works on objects that are within 1.5 x 10-15 m of each other—about the diameter of a
nucleus.
Because the strong force has a very limited range, any nucleus that is too big will be
unstable; its diameter will be larger than the range of the strong force.
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The nucleus on the left is stable because it is small; its diameter is smaller than the range of the strong
force. The larger nucleus on the right exceeds the range of the strong force and is therefore less stable.
*Chemistry 22 Textbook, Dr. Van Geel
1. List the four fundamental forces.
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2. Why do larger nuclei tend to be unstable?
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3. Complete the table shown below for the following atoms.
Isotope
14
6
C
12
6
C
Atomic #
Mass #
# Protons
# Neutrons
# Electrons
Oxidation #
4. What term can be used to describe these two atoms? Explain.
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Transmutation is the transformation of the nucleus of an atom so that the atom is changed
from one element into a different element. This can be accomplished through many types
of reactions, including fission and fusion.
Nuclear transformations always obey two fundamental conservation laws: (1)
mass number is conserved and (2) electrical charge is conserved. Energy and mass are
not conserved, but can be inter-converted according to Einstein's equation, E = mc2.
Fission:
The process of fission occurs when a nucleus splits into smaller pieces. Fission
can be induced by a nucleus capturing slow moving neutrons, which results in the
nucleus becoming very unstable.
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The following equations represent fission reactions, where n = neutron.
235
141
U + 1 n
Ba + 92 Kr + 1 3n
92
235
92
0
U +
1
0
56
n
131
50
36
Sn +
103
42
0
Mo +
1
0
2n
5. Explain why each of the equations shown above are classified as nuclear
fission.
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6. State the Law of Conservation of Mass.
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7. Both of the equations shown above adhere to the Law of Conservation of
Mass. Explain why.
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Fusion:
Fusion occurs when 2 nuclei join together to form a larger nucleus. Fusion is
brought about by bringing together two or more small nuclei under conditions of
tremendous pressure and heat.
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The following equations represent fusion reactions, where p = proton.
2
1
H +
3
1
H
4
2
He +
1
1
p
2
1
H +
1
2
1
H
3
2
H
+
1
1
p
8. How is nuclear fusion1 different from nuclear fission? Identify TWO ways.
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9. Does the Law of Conservation of Mass appear to hold true for these reactions?
Explain.
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10. Nuclear Fusion can only happen under conditions of high pressure or heat.
Why would so much energy be required to force two hydrogen nuclei
together?
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7.7 Nuclear Power
Nuclear Weapons
When a neutron strikes a uranium-235 nucleus, the following nuclear reaction occurs.
This is called a fission reaction: a reaction in which a nucleus splits into smaller nuclei.
Notice that this reaction also produces three more neutrons. If those neutrons go on to
strike other uranium nuclei, those nuclei split and produce more neutrons, which split
more nuclei, which makes more neutrons, which split more nuclei, and so on—a fission
chain reaction occurs.
The lines in this figure represent the paths of
neutrons. Each branching point is where a uranium
nucleus has split, producing three more neutrons.
Each time a uranium nucleus splits a tremendous
amount of energy is released. If a large amount of U235 undergoes a fission chain reaction all at once, the
result is a nuclear explosion. This kind of sustained
chain reaction is only possible if the piece of uranium
is large enough. If the piece of uranium is too small,
the neutrons end up flying out into empty space
instead of colliding with nuclei inside the piece of uranium. In other words, if the chunk
of uranium is too small, the chain reaction doesn’t happen, and no explosion occurs—it is
a subcritical mass of uranium. If it is just large enough to sustain the chain reaction, it is
called a critical mass. If it is larger than the critical mass, it is a supercritical mass. A
nuclear bomb starts with two or more subcritical masses of uranium. As soon as they are
brought together, they form a supercritical mass, which explodes.
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For the bomb to work, the subcritical masses must be propelled together extremely
rapidly. This is done with a conventional (non-nuclear) explosive. The bombs the United
States dropped on Japan in World War II were this type of bomb (they are called “atomic
bombs” or “A-bombs”).
Modern nuclear weapons use nuclear fusion instead of nuclear fission. Nuclear fusion
happens when two small nuclei come together to form a larger one.
This process also releases an enormous quantity of energy. In fact, fusion releases much
more energy than fission, and fusion bombs (called “hydrogen bombs” or “H-bombs”)
are thousands of times more powerful than fission bombs.
For two nuclei to get close enough to fuse, they must be moving extraordinarily fast.
Remember: Heat is the energy of moving molecules; the hotter something is, the faster its
molecules are moving. So fusion reactions require very high temperatures to occur.
Generating this amount of heat requires a fission reaction! So a fusion bomb is actually
two bombs in one. A small fission bomb explodes first, which heats up a quantity of
hydrogen enough to make it fuse.
Fusion reactions also power the stars; our sun is an enormous fusion reactor. Small atoms
such as hydrogen and helium fuse inside the sun, making larger atoms and releasing huge
quantities of heat and light.
*Chemistry 22 Textbook, Dr. Van Geel
11. Why is a nuclear chain reaction useful for Nuclear weapons?
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12. What is it so important to have a “critical mass” for maintaining a chain
reaction?
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13. How much greater is the amount of energy generated by a nuclear fusion
reaction compared to a nuclear fission reaction?
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14. Why does nuclear fusion happen so easily inside a star?
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Nuclear Reactors
A fission reaction can be slowed down to a
consistent, stable, rate; it doesn’t have to happen
all at once in an explosion. This is what happens
in a nuclear reactor; a controlled fission reaction
generates electricity. The uranium is shaped into
fuel rods. Each fuel rod has some fission going
on inside it, sending neutrons to neighboring
rods, causing their atoms to undergo fission, as
shown at right.
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If nothing intervened, the fuel rods would overheat
and cause a meltdown. To slow down the rate of
reaction, control rods are lowered between the fuel
rods. The control rods are made out of a material that
can absorb neutrons. When the control rods are
between the fuel rods, the neutrons of one fuel rod
can’t cause as much fission in a neighboring fuel rod,
and the whole reaction slows down.
A schematic of a basic nuclear reactor is shown below. The fuel rods and the control
rods make up the reactor core. When the reactor is active, the fission chain reaction
heats a liquid coolant surrounding the rods. The hot coolant is pumped through pipes to
boil water in the steam generator; the pressurized steam flows through the steam line to
the turbine, which powers the generator. The steam is then cooled and condensed by
water from the cooling tower, and then pumped back to the steam generator. So a nuclear
power plant is nothing more than a very sophisticated steam engine!
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*Chemistry 22 Textbook, Dr. Van Geel
15. Why do you think a chain reaction is not as desirable in a nuclear power
plant?
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16. How is the fission reaction controlled in a nuclear power plant? Explain.
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17. Explain the statement that a Nuclear Power Plant is nothing more than a
sophisticated steam engine.
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Synthesis Questions:
A. Describe the strong nuclear force.
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B. Describe the difference between nuclear fission and nuclear fusion.
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C. How is the use of nuclear fission in a nuclear weapon difference from the use of
nuclear fission in a power plant?
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D. The following equation represents a nuclear fission reaction. Identify X in this
equation. (Hint: start with the atomic mass and the atomic number.)
235
92
1
0
U +
152
60
n
Atomic Mass = ________
Nd +
X
Atomic Number = ________
+
1
0
3n
Element = ________
E. Identify each of the following as nuclear fission or nuclear fusion.
Fission or Fusion?
2
1H
+ 21H  31H + 11p
235
92U
+ 10n  14156Ba + 9236Kr + 3 10n
235
92U
+ 10n 
3
2He
138
54Xe
+ 32He
4
+ 9538Sr + 3 10n
2He
+ 2 11H