Nuclear Fission
Jean Brainard, Ph.D.
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Printed: November 29, 2016
AUTHOR
Jean Brainard, Ph.D.
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C HAPTER
Chapter 1. Nuclear Fission
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Nuclear Fission
Learning Objectives
•
•
•
•
Outline what happens during nuclear fission.
Describe a nuclear chain reaction.
Explain how nuclear fission can be used to produce energy.
List pros and cons of using nuclear energy.
Steam rises from the cooling towers of this nuclear power plant. The steam is only harmless water vapor. Unlike
a power plant that burns fossil fuel, a nuclear power plant doesn’t release pollution into the air. That’s because a
nuclear power plant produces power by nuclear fission, a type of nuclear reaction.
What Is Nuclear Fission?
Nuclear fission is the splitting of the nucleus of a radioactive atom into two smaller nuclei. This type of reaction
releases a great deal of energy from a very small amount of matter. Fission of a tiny pellet of radioactive uranium235, like the one pictured in the Figure 1.1, releases as much energy as burning 1,000 kilograms of coal!
Q: What causes the nucleus of uranium-235 atom to fission?
A: Another particle collides with it.
How Nuclear Fission Occurs
The Figure 1.2 shows how nuclear fission of uranium-235 occurs. It begins when a uranium nucleus gains a neutron.
This can happen naturally when a free neutron strikes it, or it can occur deliberately when a neutron is crashed into
it in a nuclear power plant. In either case, the nucleus of uranium-235 becomes extremely unstable with the extra
neutron. As a result, it splits into two smaller nuclei, krypton-92 and barium-141. The reaction also releases three
neutrons and a great deal of energy. It can be represented by this nuclear equation:
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FIGURE 1.1
235 U
92
141
+ 1 neutron → 92
36 Kr + 56 Ba + 3 neutrons + energy
Note that the subscripts of the element symbols represent numbers of protons and the superscripts represent numbers
of protons plus neutrons.
FIGURE 1.2
Nuclear Chain Reaction
The neutrons released when uranium-235 fissions may crash into other uranium nuclei and cause them to fission as
well. This can start a nuclear chain reaction. You can see how this happens in the Figure 1.3. In a chain reaction,
one fission reaction leads to others, which lead to others, and so on. A nuclear chain reaction is similar to a pile of
wood burning. If you start one piece of wood burning, enough heat is produced by the burning wood to start the rest
of the pile burning without any further help from you.
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/5018
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Chapter 1. Nuclear Fission
FIGURE 1.3
Using Energy from Nuclear Fission
If a nuclear chain reaction is uncontrolled, it produces a lot of energy all at once. This is what happens in an atomic
bomb. However, if a nuclear chain reaction is controlled, it produces energy much more slowly. This is what occurs
in a nuclear power plant. The reaction is controlled by inserting rods of nonfissioning material into the fissioning
material. You can see this in the Figure 1.4. The radiation from the controlled fission is used to heat water and turn
it to steam. The steam is under pressure and causes a turbine to spin. The spinning turbine runs a generator, which
produces electricity.
Pros and Cons of Nuclear Energy
In the U.S., the majority of electricity is produced by burning coal or other fossil fuels. This causes air pollution
that harms the health of living things. The air pollution also causes acid rain and contributes to global warming.
In addition, fossil fuels are nonrenewable resources, so if we keep using them, they will eventually run out. The
main advantage of nuclear energy is that it doesn’t release air pollution or cause the other environmental problems
associated with the burning of fossil fuels. On the other other hand, radioactive elements are nonrenewable like
fossil fuels and could eventually be used up.
The main concern over the use of nuclear energy is the risk of radiation. Accidents at nuclear power plants can
release harmful radiation that endangers people and other living things. Even without accidents, the used fuel that is
left after nuclear fission reactions is still radioactive and very dangerous. It takes thousands of years for it to decay
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FIGURE 1.4
until it no longer releases harmful radiation. Therefore, used fuel must be stored securely to protect people and other
living things.
MEDIA
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URL: https://www.ck12.org/flx/render/embeddedobject/82364
Summary
• Nuclear fission is the splitting of the nucleus of an atom into two smaller nuclei. This type of reaction releases
a great deal of energy from a very small amount of matter. It begins when the nucleus of a radioactive atom
gains a neutron.
• In uncontrolled nuclear fission, one fission reaction starts a chain reaction, in which neutrons produced in one
reaction cause other reactions, which cause more reactions, and so on.
• Energy released by nuclear fission is used to produce electrical energy in nuclear power plants. Production of
nuclear energy doesn’t produce air pollution but it poses the risk of accidents that release harmful radiation.
Review
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What is nuclear fission?
Outline how nuclear fission occurs.
Describe a nuclear chain reaction.
How is nuclear fission controlled in a nuclear power plant?
Compare and contrast the use of fossil fuels and nuclear fission to produce energy.
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Chapter 1. Nuclear Fission
Explore More
Watch the video about nuclear fission and then answer the questions below.
MEDIA
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URL: https://www.ck12.org/flx/render/embeddedobject/193511
1. How was nuclear fission discovered?
2. Describe in detail what happens to a uranium-235 nucleus when it is struck by a neutron, and explain why the
nucleus splits in two. Your answer should include the role of nuclear forces in the reaction.
References
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Courtesy of the U.S. Nuclear Regulatory Commission. Sample of Uranium-235 . Public Domain
Christopher Auyeung. Diagram illustrating nuclear fission . CC BY-NC 3.0
Christopher Auyeung. Diagram illustrating how a chain reaction occurs . CC BY-NC 3.0
Christopher Auyeung. Diagram illustrating how a nuclear power plant works . CC BY-NC 3.0
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