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Main Nuclear Physics Nuclear Fission LessonIV_2_1 Nuclear Physics

Nuclear Fission - Lesson IV.2.1

Key Terms:

fission | moderator | nuclear mass defect |chain reaction |enrichment |control rods |nuclear reactor |critical mass | IL, NUM, CCT

Fission is the splitting of a heavy nucleus into two or more nuclei.

  • A slow moving neutron is captured by the nucleus
  • The nucleus becomes unstable and splits.
  • Two or more fast moving nuclei form and are released
  • A large amount of energy is released.
An example of a typical fission:

Click to watchQuicktime Video

In order for fission to take place the neutrons must be slow moving. A moderator is a medium, such as graphite, heavy water, and beryllium, that causes the neutrons to travel more slowly.

A typical fission releases 200 MeV of energy. This energy is composed of the following:

170 MeV of kinetic energy of fission fragments.
5 MeV of kinetic energy of neutrons.
15 MeV of energy beta particles and gamma rays.
10 MeV as energy of antineutrinos.
  • The fission of 1 kg of uranium fuel produces 9 x 1016 J of energy compared to 3 x 107 J of energy produced when 1 kg of coal is burned.

Mass is not conserved in a nuclear reaction. The products have a slightly lower mass due to the nuclear mass defect.

This nuclear mass defect can be used to determine the nuclear binding energy which held the heavier nucleus together and was released during fission.

where E = energy released
c = speed of light = 3.0 x 108 m/s
m= nuclear mass defect

You can calculate mass defect by:
m = masses of parent atom and neutron - masses of daughter atoms and neutrons emitted.

Sample Problem

In a fission reaction, the loss in mass is 0.025 g. How much energy is produced?


E = energy released = ?
c = speed of light = 3.0 x 108 m/s
m= nuclear mass defect = 0.025 g = 2.5 x 10-5 kg

Check Point

The fast moving neutrons, released during fission, can cause other nuclei to undergo fission if they are slowed down by a moderator.

A sustained fission reaction caused in this way is called a chain reaction.

The fissionable isotope of uranium ore is Uranium-235. Uranium ore contains about 0.7% U-235. To increase the likelihood of sustaining a chain reaction the concentration of U-235 must be increased through a process called enrichment.

Nuclear Reactor

  • Produces a chain reaction at a controlled rate.
  • The rate is controlled by control rods, often made of cadmium, which absorb neutrons.
  • The energy produced is used to drive turbines which generate electricity.
Critical mass is the minimum mass of fissionable material that will produce a nuclear explosion.

  • A mass grater than the critical mass is needed to produce a sustainable nuclear chain reaction.

Atomic Bomb
  • An explosion results when two or more sub-critical masses ( less than critical mass) of fissionable material are brought together rapidly.
  • Produces devastating destruction.
  • Produces radioactive contamination of the environment.
  • The first atomic bombs were developed by the United States during the Manhattan Project in WW II.
 Buster Charlie - Date: 30.Oct.1951 - Yield: 14kt
This research project is to be on the atomic bomb. Students are to answer the following questions within the body of the project.
Who developed the first atomic bombs?
Why were they developed?
When and where were these bombs used?
Describe the resulting devastation.
What possible alternatives could have been used?
Do you believe the bomb should have been used?
Do such weapons threaten our existence today? Explain.
Additional information is encouraged.
Your project may be presented in a variety of ways: traditional paper, news broadcast, newspaper, skit, interview, etc. Be creative!


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