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3.65: Nuclear Fission

Difficulty Level: At Grade Created by: CK-12
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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 uranium-235, like the one pictured in the Figure below, 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 below 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:

\begin{align*}\mathrm{^{235}_{92} U}\end{align*} + 1 neutron → \begin{align*}\mathrm{^{91}_{36} Kr}\end{align*} + \begin{align*}\mathrm{^{142}_{56} Ba}\end{align*} + 3 neutrons + energy

Note that the subscripts of the element symbols represent numbers of protons and the superscripts represent numbers of protons plus neutrons.  

To see an animated version of uranium-235 fission, go to this URL: http://www.classzone.com/books/earth_science/terc/content/visualizations/es0702/es0702page01.cfm?chapter_no=visualizationNuclear_Fission_of_Uranium-235

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 below. 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. You can see another example of a chain reaction at this URL: http://www.youtube.com/watch?v=0v8i4v1mieU

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 below. 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 until it no longer releases harmful radiation. Therefore, used fuel must be stored securely to protect people and other living things. You can learn more about the problem of radioactive waste at this URL: http://www.youtube.com/watch?v=OPQ97LVRuuM


  • 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.


  • nuclear fission: Splitting of the nucleus of an atom into two smaller nuclei.


Watch the video about nuclear fission at the following URL, and then answer the questions below. http://www.youtube.com/watch?v=IJsmaBqR5xM

  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.


  1. What is nuclear fission?
  2. Outline how nuclear fission occurs.
  3. Describe a nuclear chain reaction.
  4. How is nuclear fission controlled in a nuclear power plant?
  5. Compare and contrast the use of fossil fuels and nuclear fission to produce energy.

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nuclear fission

Splitting of the nucleus of an atom into two smaller nuclei.

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Date Created:
Nov 14, 2012
Last Modified:
Oct 23, 2016
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