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Mouse Trap Reactor

The Mouse Trap Reactor is a visual representation of a chain reaction in a confined space. Useful as an analogy for an uncontrolled nuclear reaction.


The splitting of a massive nucleus into two fragments, each with a smaller mass than the original is known as nuclear fission. A typical example of nuclear fission is the splitting of a Uranium-235 nucleus. This is a reaction that is used in nuclear reactors to generate heat by which steam is produced and used to turn turbines that generate electricity. The fission of Uranium 235 begins when the Uranium 235 captures a slow moving neutron and forms an unstable “compound nucleus”. The compound nucleus quickly disintegrates into a Barium-141 nucleus, a Krypton-92 nucleus, two or three neutrons (2.5 average), and a tremendous amount of energy (~200MeV per fission).

Because the Uranium-235 fission reaction produces 2 or 3 neutrons, it is possible for it to initiate a series of subsequent fission reactions. Each neutron released can initiate another fission event, resulting in the emission of more neutrons, followed by more fission events, and so on. This is a chain reaction – one event triggers several others, which in turn trigger more events, and so on. In a nuclear power plant the chain reaction is controlled by restricting the number of neutrons available to collide with the Uranium. This is accomplished by absorbing some of the released neutrons with various materials. In an uncontrolled chain reaction (such as an atom bomb explosion) there is nothing to control the number of neutrons being released, so the rate of the chain reaction increases dramatically.

This video features ANS member, Matt Dennis, demonstrating the “Mouse Trap Reactor”. The activity, which is essentially a 3’x4’x4′ cube with clear plexiglass sides, with rows of mousetraps screwed into the bottom, covering the whole area. Ping pong balls are placed on each mousetrap after the traps are set.  You can cycle through large groups of students and give a presentation prior to allowing one lucky student to throw an additional ping pong ball into the “reactor” which starts a fast and loud chain reaction. We suggest discussing fission as the splitting of the atom and the source of power, in the form of heat, used in nuclear power plants. Depending on the audience’s age level, you can discuss the ping pong balls as neutrons that are released in fission and carry the chain reaction to other mouse traps or the fissile atoms, Uranium 235 or Plutonium 239.



  1. Arm the mousetraps and place them in a 10 by 10 grid on the reactor base.
  2. While positioning the traps, hold them by their sides on the end opposite the trigger paddle and alternate the direction of the trigger paddle.
  3. After positioning a row of mousetraps, place a cardboard partitions around it to avoid triggering armed traps.
  4. Once the reactor is in the classroom, carefully remove the reaction chamber and load the balls onto the armed traps.
  5. Once the reactor chamber is in place, a trigger ball (representing a neutron) is dropped through a hole in the top of the reactor where it triggers the chain reaction. The whole reaction takes about 2 seconds.


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