Ping-Pong Ball Chain Reaction

Author: Jerry Silver

The Idea

This is a fun and simple demonstration that will help you understand how a nuclear chain reaction occurs. It was used years ago in a Walt Disney film called Our Friend the Atom and appeals to all readers including the youngest and technically least sophisticated.

What You Need

  • 24 spring-type mousetraps
  • 49 ping-pong balls
  • enclosure with at least one transparent side (a large fish tank with glass sides and a glass bottom can work well)
  • optional: 2 mirrors the size of, or larger than, the side of the enclosure


  1. Set the traps (Figure 123-1).
  2. Carefully place two ping-pong balls on each of the mousetraps (where the cheese would have gone). See Figure 123-2.
  3. Lay out the mousetraps in an array that will fit into the enclosure, such as a 6 × 4 array. Obviously, you need to be extremely gentle and avoid sudden motions to prevent a premature release of the mousetrap. Any mishap will likely take other mousetraps out with it.
  4. Either lower the enclosure over the mousetraps or develop a way to bring the mousetraps into the enclosure. You may need to experiment with different methods of loading the mousetraps. You may prefer to place the ping-pong balls after, rather than before, moving the traps. You may want to develop a wooden or foamboard template that protects the trap's trigger mechanism while you are placing the ping-pong balls or glue the traps to a board.
  5. Ping-pong ball chain reaction

    Ping-pong ball chain reaction

  6. With the ping-pong ball loaded on the mousetraps in the enclosure, you are ready to initiate the chain reaction. So far, you have used 48 ping-pong balls, so one should be left. The remaining ball is the neutron that starts the chain reaction.

Expected Results

As in a nuclear-fission chain reaction, a neutron (the starter ping-pong ball) creates the first fission reaction. This event is simulated by the mousetrap releasing two additional ping-pong balls. These, in turn, potentially each release two more balls (neutrons) initiating a doubling of the available neutrons with each fission. As additional ping-pong balls are released, the rate of the reaction accelerates. This chain reaction is simulated by rapidly releasing ping pong balls, which in turn releases other ping-pong balls to continue the reaction. The aftermath of this is shown in Figure 123-3.

Ping-pong ball chain reaction

Why It Works

Nuclear fission occurs in nature when an isotope of a nuclear material absorbs a neutron and become unstable. The nucleus splits, forming two lighter "daughter" nuclei and a spray of free neutrons that produces the cascading effect known as a chain reaction. There also needs to be a critical mass for this process to become self-sustaining.

Other Things to Try

It would be interesting to capture a video image of this simulated nuclear reaction and view it in slow motion.

The Point

Nuclear fission is initiated by a free neutron that causes a nucleus (such as a uranium-238 nucleus) to split and release additional neutrons. This is the basis of nuclear power, which currently provides about one-fifth of the electricity in the United States.

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