Human Chain Reaction


In this exploration people play the role of uranium nuclei and create a chain reaction.


To Do and Notice

Give each person two Ping-Pong balls.

Have the people stand in a roughly circular group with about two arm-lengths between each of them.

Tell them to throw their two balls in different directions over their shoulders when they are hit by a ball. This is a two-dimensional model so remind them to throw their balls in a horizontal plane. (We have them throw the balls over their shoulders so that they cannot aim at other people.)

Toss a ball into the group and watch what happens.

What's Going On?

Each person plays the role of a uranium 235 nucleus. When the nucleus is struck by a neutron it fissions (in this model we do not ask the participants to split in half.) and emits 2 to 3 neutrons. If these neutrons hit another nucleus it may also fission.

This can result in a chain reaction where one initial neutron becomes two which then become 4 and so on. In a nuclear bomb there may be over 60 doublings. Each fission is accompanied by the release of 200 million electron volts, 200 MeV.

Going Further

Do the exploration in a large space such as a gymnasium. Experiment with different numbers of people and with different spacings between the people. At the end of each exploration note the number of people who are still holding Ping-Pong balls. These are the nuclei that have not fissioned. When the people are widely separated the reaction does not grow. It takes a critical mass of nuclei to create an exponentially growing chain reaction.


The energy released during a nuclear fission comes from the electrostatic repulsion of the two halves of the nucleus.

Scientific Explorations with Paul Doherty

© 2005

27 July 2005