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Chain Reaction – Critical Mass



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.

There are two parameters needed to create a critical mass, the number of atoms and the spacing of the atoms. In this demonstration each student represents a uranium atom inside of a nuclear reactor. Each uranium atom releases two neutrons when it fissions. For this demonstration, the more students that participate, the better the results.

Refer to Enrico Fermi’s quotes:
“The event was not spectacular, no fuses burned, no lights flashed. But to us it meant that release of atomic energy on a large scale would be only a matter of time.” -Enrico Femi

“We all hoped that with the end of the war, emphasis would be shifted decidedly from the weapon to peaceful aspects of atomic energy” -Enrico Femi

Students gain a better understanding of critical mass and how a chain reaction can become uncontrolled. Students are able to visualize what is meant by subcritical, critical, and supercritical mass. By extension, this experiment is a useful analogy to nuclear fission. This experiment is best used by students working in groups.

Indoor or outdoor activity.

Grade Level:
5-12th grade

Time Required:
20-30 minutes


Part One-Procedure:
Ten students will be uranium-235 “atoms.” Rest of class observes and records what is happening. The “atoms” stand in a circle about three feet apart. Each student holds two balloons (representing neutrons) in position to toss. Tell the students you will throw your balloon up into the air. If it hits someone, that person should throw his or her balloons up in the air. The student or students hit by those balloon toss their balloons and so on. Discuss what happens. Not all students are hit. That is what happens in a nuclear reactor.

Part Two-Procedure:
Choose ten other students to be “atoms” to maximize participation. This time the “atoms” stand in a circle only one foot apart. Follow the same procedure of balloon tossing as in Part One. What happens this time? Many more “atoms” are hit. Tell students they have just demonstrated an uncontrolled chain reaction. In a nuclear reactor, the reaction is controlled by control rods. These are special rods that go in between groups of fuel rods (which have fuel pellets stacked in them) inside the reactor. The control rods help to start (when they are removed), stop (when they are fully inserted), increase or decrease (when they are partially removed or inserted) the fission process.

Part Three-Procedure:
Explain that students will now demonstrate a controlled reaction. Use the same students to be atoms or select a new group. Choose one (or more) additional student(s) to be a control rod. His or her job is to stand inside the “atoms” group and try to grab or bat away the falling balloons before they hit a student. (Because there are now control rods in your demonstration, the first balloon may have to be thrown several times before it hits a student.) After all the balloon are thrown, discuss what happened. Fewer students should have been hit because the control rods intercepted some of the “neutrons.” Students can see how the rods slow down and can even stop a chain reaction. When that happens, the fission process will stop very quickly.

Analysis and Results:

  1. Have the students measure the amount of time needed for each of the procedures.
  2. Have the students describe the difference in the activity according to their distance from one another.


  1. Have the students describe how this activity demonstrates the way a nuclear fuel in a nuclear reactor could provide a sustained heat source for generating electricity.
  2. Have the students discuss why they were given 2 particles while the instructor started with only one.
  3. Have the students discuss the possible differences using only half the class in the activity.

To Share or Not to Share.
Ask students to write a persuasive, logical argument that answers the question:
Should scientists have shared their knowledge of their accomplishment of splitting the atom and being able to release nuclear energy?

The argument should address the many improvements in our lives that have resulted (refer to A Day with the Atom) and the fact that the discovery also resulted in the development of the atomic bomb and the fear of nuclear war. Discuss the basic elements of organization and clear writing before they begin.

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