Objective: This demonstration will teach the principles of chain reactions.
Review of Scientific Principles:
A nuclear reactor produces energy by causing a fission reaction of a heavy unstable atom like 235Uranium into smaller elements we call fission products. The reaction is started by hitting the atom with a slow neutron. Then, a chain reaction begins as each atom breaks into fission products and more neutrons. So, if you start with one atom of Uranium and each atom gives off 2 neutrons (for example) when it undergoes fission, then after each fission reaction you have 2 new neutrons which could cause fission events of their own. (See Figure 7 in Scientific Principles for a review.)
This type of reaction can be simulated with mouse traps and ping pong balls. By setting up several mouse traps all loaded with ping pong balls and hitting one of them with a free ping pong ball, you can see a chain reaction for yourself!
Safety note: Mouse traps can really hurt your fingers! Be careful both when setting up this demo and when taking it down.
Materials and Supplies:
- 24 mouse traps for each group
- 24 ping-pong balls for each group plus a few extra for ones lost
- 1 foot of string
- 1 containment box
- Plexiglas containment box
- a Plexiglas box open at one end around (2' x 2' x 2')
- Cardboard containment box
- a cardboard box around (2' x 2' x2')
- a box cutter
- a transparency
- Make the containment box. See teacher's notes for instructions.
- Set a mouse trap and place on smooth surface.
- Carefully place a ping-pong ball on the trap so that it rest on the metal snapping piece and the holding arm.
- Repeat steps 1 and 2 starting with the traps away from your body and finishing with the traps toward your body. Laying out the traps in the pattern shown below works particularly well.
- On the last trap attach a string to the holding arm before setting.
- Carefully place the containment vessel to cover the traps.
- Pull swiftly on the string and watch as the traps go off.
Construction of a Plexiglas containment vessel requires time and patience. Consult your Plexiglas dealer for instructions on how to work with Plexiglas.
Construction of cardboard containment vessel
- Cut the four, top lids off the box.
- Cut a 7.5 inch wide by 10 inch high hole in the side of the box. The hole should be 1/2 from the open end and an inch away from the corner.
- Tape the transparency to the inside of the box so that it covers the hole made in step 2.
- Have the students come up with ways of simulating control rods. Any original idea that would remove ping pong balls from the reaction would be appropriate. An example is hanging strips of tape from the top of the containment box.
- This would be a great contest or science fair project. Encourage students to experiment with different shapes of trap layouts, different densities of traps, or different number of balls on each trap.
- Tape the mouse traps to the surface so that only ping-pong balls set off traps.
- Use in conjunction with the accompanying explanations in the nuclear section for best results.