In-class group activity 16:  Fission

Some nuclear reactions do not occur spontaneously, but require external sources of energy, in the form of "collisions" with outside particles.  "Activation energy" has to be provided before a much larger amount of "reaction energy" is released. Nuclear fission can be "activated" when a slow neutron collides with a fissionable nucleus.

Use an on-line simulation from the University of Colorado PhET group to explore nuclear fission. 
Link to the simulation http://phet.colorado.edu/en/simulation/nuclear-fission.  Click "Run Now!".

Open a Microsoft Word document to keep a log of your procedures, results and discussions.

• Click the Fission: One Nucleus tab.

  • Before the gun is fired, is the material stable?

  • What type of "bullet" does the gun fire?

  • What happens to the nucleus when it is hit?

• Click the Chain Reaction tab.

  • Add some uranium-238.  Is uranium-238 “fissionable”?  How does firing the gun on a uranium-238 nucleus change it?  (Note you can aim the gun.)

  • Reset the sim and add ~50 fissionable uranium-235 nuclei.  Fire the gun.  Describe what happens and why it happens.

  • Naturally occurring levels of the U-235 isotope are about 0.72%, with the majority being U-238.  Round the level up to 1% U-235 (one atom of U-235 and 99 atoms of U-238).  Use the simulation to find out if naturally derived uranium is able to start a chain reaction,

  • Use the simulation to find a minimum ratio of U-235 to U-238 that can start a chain reaction?  Compare your mixture to “weapons-grade” enriched uranium (about 80%-85% U-235).

  • Use the simulation to make a nuclear weapon.  What conditions are needed?  (Check the box “containment vessel”, and determine the level of enrichment needed.)

• Click the Nuclear Reactor tab.

  • What is needed to start the nuclear reactor?

  • What does adjusting the control rods accomplish? 

  •  Without the control rods in position, what happens?

Watch the linked video. Is this a good analogies of a nuclear chain reaction? If mousetraps and Ping-Pong balls are used to illustrate a fission chain reaction, what do each represent?