This model simulates a nuclear reactor. Atoms decay at random and emit two neutrons each. The neutrons travel a certain distance and/or collide with other atoms, causing them to decay and emit neutrons in turn. Absorbers can be placed into the reactor, simulating control rods.
For detailed instructions on using the program, take a look at the info window from the Nuclear Reactor model.
Click on one of the pictures to see a quicktime movie of the model: