This model is a computer simulation of a nuclear reactor, where the yellow particle(s) represent a neutron and the red particles represent uranium, the reactors fuel. The power output of the reactor can be manipulated according to the position of the blue control rods which absorb neutrons. The plots to the right represent power output from each simulation.
For detailed instructions on using the program, take a look at the info window from the Fission model.
Click on one of the pictures to see a quicktime movie of the model: