In a nuclear fission reaction in a nuclear
power plant, the radioactive element Uranium-235
is used in a chain reaction.
The fission of U-235 splits off two neutrons,
which in turn strike two U-235 atoms.
Two neutrons are split from each of the
two U-235 atoms. Each of these neutrons
then go on to strike another U-235 atom.
Each of those atoms are split releasing
two neutrons, which go on and hit more Uranium
The chain reaction continues on and on,
getting bigger and bigger with each split.
The things that slow down a chain reaction
are the control rods. A control rod is made
up of cadmium or boron, which absorb neutrons.
If you insert the control rod between the
uranium atoms, the amount of neutrons available
to cause more splits is reduced.
In the second line up of dominoes, the ruler
served as a control rod. Putting it between
two dominoes breaks the chain reaction similar
to what happens in a nuclear reactor.