The term splitting an atom seems like hitting a bunch of billiard balls to cause the atom to fly apart. What really happens is you want the atom to absorb the

The term splitting an atom seems like hitting a bunch of billiard balls to cause the atom to fly apart. What really happens is you want the atom to absorb the neutron and its kinetic energy so as to make the atom's energy higher than the strong force's ability to hold the it together. If the neutron's kinetic energy is too low, the atom will just absorb it. It may become radioactive and decay over time, but not necessarily in a speedy chain reaction way; however, going too fast isn't good either. Atoms become inelastic the higher the neutron's energy. If it's too high, the atom is effectively transparent to the neutrons and won't capture them. So, a "neutron moderator" slows the neutrons to an energy that is low enough to allow capture and fission, but not capture and no fission.