2 Responses

1. They use “control rods” which contain material that has a high “nuetron cross section”. This means that the high energy nuetrons created when an atom fissions get slowed down or absorbed (by collisions with nucleii) before they hit other fissionable atoms. It is the impact of these nuetrons that trigger fission in additional atoms (say U atoms) and when one nuetron splits an atom, that split creates several nuetrons and each of those nuetrons split several more atoms which generate more nuetrons which split more atoms and that is what the chain reaction is. If the chain reaction is allowed to run (without any control rods), the temperature of the fuel increases very rapidly until it melts (and it starts melting everything around it, i.e. the “China Syndrome”) and it is likely to explode. That is exactly what happened at Chernobyl in the USSR (what is today Ukraine).

2. Fission occurs when a radionucleus absorbs a neutron beomes unstable and then cleaves ( fissions ) with release of energy and more neutrons . Control rods that can be inserted into the reactor “pile” function by absorbing the neutrons and interferring with the chain reaction nature of fission. Certain elements such as boron and cadmium have excellent neutron capture qualities and are used in making control rods for reactors. These rods slow the fission rate and are used to prevent the fissioning from getting out of control