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The Basic Physics Behind a Nuclear Reactor
The nuclear reactor operates on the principle of nuclear fission, an event that occurs when an atom is struck by a neutron and flies apart.
The figure at right depicts a neutron heading towards a fissile atom (one that is capable of supporting fission). Most often, this is Uranium-235 or Plutonium-239, where the number after the element name indicates the total number of neutrons and protons present in the nucleus of the atom.
When the high-velocity neutron strikes the nucleus, the atom sheds more neutrons and breaks up into several other particles. The motion of all these particles and atoms flying around and colliding creates the heat that is used in power plants to generate electrcity.
Roughly speaking, when the same number of neutrons are created at a given time as are created in a previous generation, a reactor is considered to be 'critical'. If the reaction produces less neutrons than the previous generation, it is 'sub-critical' and, as can be inferred, if the reaction produces more neutrons than the previous generation, it is said to be 'super-critical'. A super-critical reaction is not necessarily bad, as it is the means used by a reactor to increase in power output. If a reactor is 'prompt critical,' however, the operator should be very concerned because the reactor is changing in power very quickly and is very likely moving faster than human reaction time. To prevent a prompt critical state, many standard procedures and regulations have been created and are rigorously enforced.
This also explains why nuclear engineering students cringe when a television show depicts a reactor going critical and then blowing up catastrophically.