Ask A Scientist© Physics Archive

Question:  What is the difference between nuclear fusion and nuclear fission?
------------------------------------------------
Answer:  Very simply, fission splits a massive element into fragments, releasing 
energy in the process.  Fusion joins two light elements, forming a
more massive element, and releasing energy in the process.  The reason
they both release energy can be understood by examining a curve
called the binding energy per nucleon curve.  Since I cannot draw the
curve for you here, I suggest you look it up.  Basically, the significance
of the curve is that it has an initial steep increase in binding
energy per nucleon as the number of nucleons increases, reaches a maximum
value, and then decreases gradually as the number of nucleon increases.
In fission, an element with a very large number of nucleons (such as
Uranium) is split, forming two fragments which each have fewer nucleons
(the total number of nucleons is always constant).  These fragments
are nearer the maximum of the curve, so the total binding energy
increases.  In fusion, very light atoms (before the maximum of the
curve) are fused into a more massive atom, neare the maximum again.