Nuclear energy is defined as the energy necessary to keep the integrity of the atomic nucleus.

In 1905, Albert Einstein developed the equation that relates mass to energy, as a part of his theory of relativity. This equation states that a certain mass (m) has a quantity of energy associated ( E) equal to the mass multiplied by the square of light speed (c).

E=mc²

This equation shows that a very little quantity of mass is equivalent to a huge quantity of energy. For example, if a milligram of mass is transformed into energy, we have that energy released is: E=0.000001Kg*90.000.000.000.000.000 = 90.000.000.000 joules = 90 giga joules.

At the moment there are two processes that have a great practical importance, because they provide a way to take advantage of the huge quantities of energy contained in the atomic nucleus, which are:

Nuclear Fission. It is the transformation of a heavy nucleus into lighter nucleus.

Thermonuclear Fusion. It is the union of two light nucleus (to extremely high temperatures) to make a heavier nucleus.

In both cases, the mass of reacting particles is bigger than products mass and the difference of mass appears as energy, according to Einstein’s equation.