The material is formed by atoms.
A nuclear reaction is a conversion of one or several atomic nuclei.
It is the chemical reaction which concerns electrons or connections between nucleons.

- we speak about fusion when two atoms light unite to form a heavier, more stable nucleus.
- we speak about fission when a heavy nucleus breaks in two also more stable light nuclei.



The heat is produced during a break of an atomic connection.
This heat allows creating a current between a warm water and a colder water which makes turn turbines.

At present, the energy produced by our power station results from the nuclear fission, the nuclear fusion is not still mastered (see article on ITER)