What is the science behind nuclear energy both fission and fusion?

Fusion is the combination of two atomic nuclei into one, while fission is the separation of one nuclei into two. Nuclear power plants use fission, where a particle (neurton) is shot at an atomic nuclei to break it apart. In the breaking apart process some of the mass of the original atom is converted into energy (e=mc2). This energy (heat) is used to heat water, expanding into steam that turns a turbine.