It uses both fission and fusion

The ratio of the total volume of a set of objects?packed?into a space to the volume of that space. The difference between the isotopic mass of a nuclide and its mass number, divided by its mass number. The?packing fraction?is often interpreted as a measure of the stability of the nucleus.

Plutonium - 239 is the best nuclear fuel

Graphite and heavy water are two common moderators used in a nuclear reactor. The function of the moderator is to slow down the neutrons to thermal energies