Fission or Fusion?
To begin, both fission and fusion generate energy through heat.
In fission reactors, atoms are split to create heat. These atoms can be considered to have a heavy nucleus. The fission of heavy elements is an exothermic reaction, and huge amounts of heat energy are released in the process. The atom with the heavy nucleus is split into two atoms, of roughly the same size, with lighter nuclei. The heat generated is used to boil water, creating steam. The steam turns a turbine to generate electricity.
In a fusion reactor, two atoms with lighter nuclei fuse together to form an atom with a heavy nucleus. This produces large amounts of energy. The reactor harnesses this energy to create electricity.
So, which is better?
Well Fusion is safer overall. This is because there is reduced radioactivity in operation and little high-level nuclear waste, ample fuel supplies, and increased safety.
But, so far, we have been unable to create sustainable energy using fusion in an economic manner. So, fusion reactors cost more to make and do not produce enough electricity to be a productive and economic source of energy, so we continue to use fission reactors.
