One drawback to its use as a power source is that among the heavy nuclear fragments it produces are radioactive elements with long half-lives. Fission is set off by bringing together a critical mass of an element such as Uranium-235. If a sufficient amount of Uranium is brought together an emitted neutron will most likely be captured, changing the Uranium to U-236, rather than escaping from the surface. This is called a chain reaction . In a power plant the chain reaction is modified with neutron absorbing rods.
Fusion is the source of the sun's energy and is the source of energy in hydrogen bombs.
Fusion is an inherently cleaner source of energy, but igniting it in a controlled way has proved problematic.