Uranium.

Nuclear energy is energy in the nucleus of an atom.
It is usually in a form of heavy metal, naturally occurring in most rocks, soil, and even in the ocean! It is found in many places in the world.
Power generated from nuclear reaction is similar to that of fossil fuels because they all use heat to turn blades to generate power.
A nuclear power plant uses uranium as fuel. Uranium pallets are combined into large fuel assemblies and placed in a reactor core. 
In that chamber, uranium atoms can be made to split, or fission, to release heat. 'Fission' is the process of splitting the uranium atom to form smaller atoms.
Uranium was first discovered by a German Scientist called Martin Klaproth in 1789.
It is believed that Uranium was named after the planet Uranus, and it provides the main source of heat inside the earth.
CO2 emissions from nuclear fuel is very low, even lower than hydro-power. Nuclear power plants are very expensive to build.
In 2011, a tsunami struck Japan and caused problems with its nuclear power plant in Fukushima.
There are laws that check that countries with unclear plants comply with safety rules and also do not misuse their plants for developing nuclear weapons.