Nuclear energy is what you get when you split an atom. The process that produces the heat in a nuclear plant is the fissioning or splitting of uranium atoms. That heat boils water to make the steam that turns the turbine-generator, just as in a fossil fuel plant. The part of the plant where the heat is produced is called the reactor core.
Nuclear power depends on a chain reaction to make heat.
In a chain reaction, particles released by the splitting of the atom go off and strike other uranium atoms, splitting those. Those particles given off split still other atoms in a chain reaction. This chain reaction gives off heat energy. This heat energy is used to boil water in the core of the reactor. So, instead of burning a fuel, nuclear power plants use the chain reaction of atoms splitting to change the energy of atoms into heat energy.