Nuclear power stations typically work by the fission of enriched uranium into lighter elements. A significant product of this reaction is energy release, most of this is in the form of kinetic energy of the fission products and resulting neutrons. Enriched uranium is a term used to refer to uranium with a higher proportion of U-235 to U-238 than would normally naturally occur. U-235 undertakes a fission reaction when bombarded by a neutron, whereas the heavier U-238 does not.
The increased kinetic energy of the fission products gives rise to a rise in temperature which is used to heat water to produce steam, which drives steam turbines thereby converting kinetic energy of the steam to kinetic energy (mechanical movement) of the turbine blades, an electrical generator then converts this mechanical movement into electricity.
This mass to energy conversion is a perfect example of Einstein’s mass-energy equivalence formula E=mc^2, whereby mass has been converted into energy via a nuclear fission reaction.