The primary type of energy source used in nuclear power is nuclear fission. This process involves splitting the nuclei of heavy atoms, typically uranium-235 or plutonium-239, to release a significant amount of energy in the form of heat. This heat is then used to produce steam, which drives turbines to generate electricity. Nuclear fission is a highly efficient energy source, producing large amounts of energy from relatively small amounts of fuel, and it does not emit carbon dioxide during operation, making it a low-emission energy option compared to fossil fuels.