Nuclear energy is carbon free, which makes it an attractive and practical alternative to fossil fuels, as it doesn’t contribute to global warming. We also have the infrastructure for it already in place. It’s nuclear waste that makes fission bad for the environment. And it lasts for so long, some isotopes for thousands of years. Nuclear fuel is comprised of ceramic pellets of uranium-235 placed within metal rods. After fission takes place, two radioactive isotopes are left over: cesium-137 and strontium-90.