Scientists have built the first-ever thorium reactor. Thorium is both more easily accessible and less dangerous than uranium—the most common fission fuel. The system also uses molten salt instead of water to cool the fission reactor, which is reportedly much safer in the event of a meltdown.Uranium (U) is the poster child for nuclear fission reactors—the most common type of nuclear reactor we have. Most fission reactors are fueled by the isotope uranium-235 (it even made its way into The Simpso...