A Civilization advance.
The development of a reasonably accurate model of the structure of the atom led physicists to attempt to intentionally split one in two. When a large, unstable atomic nucleus splits--fissions--the result os two or more smaller, more stable nuclei. Other by-products of this fission are a tremendous amount of energy and fast neutrons, plus a lingering, deadly radioactivity. The first intentional use of nuclear fission was in warfare, where the incredible power of an unconfined fission reaction produced an explosion rivaled only by the largest known volcanic eruptions.
Prerequisites: Mass Production and Atomic Theory.
Allows for: Nuclear Power.