In 1896, a French scientist named Henri Becquerel discovered that uranium is a source of energy when a uranium-bearing crystal in his lab unexpectedly fogged a photographic plate. According to Duke University, the crystal and the plate had both been kept in an opaque box, ruling out other energy sources.
Nuclear energy as a viable technology has its origin with Hungarian physicist Leo Szilard. Szilard first articulated the concept of the atomic chain reaction in 1933, according to Wikipedia. Along with Enrico Fermi, Szilard would eventually acquire a patent for the first nuclear reactor. In 1939, along with Albert Einstein, he co-authored a letter to President Roosevelt that eventually inspired the Manhattan Project.