Henri Becquerel used uranium in his discovery of radioactivity. In 1896, he found that uranium salts could darken photographic plates even when the plates were wrapped in opaque material and kept away from sunlight.
Becquerel had been investigating whether phosphorescent substances might emit X-ray-like radiation after exposure to light. He placed uranium compounds over covered photographic plates, but cloudy weather forced him to leave some of the prepared materials in a dark drawer. When he developed the plates, they were strongly exposed anyway.
Further experiments showed that the radiation did not come from phosphorescence or sunlight. It was an intrinsic property of uranium itself, emitted spontaneously. This discovery established the phenomenon later named radioactivity and opened the field of nuclear physics.
A frequent mix-up is to credit Becquerel with discovering radium or polonium. Those elements were identified later by Marie and Pierre Curie during their investigation of unusually radioactive uranium ore. Becquerel’s original discovery involved uranium, not radium, polonium, or thorium.