Which scientist discovered radioactivity in uranium?

Henri Becquerel, in 1896, and it was an accident twice over. He was testing whether phosphorescent minerals emitted X-rays after sunlight exposure, wrapped photographic plates against uranium salts, and was defeated by a run of cloudy weather in Paris. He put the lot in a drawer.

He developed the plates anyway some days later, expecting faint images, and found strong ones. The uranium had been fogging them in complete darkness with no sunlight involved, which meant the energy was coming from the substance itself. That was genuinely disturbing: it appeared to be energy from nowhere, and it took several years and the Curies' work to establish that atoms were transforming.

Marie Curie named the phenomenon radioactivity and shared the 1903 Nobel Prize with Becquerel and Pierre. Becquerel also supplied one of the earliest health warnings by carrying a tube of radium in his waistcoat pocket for a few hours and getting a burn that took weeks to heal - which he published, and which was widely read as an interesting curiosity rather than as a warning, for another forty years.