Dr Mark Mills (1917-1958), American nuclear engineer, giving evidence before the Congressional Joint Atomic Energy Committee hearings on atomic radioactive fallout. Dr Mills was also a leading US weapons designer and deputy director of the University of California's radiation laboratory at Livermore at the time of the hearings. He was concerned that in the event of a nuclear reactor accident, a chemical reaction (rather than a nuclear one) could destroy the reactor, sending radioactive material into the environment. Photographed on 27 May 1957.