Numerous photographs of various thermonuclear bomb exteriors have been declassified. Redwing Cherokee was the first US thermonuclear bomb airdrop test. He calculated the dynamics of the first Soviet thermonuclear bomb, including predicting the yield. The first series of Operation Grapple tests involved Britain's first airdrop of a thermonuclear bomb. North Korea claimed to have tested its miniaturised thermonuclear bomb on 6 January 2016. The Mark 15 was the first lightweight US thermonuclear bomb. The People's Republic of China exploded its first thermonuclear bomb in June 1967. In 1954, Sakharov worked on a third concept, a two-stage thermonuclear bomb. It was the first test of the TX-17 thermonuclear weapon, the first deployed thermonuclear bomb. The first thermonuclear bomb, Ivy Mike, used liquid deuterium (hydrogen-2), for nuclear fusion.