Wilson was a United States nuclear weapons-development test detonated on 18 June 1957 at the Nevada Test Site, in Area 9a on Yucca Flat. The device was suspended beneath a balloon at a height of roughly 152 metres and produced a yield of about 10 kilotonnes, slightly above its 8-kilotonne prediction. It was sponsored by the University of California Radiation Laboratory (UCRL, later Lawrence Livermore) as the fourth detonation of Operation Plumbbob, the sixth and largest test series conducted at the Nevada site. The shot was the proof test of the XW-45 "Swan" device, a compact, lightweight gas-boosted fission primary built around a composite plutonium–uranium pit. The Swan was developed for use inside thermonuclear weapons, and the same primary design was fired in several later Plumbbob shots within mock thermonuclear assemblies. Like other UCRL events in the series, the test was named after a North American mountain. Wilson is best understood within the toll of the series it belonged to. Operation Plumbbob released about 58,300 kilocuries of radioiodine (iodine-131) into the atmosphere — more than twice the amount of any other continental U.S. test series. The National Cancer Institute later estimated that this exposed the American public to roughly 120 million person-rads of thyroid tissue radiation, about a third of the total from all continental testing, and projected that it would eventually cause on the order of 38,000 thyroid cancers and some 1,900 deaths. The series also brought around 16,000 military personnel to the site for the Desert Rock VII and VIII exercises, manoeuvring close to ground zero.