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The W-71 nuclear warhead was a US thermonuclear warhead developed at Lawrence Livermore National Laboratory in California and deployed on the LIM-49A Spartan, a component of the Safeguard Program, an anti-ballistic missile defense system briefly deployed by the US in the 1970s.
The W71 warhead had a yield of around 5 megatons of TNT (21 PJ) optimized for the production of thermal X-rays and minimal debris in an exoatmospheric detonation. The target would be damaged or destroyed by near-instantaneous x-ray vaporization of its surface resulting in an inward-propagating shock wave. The warhead package was roughly a cylinder, 42 inches (1.1 m) in diameter and 101 inches (2.6 m) long. The complete warhead weighed around 2,850 pounds (1,290 kg).
Probably because of unique design features associated with x-ray production and emission from the thermonuclear component of the bomb, it was decided to conduct a full-yield test of the W71. A prototype was successfully tested in Project CANNIKIN in the world's largest underground nuclear test, on Amchitka Island in the Aleutian Islands off Alaska. The W71, mounted in a Spartan missile body, was lowered 6,150 feet (1,870 m) down a 90 inches (2.3 m) diameter borehole into a man-made cavern 52 feet (16 m) in diameter. A 264 feet (80 m) long instrumentation system monitored the detonation. The full yield test was conducted at 11:00am local time November 6, 1971 and resulted in a vertical ground motion of more than 15 feet (4.6 m) at a distance of 2,000 feet (610 m) from the borehole, equivalent to an earthquake of magnitude 7.0 on the Richter scale. A mile (1.6 km) wide and 40 feet (12 m) deep crater formed two days later.
Film of the test has been declassified and can be seen in the third of the Welcome To Ground Zero.
Thirty units were produced in 1974 and 1975. The weapons went into service, but were then taken right back out of service in 1975 and the warheads stored until 1992 when they were dismantled.