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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 missile, a component of the Safeguard Program, an anti-ballistic missile(ABM) defense system briefly deployed by the US in the 1970s. The warhead was designed to intercept a cloud of incoming enemy warheads after their carrying missile's boost phase, at altitudes comparable to low earth orbit where there is practically no air, an environment which permits the passage of essentially all of the X-rays generated by the nuclear explosion. As part of the Safeguard ABM program the W71 was to serve as the first line of defense and as a complement to the other Safeguard missile-warhead pairing, the Sprint missiles enhanced neutron radiation warhead, the W66.
The W71 warhead had a yield of around 5 megatons of TNT (21 PJ) optimized for the production of thermal soft 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). The Spartan warhead(W71) produced great amounts of x rays, and needed to minimize fission output and debris to reduce the radar blackout effect that fission products and debris produce on Anti-Ballistic Missile radar systems.
Probably because of unique design features associated with x-ray production and emission from the thermonuclear component of the warhead, it was decided to conduct a full-yield test of the W71, in preparation for this full-yield test, a calibration test known as Milrow of Operation Mandrel was conducted in 1969. Despite political and pressure group opposition to both tests, and in particular the full yield W71, coming from then US Senator Mike Gravel and the nascent Greenpeace, a Supreme Court decision led to the test shot getting the go-ahead, and a W71 prototype was successfully tested in Project CANNIKIN of Operation Grommet 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 Atomic Journeys documentaries 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.
The design predecessors to the W71 were the explosive devices developed for Operation Plowshare-the US development of possible peaceful uses of nuclear explosives- which similarly required "clean bombs"/reduced outputs of residual radiation by reducing the "Fission fraction", or the quantity of fission products per kiloton and reducing the neutron induced radioactivity of the device casing and the soil to make peaceful applications such as earth moving environmentally practical. The design approaches to reduce residual radiation in these early efforts proved critical to the development of warhead concepts that were deployed on the Spartan missile system in the early 1970s.
Using the same principle of nuclear detonation generated soft x-rays to destroy incoming missiles and warheads, the Project Excalibur x-ray lasing concept of the Strategic Defense Initiative can be seen as the Spartan's W71 conceptual descendant.