W71

The W71 nuclear warhead was a US

The W71 warhead had a yield of around 5 megatons of TNT (21 PJ). 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).[2]

The W71 produced great amounts of x-rays, and needed to minimize fission output and debris to reduce the

The W71 design emerged in the mid-1960s as the result of studies of earlier high-altitude nuclear tests carried out before the Partial Nuclear Test Ban Treaty of 1963. A number of tests, especially those of Operation Fishbowl in 1962, demonstrated a number of previously poorly understood or underestimated effects. Among these was the behaviour of x-rays created during the explosion. These tended to react with the atmosphere within a few tens of meters at low altitudes (see rope trick effect). At high altitudes, lacking an atmosphere to interact with, the mean free path of the x-rays could be on the order of tens of kilometers.^[4]

This presented a new method of attacking enemy nuclear reentry vehicles (RVs) while still at long range from their targets. X-rays hitting the warhead's outermost layer will react by heating a thin layer of the material so rapidly that shock waves develop that can cause the heat shield material on the outside of the RV to separate or flake off. The RV would then break up during reentry.^[5] The major advantage of this attack is that it takes place over long distances, as great as 30 kilometres (19 mi), which covers the majority of the threat tube containing the warhead and the various radar decoys and clutter material that accompanies it. Previously the ABM had to approach within less than 800 feet (240 m) of the warhead to damage it through neutron heating, which presented a serious problem attempting to locate the warhead within a threat tube that was typically at least a kilometer across and about ten long.^[4]

Bell received a contract to begin conversion of the earlier

To maximize the production of x-rays, the W71 is reported to have used a gold tamper,^[citation needed] rather than the usual depleted uranium or lead. The lining normally serves the primary purpose of capturing x-ray energy within the bomb casing while the primary is exploding and triggering the secondary. For this purpose, almost any high-Z metal will work, and depleted uranium is often used because the neutrons released by the secondary will cause fission in this material and add a significant amount of energy to the total explosive release. In this case the increase in blast energy would have no effect as there is little or no atmosphere to carry that energy, so this reaction is of little value. The use of gold may have been to tailor its transparency to x-rays.^[6] In Congressional testimony on potential dismantling of the W71, a DOE official described the warhead as "a gold mine".^[7]

In 2008, the United States Department of Energy declassified the fact that the radiation case of the W71 contained thorium metal.^[8]

Under good conditions, the W71 warhead had a lethal exo-atmospheric radius as much as 30 miles (48 km),^[9] although it was later stated to be 12 miles (19 km) against "soft" targets, and as little as 4 miles (6.4 km) against hardened warheads.^[10]

There were 30 to 39

Prior to the W71 test, a calibration test known as

The W71 was lowered 6,150 feet (1,870 m) down a 90-inch-diameter (2.3 m) borehole into a man-made cavern 52 feet (16 m) in diameter. A 264-foot-long (80 m) instrumentation system monitored the detonation. The full yield test was conducted at 11:00 am 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 1-mile-wide (1.6 km) and 40-foot-deep (12 m) crater formed two days later.

• Amchitka Milrow and Cannikin tests