America’s loose nukes

PAUL CROCK/AFP/Getty Images

Last week, the Pentagon admitted that a B-52 had mistakenly flown nuclear-armed cruise missiles across the United States. And worse, for almost fourteen hours no one—at the base of departure, on the bomber itself, or at the base of arrival—had any idea something was wrong. Officials have assured the public that there was no danger of a nuclear explosion, even if the plane had crashed.

The specific warheads carried by U.S. cruise missiles belong to the W80 family, in this case the W80-1. (Other versions of the W80 are designed for use with Tomahawk cruise missiles, which are launched from submarines.) There are about 1,450 of these warheads in the active stockpile, with another 360 or so in the inactive stockpile. They have “dialable” (variable) yields of up to 150 kilotons, or about 10 times that of the Hiroshima bomb. And, as mentioned by the Pentagon, they have several safety measures built in to prevent accidental detonation.

First, the actual detonation system is physically protected by an “exclusion zone,” which isolates it from electric (and to some extent physical) shocks. The exclusion zone can be connected to the rest of the warhead’s electronics by a “strong link,” which does not physically connect until the warhead is armed.

An accident—fire, lighting strike, crash, etc.—could breach either or both of these safeguards, so the electronics inside the exclusion zone also contain safeguards, called “weak links.” These are electronic links designed to fail under lower stress than either the exclusion zone or the strong link. This ensures that, for instance, if the exclusion zone collapses, the weak links will as well and the nuclear core will remain inert.

And beyond those nested safety systems, most U.S. warheads have other safeguards, including insensitive high explosives that will not detonate easily due to mechanical shock. The biggest worry with this incident was not technical, but organizational: How did nuclear warheads get loaded onto a plane and flown across the country before anyone even noticed they were gone?