The superfuze gives U.S. nuclear–equipped subs greater destructive power with fewer warheads.

JOHN KOWALSKI/U.S. NAVY

More precise U.S. nukes could raise tensions with Russia

U.S. weapon designers may deserve a pat on the back for the sheer cleverness of an improved targeting system that is turning aging nuclear warheads into surgically precise weapons. But a new analysis warns of risky consequences. The fix, which has been developed quietly over 2 decades and is now being deployed on U.S. submarine–launched ballistic missiles, makes a small adjustment to the height at which a warhead explodes. The result is a dramatic improvement in the odds that the blast will destroy its target.

To Russia, whose defensive radars provide very short warning of a ballistic missile attack, the fix could raise fears that the United States is capable of launching a first strike that would knock out Russia’s silo-based nuclear missiles before they can be launched. That undermines nuclear deterrence and creates “a deeply destabilizing and dangerous strategic nuclear situation,” according to the report in the 1 March issue of the Bulletin of the Atomic Scientists (BAS).

The tweak to a nuclear weapon’s fuze, or detonation control, could add to tensions that are rising on several fronts. Earlier this month, U.S. officials confirmed that Russia has deployed a new missile in violation of the Intermediate-Range Nuclear Forces (INF) Treaty, a 1987 pact affecting Europe. At the same time, Russia has been leaking information about its plans for a new seagoing robotic bomb designed to hit U.S. ports. And in December 2016 the Pentagon’s Defense Science Board recommended resurrecting small, low-yield nuclear weapons of the sort that were eliminated from the U.S. arsenal (but not Russia’s) starting in 1991 because they might lower the threshold for nuclear war. President Donald Trump, meanwhile, tweeted in December 2016 that the United States must “greatly strengthen and expand its nuclear capability.” Trump’s proposed 2018 budget would boost funding for weapons-related work at the National Nuclear Security Administration by 11%, to $14.3 billion.

The targeting change is part of the nuclear stockpile stewardship plan that began a decade ago and is aimed at maintaining the U.S. nuclear deterrent without the need to develop and test new weapons. Many details of the fix are secret. But the BAS article explains how the new “superfuze” works.

Shortly before a warhead arrives at its target, the superfuze uses radar to gauge the distance remaining on the ballistic path, taking into account any drift off track. The old technology set the detonation at a fixed height at or near the ground; course errors could shift the center of the blast away from the target (see diagram). But the new system adjusts the detonation altitude so that the blast is triggered at a higher point to keep it in the target’s so-called “lethal volume.” Within this zone, the authors say, a 100-kiloton warhead will destroy a hardened structure with 86% certainty. The public has “completely missed [the superfuze’s] revolutionary impact on military capabilities,” the report’s authors contend.

To tell the superfuze story, Theodore Postol, a physicist and missile expert at the Massachusetts Institute of Technology in Cambridge, gathered technical clues from U.S. Navy patents issued in 1976 and 1984, when the idea first surfaced. Hans Kristensen, who directs the Nuclear Information Project at the Federation of American Scientists in Washington, D.C., sifted through weapons history and policy statements. And Matthew McKinzie, a nuclear physicist at the Natural Resources Defense Council in Washington, D.C., vetted the math. 

Lethal impact

Warheads that would otherwise overfly a target can now, with a fuze that adjusts the height of detonation, have a high probability of destroying the target.

CREDITS: (GRAPHIC) V. ALTOUNIAN/SCIENCE; (DATA) THEODORE A. POSTOL

The trio found few signs that U.S. policy leaders grasped the importance of the targeting fix. Kristensen points to a rare public discussion in a 1994 study by the U.S. departments of defense and energy. It concludes that, by switching out the fixed-height fuze on a garden-variety U.S. warhead—the W76, with a 100-kiloton blast—to use one that adjusts the detonation height just before arriving, the system could draw a bead on Russian targets more effectively.

More disturbing, say the BAS authors, the superfuze makes it possible to rely on the updated W76 to destroy “hardened targets” such as heavily reinforced missile silos. Sending just two superfuze-enhanced W76 warheads against a fixed missile silo, according to Postol, would be certain to destroy the missile. Previously only larger warheads, or a larger number of W76s, could reliably do so. In a stroke, the superfuze gives U.S. nuclear submarines greater destructive power with fewer warheads and more flexibility—a new capability. “It’s a pretty amazing accomplishment,” Kristensen says. 

The BAS authors calculate that by the end of 2016, U.S. weapon facilities had already produced roughly 1200 of a planned 1600 W76s armed with the superfuze. Of these, they say, “about 506” are now deployed on ballistic missile submarines. They estimate that potentially 272 such warheads, with two sent against each target, could eliminate “all 136 Russian silo-based ICBMs [intercontinental ballistic missiles].” That would leave a thousand more updated warheads—as well as more powerful warheads with the older fuze—in reserve, for use against targets like mobile missile units, command centers, or deep bunkers. Potentially hundreds more warheads in the stockpile could be updated in the future as well. 

Although the analysis cites few government sources, it is solid, says Philip E. Coyle III, a former Pentagon weapons test director now at the Center for Arms Control and Non-proliferation in Washington, D.C. Richard Garwin, an IBM physicist in Yorktown Heights, New York, who has been involved in weapons design and control for decades, also judges the analysis to be “true.” But Thomas Karako, a senior fellow at the Center for Strategic and International Studies in Washington, D.C., dismisses what he calls its “breathless exposé language” and the authors’ concerns about unnerving Russia. Karako argues that the Russian government has violated or ignored not only the INF Treaty, but also a series of other U.S.-Russian agreements made in the 1990s to scrap shorter-range nuclear weapons. He calls it a “unique neurosis of Western arms controllers” to try to block improvement of their own technology. 

Russians see this technological advance as one of many reasons to be wary of U.S. intentions, Kristensen argues. He cites a diatribe delivered by Russian President Vladimir Putin last year to a group of journalists in St. Petersburg, Russia, in which Putin complained about a risky expansion of nuclear weapons systems across the board. “Your people,” Putin said, referring to the United States and its allies, “do not feel a sense of the impending danger—this is what worries me.”

Eliot Marshall is a science journalist in Washington, D.C.