Buried Truths

Debunking the nuclear “bunker buster”

According to Defense Department estimates, there are perhaps 10,000 underground military installations in the world. Most, no doubt, are crude ammo dumps, but some are literally subterranean fortresses. The most dazzling is the complex beneath Russia's Yamantau Mountain, begun under Brezhnev but completed only recently; tunneled sideways into the Urals southeast of Moscow, the complex sits below thousands of feet of quartz, insulated from an American ICBM attack. China, too, has an extensive system of underground bunkers and command shelters, including hundreds of fortified missile silos, complete with living quarters, that are scattered throughout its 3.7 million square miles. In North Korea the reliance on tunnels and bunkers is even more obsessive. Underground weapons factories there are believed to employ as many as 20,000 workers; the nation has tunneled under the DMZ and into South Korea, has dug in upward of 10,000 pieces of artillery along the border, and even has built underground airstrips. Iran possesses an underground uranium-enrichment plant at Natanz that can be expanded or altered in secrecy. Libya's vast chemical-weapons plant at Tarhunah, though now apparently in disuse, is still standing, still underground, and could be quietly reopened by a lapsed Qaddafi. Without satellite surveillance showing its construction and burial, the world might still be ignorant of Syria's As-Safirah chemical-weapons factory. These installations, and countless others unknown around the world, constitute the last class of targets that America's current arsenal cannot credibly threaten with swift annihilation.

Since the Cold War, the U.S defense community has become obsessed with the problem of bunkers and how to destroy them. The solution put forward has, of course, been expensive new weaponry. Soon after the attacks of September 11, 2001, the Bush Administration made a push for new nuclear programs, the most conspicuous of which was the Robust Nuclear Earth Penetrator (RNEP), designed to destroy deeply buried bunkers. During the first presidential debate this fall, John Kerry made much of his opposition to the program. “Right now the President is spending hundreds of millions of dollars to research bunker-busting nuclear weapons,” he said. “We're telling other people, ‘You can't have nuclear weapons,’ but we're pursuing a new nuclear weapon that we might even contemplate using. Not this president. I'm going to shut that program down, and we're going to make it clear to the world we're serious about containing nuclear proliferation.”

But Kerry's gesture, while well intentioned, was largely empty. The United States already has a nuclear bunker-buster, the B61-11, which was tested during the first Bush Administration and deployed in 1997 under Bill Clinton. In the eyes of potential adversaries, the B61-11 is inherently a weapon that, in Kerry's phrase, “we might even contemplate using”: alone in our vast nuclear arsenal, the B61-11 is tailored not to deter a full-on attack by a large nuclear competitor but to preemptively strike a smaller state. If, as Kerry rightly argues, the RNEP is a provocation to non-nuclear states in their efforts to acquire weapons of mass destruction, it is hardly more so than the bunker buster we already possess.

Were the effectiveness of bunker busters to be demonstrated, the weapons might conceivably be worth the risk and expense. But in fact, even a cursory consideration of the science shows that bunker-busting nuclear weapons are a wasteful and dangerous delusion.

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A bunker-busting weapon is one that will, in principle, penetrate its target and explode only afterward. This is accomplished, again in principle, with fuses that delay detonation until a predetermined point after impact. All current bunker busters, including the nuclear B61-11 and the conventional GBU-37, are “gravity bombs,” meaning that they have no propulsion system. They rely instead on the natural acceleration of gravity, as well as on their javelin-like construction: the B61-11, for example, is roughly twelve feet in length but only thirteen inches in diameter, and is capped with a pointed nosecone. Such a design is supposed to allow bunker busters to destroy targets several hundred feet down.

In real-world conditions, though, it has been difficult to make the weapons perform as advertised. Dropped from moving airplanes, gravity bombs often strike the earth at a considerable angle, which increases the tendency of their trajectory, while underground, to bend back up toward the surface. If the angle of attack is particularly shallow, a penetrator will actually come back up out of the ground, skipping along the battlefield. And even when they do strike at a useful angle, they cannot be made to penetrate deeply enough to destroy any but the shallowest of bunkers. The Defense Department's Nuclear Posture Review for 2001 laments that the B61-11 “cannot survive penetration into many types of terrain in which hardened underground facilities are located.” This is a generous analysis: the “terrain” referred to is the hard rock under which valuable targets are almost always buried. When dropped from a height of 40,000 feet, the B61-11 was able to penetrate three meters at most into the Alaskan tundra, and not at all into hard rock (that is, without self-destructing).

The inadequacy of the B61-11 is due not to a particularly poor construction but rather to the basic limitations of bomb-making steel. In the test drops performed in Alaska, the B61-11 reached roughly 300 meters per second at impact. In order to penetrate reinforced concrete, it would need to be traveling at approximately 500 meters per second. At around 900 meters per second, the shock wave generated by the missile's slamming into the ground will deform it severely; at 1,200 meters per second, the missile will in most cases break into pieces. To penetrate granite—ubiquitous in mountainous bunkers, and believed to be common above any truly valuable bunker—a penetrator would have to attain upward of 3,000 meters per second, at which speed it would certainly be crushed.

Robert Nelson of Princeton University has demonstrated that because of the limitations imposed by the yield strength of the steel used in casings, no bunker buster can ever go fast enough to penetrate reinforced concrete deeper than five times its length without destroying itself in the process; and even this number is too high for any real-world scenario. What is more, the length of the bomb cannot be increased much, for two reasons: there are no aircraft capable of carrying a weapon much longer than the ones that are currently deployed; and as length increases, so does the tendency of the bomb to snap in two on impact.

Raymond Jeanloz, a member of the National Academy of Sciences committee advising Congress on earth penetrators, expresses frustration at the defense community's obliviousness to existing research. “A lot of the information is already in house,” Jeanloz said in an interview. “Why don't [they] come back to Congress with a really good plan that has a good chance of working, rather than asking for a bunch of money where it's not even clear [they've] reviewed the information [they] already have?” The answer lies, no doubt, in the seductiveness of the bunker-buster idea, whereby a bomb, after being dropped from the safety of 40,000 feet, eliminates in a clean, swift, and invisible blast the most intractable problem facing the U.S. military. This seems to be a fantasy too powerful to abandon.

The most stubborn part of the fantasy is that a “low-yield” bunker buster could be employed as a “clean” nuclear weapon, whose explosion and fallout would be contained underground. This aspiration is most explicitly laid out in a report from the Defense Department Science Board entitled “Future Strategic Strike Forces,” which imagines that “[p]enetration [by a nuclear bunker-buster] to a depth of 50 to 55 meters would enable disablement of 100-meter-deep underground facilities by contained 400-ton explosions.” Let us, for the moment, forget the fact that 50 meters is more than twice the depth it is physically possible for any penetrator, real or idealized, to burrow into rock. According to the government's own guidelines, drawn up during the decades in which it tested nuclear weapons under the Nevada desert, a 400-ton explosion would have to occur a full 600 meters underground in order to be “contained.” These guidelines also stipulate a carefully sealed burial shaft to contain the blast, not a maw. Even the B61-11, at its current, inadequate impact speeds, does not burrow a clean rabbit-hole in the ground but rather kicks up a crater like a meteorite; any faster-moving penetrator would do so to a still greater degree.

Even supposing that the missile's point of entry were miraculously neat, a nuclear blast at the depths a real missile could attain would invariably breach the surface of the earth, expelling a hot fallout cloud in what is known as a “base surge.” Base surges are more dangerous than traditional fallout clouds because they are more toxic, containing irradiated particles of dirt and rock. They also spread more quickly, sweeping across the surface of the earth in every direction, outward rather than upward. Bunkers are usually built in urban areas, so many thousands of deaths would be a virtual certainty. Even a 1-kiloton bunker buster—a relative firecracker, with a tiny fraction of the explosive power of the high-yield RNEP—detonated at fifty feet underground could eject about 1,000,000 cubic meters of radioactive soil.

Finally, it is entirely unclear whether even such a catastrophic blast would, as the Science Board claims, “enable disablement” of an installation. A well-designed granite bunker could withstand four times the shock produced by such an explosion. If the bunker housed weapons of mass destruction, studies have shown that a canister of, say, mustard gas could be insulated from the heat of the blast by a few meters of earth, and thereby escape being vaporized. Cushioning the canister from the shock wave is more difficult, and in the likely event that a canister is ruptured but not destroyed, the chemical agent would escape the shattered container into the earth; a split second later it would be blasted up into the air, carried away in the fallout cloud.

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During the Cold War, it eventually became clear that the war nuclear weapons were built to fight could never be joined. Yet the United States and the Soviet Union kept adding more weapons to their arsenals, long after the absolute destruction of each was at the fingertips of the other. The mechanics of Mutual Assured Destruction guaranteed that if either side launched a nuclear attack, both sides would be destroyed, and the world as well. From their former role as war-fighting weapons, nuclear weapons were reconceived as having the sole purpose of preventing their own use, so that any addition to the nuclear stockpile could be justified by the word “deterrence.” It was a tortured, grim logic, but practical.

Deterrence remains the government's public justification for building more nuclear weapons, but the term has undergone semantic drift. What today is passed off as deterrence by proponents of low-yield bunker busters and the RNEP is not, as it once was, the demonstrable ability of nuclear weapons to prevent nuclear war but the unproven power of unworkable weapons to bully other countries into abjuring any action at all deemed offensive by the United States.

Even supporters of the new projects concede that nuclear weapons don't seem to work as well in the new deterrence as they did in the old. “Sometimes they just don't deter, do they?” Joseph Howard, a nuclear scientist at Los Alamos and an early and influential voice in favor of low-yield nuclear weapons, told me recently. “We're in the new world order, and I think a very low-yield penetrator offers us some versatility. On the other hand, I don't know what to do against some of these other diaperheads. . . . The problem is that, whatever the rogue nation is, whoever the rogue leader is, it seems like it could be very, very tough to deter them with any type of rational means we deterred with during the Cold War.”

The technology of bunker busters may yet be improved, but only slightly; and what advances can be made against the hard limits of earth penetration are not enough to warrant the hundreds of millions of dollars needed to realize them. Even if earth penetrators could be made to perform at their theoretical limit, the only gain would be a temporary advantage over countries that have not yet dug bunkers at a depth that no weapon, no matter how massive, could ever reach. As soon as that comparatively easy engineering feat is completed, the nuclear weapon that spurred it on will have brought about its own obsolescence. If we are developing nuclear weapons that our government says we might use, there is no incentive for smaller countries not to go after their own weapons as quickly and quietly as possible—down in the very bunkers we are unable to destroy.

About the Author

Benjamin Phelan is a writer in New York City.