So far as we know, the US government has not yet built nuclear "bunker-buster" bombs. Bunker busters are aimed at destroying facilities located underground. The US does have conventional bunker busters in its arsenal, such as the Guided Bomb Unit-28 (GBU-28):
GlobalSecurity.org describes the GBU-28 this way:
The Guided Bomb Unit-28 (GBU-28) bomb is designed to penetrate hardened targets before exploding, capable of penetrating 100 feet of earth or 20 feet of concrete. The GBU-28 was initially developed in 1991 for penetrating hardened Iraqi command centers located deep underground. This "bunker buster" was required for special targets during the Desert Storm conflict and was designed, fabricated and loaded in record time. The GBU-28 is a laser-guided conventional munition that uses a modified Army artillery tube as the bomb body. They are fitted with GBU-27 LGB kits, 14.5 inches in diameter and almost 19 feet long. The operator illuminates a target with a laser designator and then the munition guides to a spot of laser energy reflected from the target.
Some considerable confusion exists in the literature concerning the weight of this bomb. Although nominally a 5,000 pound bomb, it appears that the actual weight is somewhat less than this ...
This new system was needed to deal with deeply buried command and control bunkers that were beyond the reach of existing systems.
The bunker buster concept earned a certain notoreity in the Gulf War of 1991:
Initially it was reported that one of the GBU-28s was used to destroy a bunker in a residential neighborhood of Bagdad, that turned out to be a shelter for hundreds of key family members of the Ba'ath Party elite. Originally over 500 casualties were reported, but that was later changed to 314. After the war it was reported that an F-117 dropped a Penetrator I-2000 on this target, and that it was not one of the GBU-28's from an F-111.
GlobalSecurity.org also notes:
Raytheon MissileSystems, Tucson, Ariz., was awarded October 19, 2005 an $18,478,880 firm fixed price contract modification for 140 production Lot 2 of the Guided Bomb Unit-28C/B guidance control units and tail kits. The Guided Bomb Unit-28C/B, or Enhanced Paveway III, provides the Air Force with an improved aerial delivery capability for the 5000-Pound class BLU-122 warhead. It possesses a global positioning system aided laser guidance capability to be compatible with F-15E and B-2A aircraft platforms. The GBU-28C/B, with BLU-122 warhead, will improve the lethality, survivability, and penetration over the previously produced GBU-28B/B weapon system. At this time, $18,478,880 has been obligated. This work will be complete May 2007.
I suppose $18 mil for 140 bombs is a bargain by Pentagon standards. Although don't fret for the stockholders of Raytheon; I'm sure they get a nice profit margin on these devices.
Don't misunderstand me here. Conventional bunker-busters are necessary weapons. Where and how they are used determines their effects. And I'm also not suggesting there's anything inherently wrong with Raytheon building weapons and making a profit on them. That's the way the system works.
But I also think it's long past time that the wider public start paying closer attention and bringing a more critical eye to how the business of war is conducted.
Another bunker buster, the EGBU-28, has enhanced GPS guidance. GlobalSecurity.org writes of it:
The new EGBU-28 (the “E” being for enhanced) replaces the GBU-37. This latest version of the “bunker buster” uses the Global Positioning System for guidance so that it can be dropped with accuracy at higher altitudes in foul weather. The amount of rock and concrete that the EGBU-28 can penetrate is classified, but Major Dick Wright, who was the weapon’s test manager in 1991, said that the older version “went through 20 feet of concrete like butter” and when dropped onto hard ground, penetrated down to 100 feet. ...
The New York Times reported on 10 October 2001 that the GBU-28 had been used in combat in Afghanistan as part of Operation Enduring Freedom. Strikes conducted on 09 October, the third day of air operations, reportedly included the first use of the 5,000-pound bomb. Prior to this time the only operational aircraft known to be certified to carry the GBU-28 was the land-based F-15E fighter/bomber. However, there were no reports that the F-15E was participating in the air campaign, and it was reported by CNN beginning on 11 October that the GBU-28 was being delivered from the B-2 stealth bomber.
The nuclear bunker buster would be a much more powerful weapon. It would also represent a new generation of nuclear weapons, and therefore a new step in the nuclear arms race that, tragically, did not end with the Cold War. Michael Levi discussed the promise and the very real problems with such a nuke in Nuclear Bunker Buster Bombs Scientific American Aug 2004 (link is behind subscription).
There are several problems with the bunker buster nuke, not least of them political and diplomatic ones. Levi's article concetrates more narrowly on military drawbacks.
Producing a new generation of nuclear weapons is destabilizing to the existiong non-proliferation regime. Other countries come immediately under pressure to build their own versions and/or counter-measures.
Levi explains why a nuclear bunker-buster has some practical appeal. Countires like Iran have been undergrounding key facilities such as bunkers housing ABC (atomic, biological, chemical) weapons and command centers. Kinetic bombs - ones that rely on the force of gravity and a sharp point for their penetrating power, can go only so deep.
He discusses the GBU-28, which he refers to as the BLU-113, or the Big-BLU, which refers to an 8-inch gun barrel body used to deliver the device. The BLU-113/GBU-28 would strike the earth at 450 meters/second when dropped from 40,000 feet. If enchanced with rocket propulsion, it could reach the maximum speed of 900 m/sec. Levi explains that with available materials, a speed greater than900 m/sec would cause the bomb to disintegrate on contact with the surface.
At 450 m/sec, the BLU-113/GBU-28 could penetrate 15 meters in limestone ground; at 900 m/sec, 30 meters. The advantage of detonating the bomb inside the earth rather than on the surface is that more of the bomb's energy strikes the target. As Levi explains:
This new breed of burrowing nuclear weapons would be more effective against subsurface targets because detonation underground enhances the ground shock a bomb can create. Much of the shock wave produced by a device exploding at or above the earth's surface reflects off the ground-air interface. That rebound sends the shock skyward, away from the buried bunker. If the same-size bomb is set off below the surface, most of the shock propagates directly to the target. Even a small amount of penetration can make an enormous difference in destructive power: a onc-kiloton device exploded one meter underground delivers more shock to a buried base or stockpile than a 20-kiloton bomb detonated aboveground.
This may seem counter-intuitive to us non-physicists. But, Levi writes, "Hard strata transmit schock waves more efficiently than loose earth". Loose earth, of course, is easier to penetrate. But the advantage of an underground detonation is considerable:
A one-kiloton bomb detonated five meters deep in granite, for example, will demolish well-built bunkers 35 meters underground, whereas a 10-kiloton device exploding under one meter of soil has a destructive radius of only five meters.
Which comes back as consideration of using nuclear weapons always does, to their essential military appeal: they pack a much greater "bang" than any similarly-sized conventional weapon possibly could.
But now come the complications. One agument for the bunker-buster that I'm sure we'll hear over and over is that it minimizes radiation. It's always important dealing with these subjects to no slip too far into the eerie world of the nuclear Jesuits. Nuclear bunker busters might produce less fallout than a nuclear weapon detonated on the surface. Conventional bombs do not produe radioactive fallout.
(I use the term "nuclear Jesuits", which I first saw in a quotation from the late John Kenneth Galbraith, to refer to the theorists of nuclear war.)
But rejoining the scenarios of the nuclear Jesuits, a successful detonation of a nuke burrowed into the earth creates a tunnel to the surface as it burrows into the earth. When it explodes, it also creates an explanding cavity around itself, some of which is very likely to burst through the surface. And radioactive waste will spew into the air.
Levi gives a sketch of what that means:
But no matter how deep a bomb digs, it will leave a hole behind it through which fallout can escape. These figures leave little doubt that use of a nuclear bunker buster would spread fallout; in an urban area, a kiloton-size bomb could kill tens of thousands.
Downsized weapons would probably offer greater utility in less populated areas. Consider a one-kiloton bunker buster bomb set off at relatively shallow depth-less than 10 meters - in wind conditions averaging 10 kilometers an hour. Although the numbers will vary slightly depending on detonation depth, geology and weapon details, the basic results will be similar. If it takes six hours for people in the vicinity to evacuate, then calculations show that nearly everyone downwind of the blast within approximately five kilometers would still be killed by fallout, and half the inhabitants eight kilometers away would die. Only if the nearest population center is 10 or more kilometers downwind will the fallout lead to few if any rapid fatalities.
Even if the number of casualties were small, extensive areas adjacent to the blast zone would be contaminated with radioactivity. As an indicator of what level of exposure people might accept, after the Chernobyl disaster in Ukraine, authorities permanently closed off the region in which residents would be expected to receive a radiation dose of two rems in their first year (a rem is a unit of damage to tissue, averaged over the body, from ionizing radiation). By those standards, in the above scenario everyone downwind within 70 kilometers would need to relocate at least temporarily. Thirty kilometers downwind, people could return after a month; 15 kilometers downwind, they would have to wait a year to return home.
Obviously, US soldiers would also be restricted in what they could do in the immediate vicinity of ground zero. (Maybe the nuke theorists will call this "underground zero".)
Levi describes the conventional alternatives to the nuclear bunker busters. This is Scientific American, not Rush Limbaugh or James Dobson, so he's not tossing absolute claims out carelessly. Bunker busters of either type are new techn ologies, so there's so inherent uncertainly about them.
But conventional alternative can be made more effective by such approaches as these: an "extending penetrator" bomb that increases its length before hitting the ground, thus increasing their penetrating depth; a Deep Digger bomb that w2ould include a rapid drilling device on the end (based on "dry drilling" techniques used for oil), and, the Hard Target Smart Fuse which would detonate the device only when it actually reached the bunker.
Levi makes a couple of other vital points that Congress and the public should keep in mind about bunker busters. One is that defenses can be devised against them. He writes that "there are buners - those below a few hundred meters - too deep to be annihiliated by the biggest nuclear bombs".
Another vital point is that in many cases, simply *sealing the tunnels to the bunkers* will be as militarily effective as destroying the bunker itself. And the tunnel entraces "will be far easier to find than the underground bunkers themselves".
This wouldn't be as beneficial in the preventive-war scenario that the administration is currently using to threaten Iran. If the idea is to simply wreck certain facilities in an expanded version of Israel's Osirik strike of 1981 on Iraq, with no follow-up invasion or combat on the ground, sealing the tunnels would be of limited effectiveness, it would seem. But when dealing with nuclear weapons especially, developing anything new for the purpose of facilitating preventive war is a bad idea, and wrong.
Even in a legitimate, defensive war, sealing tunnels wouldn't be sufficient in all cases. Levi writes:
Against headquarters facilities, the simple sealing approach will probably not succeed. Even if all the entrances are blocked, communications lines may survive, permitting the base to continue to function. To destroy or cripple power and communications lines leading into the target, U.S. forces may need to physically bomb or to employ the Pentagon's E-bomb, which emits a powerful microwave pulse. It is hard to imagine a scenario, however, in which a nuclear weapon would be of much greater value than a conventional bomb for merely taking a bunker out of service.
Levi also addresses the use of bunker busters against stores of biological or chemical weapons. To put it briefly, both nuclear and conventional weapons can generate enough sustained heat to neutralize biological weapons. But neither by themselves can reliably neutralize chemical weapons; some specialized neutralizing agent would have to be included in the bomb to do that . The force of a nuclear blast would be more likely to release chemical agents into the atmosphere than conventional explosives.
This makes me wonder whether it would ever make sense to blow up an underground bunker thought to house chemical or biological weapons, as opposed to sealing the tunnels.
Levi concludes by explaining that making bunker buster nukes reliable is unlikely without resuming nuclear testing, further undermining international nuclear nonproliferation arrangements. Unfortunatley, doing just that has been an obvious is unstated goal of the Bush administration hardliners who have mostly dominated foreign and military policy so far.