There’s been a lot of talk recently about the Seymour Hersh article in the New Yorker discussing the White House’ plans for stopping Iran’s nuclear program, which claims:
One of the military’s initial option plans, as presented to the White House by the Pentagon this winter, calls for the use of a bunker-buster tactical nuclear weapon, such as the B61-11, against underground nuclear sites.
Someone at the Seed office asked the question, “Is a nuclear weapon really the only way to destroy an underground bunker?”
I decided to look into it….
I emailed several scientists who’ve worked on the problems surrounding bunker-busting nuclear weapons and asked them, leaving aside moral and political considerations, whether tactical nukes were the only feasible way to destroy an underground facility like the one at Natanz.
Several were out of town, but the general consensus among those who responded was that collapsing a reinforced underground bunker facility with conventional weapons is difficult, if not impossible.
“I suspect repeated attacks with penetrating [conventional] weapons would be needed,” said Michael May, a former director of Lawrence Livermore National Laboratory, one of the US’ three nuclear weapons labs.
“If the goal is to actually destroy any equipment buried 75 feet or more below hardened concrete, then it is correct that no conventional weapon would be able to do that,” said Robert Nelson, a physicist at Princeton and senior scientist at the Union of Concerned Scientists.
In either case, both agreed that conventional weapons could be used to collapse entrances and exits to a bunker facility, effectively sealing it off.
“Conventional weapons could at least render the site inoperative by cutting off the electricity, access and other connections,” said May.
So what would a nuclear bunker buster do to an underground bunker that a conventional one won’t?
“Nuclear weapons detonated on the ground would likely fracture or spall the concrete reinforcement,” wrote May. “Depending on the yield, the crater could engulf some of the facility itself. Any sensitive equipment within a hundred meters or more, again depending on yield, would probably be destroyed. The local fallout would deny access to the area for some time, perhaps weeks or months or longer, depending on yield and workers’ tolerance of radiation levels. There would also be more distant fallout.”
There are a number of problems with using bunker-busting nuclear weapons. First of all, collapsing a buried and reinforced bunker takes a relatively direct hit. But the intelligence we have on most underground bunkers merely indicates where the entrances, exits and ventilation shafts are, which give scant clues, at best, to the location of the bunkers themselves.
Furthermore, the idea that bunker-busters burrow down deep before exploding is misleading. The weapons are designed to penetrate the ground, but relative to the depths at which bunkers can be placed, bunker-busters actually don’t bury themselves very deep before exploding. Not deep enough, anyway, to contain the ensuing explosion that vents to the surface spreading plenty of fallout, which can disperse over a distance of up to 1000 miles.
Articles by May (with then-grad student Zachary Haldeman) and Nelson from a 2004 issue of the journal, Science and Global Security, also debunk the idea that nuclear weapons could be used to neutralize bioweapons or chemical weapons stored in bunkers.
While nuclear explosions get very hot and emit a lot of radiation, they do it in an exceedingly brief window. In bunker busting conditions, a tactical nuke would apparently be sufficient only to sterilize biological warfare agents within at best a 10-30 meter radius (May and Haldeman). That’s not much margin for error, suggesting the need for targeting information far more specific even than what it would take to collapse the bunker in the first place.
Furthermore, many chemical warfare agents would remain relatively unaffected, requiring high temperatures for far longer sustained periods for neutralization. And, in both cases, much of whatever chemical or biowarfare agents were left undestroyed would be vented into the atmosphere, causing a threat larger even than the direct effects of the nuclear weapon.
Of course, a uranium enrichment facility like Natanz doesn’t deal with biological or chemical weapons. But the uranium processed there has been converted into gaseous form for centrifugation. So I asked, what would have if that vented into the atmosphere?
“I don’t know how much UF-6 there is at Natanz, but in any case it would be a small contribution to total radioactivity and a very small one to any nuclear explosion fallout. Uranium has a very long half-life and is only mildly radioactive. On the other hand, UF-6 is chemically corrosive and uranium, like other heavy metals, is not a good thing to breathe in the first place,” said May.
Note: For more information, I recommend this presentation from the Union of Concerned Scientists, which Dr. Nelson helped put together. The preceding image is taken from this animation.