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Iran’s underground nuclear sites not immune to U.S. bunker-busters, experts say

By Joby Warrick,

Western spy agencies for years have kept watch on a craggy peak in northwest Iran that houses one of the world’s most unusual nuclear sites. Known as Fordow, the facility is built into mountain bunkers designed to withstand an aerial attack. Iran’s civil defense chief has declared the site “impregnable.”

But impregnable it is not, say U.S. military planners, who are increasingly confident about their ability to deliver a serious blow against Fordow should the president ever order an attack.

U.S. officials say they have no imminent plan to bombard the site, and they have cautioned that an American attack — or one by its closest Middle Eastern ally, Israel — risks devastating consequences such as soaring oil prices, Iranian retaliation and dramatically heightened tension in a fragile region.

Yet as a matter of physics, Fordow is far more vulnerable than generally portrayed, said current and former military and intelligence analysts. Massive new “bunker buster” munitions recently added to the U.S. arsenal would not necessarily have to penetrate the deepest bunkers to cause irreparable damage to infrastructure as well as highly sensitive nuclear equipment, probably setting back Iran’s program by years, officials said.

The weapons’ capabilities are likely to be a factor in discussions with a stream of Israeli leaders arriving in Washington over the next week. The Obama administration will seek to assure the visitors, including Defense Minister Ehud Barak and Prime Minister Benjamin Netanyahu, of U.S. resolve to stop Iran if it decides to build a nuclear bomb.

Israel has signaled in recent weeks that it may launch a preemptive strike against Iranian nuclear facilities, with little or no warning to its U.S. allies. The White House has urged Israel not to move hastily; making clear that the United States has the capacity to disrupt the Iranian program may give the Israelis reason to pause.

‘How many turns do you get?’

In arguing their case, U.S. officials acknowledged some uncertainty over whether even the Pentagon’s newest bunker-buster weapon — called the Massive Ordnance Penetrator — could pierce in a single blow the subterranean chambers where Iran is making enriched uranium. But they said a sustained U.S. attack over multiple days would probably render the plant unusable by collapsing tunnels and irreparably damaging both its highly sensitive centrifuge equipment and the miles of pipes, tubes and wires required to operate it.

“Hardened facilities require multiple sorties,” said a former senior intelligence official who has studied the formerly secret Fordow site and who agreed to discuss sensitive details of U.S. strike capabilities on the condition of anonymity. “The question is, how many turns do you get at the apple?”

U.S. confidence has been reinforced by training exercises in which bombers assaulted similar targets in deeply buried bunkers and mountain tunnels, the officials and experts said.

U.S. officials have raised the necessity of multiple strikes as they warn Israel against a unilateral attack on Iran’s nuclear installations, the officials said. While Israel is capable of launching its own bunker-buster bombs against Fordow, it lacks the United States’ more advanced munitions and the ability to wage a bombing campaign over days and weeks, American officials and analysts said.

The U.S.-Israeli rift over the urgency of stopping Iran’s nuclear progress stems in part from the belief among some Israeli officials that their window for successfully attacking Iran’s nuclear installations is rapidly closing as Tehran moves key assets into bunkers. Barak, in a speech this month, spoke of Iran’s progress in creating a “zone of immunity” for its nuclear program.

To U.S. military planners, a “zone of immunity,” if it exists at all, is years away. The Obama administration, while not ruling out a strike, regards military action as a last resort, preferring to allow more time for changing Iran’s behavior through economic and political pressure.

U.S. officials also remain unconvinced that Iran has decided to build a nuclear bomb, though they think it is pursuing the capacity to do so. Iran says its nuclear program is for peaceful energy production.

Under a mountain

Fordow is in the barren hills of northwestern Iran just outside Qom, an ancient city that is the spiritual home of the 1979 revolutionary movement. U.S. intelligence officials think tunneling began nearly a decade ago for what was intended to be a secret uranium-enrichment site that would operate parallel to the country’s much larger, declared enrichment plant at Natanz.

The CIA began monitoring the site at least four years ago, and in 2009, President Obama, flanked by other world leaders, publicly exposed the partially built facility and demanded that Tehran come clean about its intentions.

Iran acknowledged that it was building a second uranium-enrichment plant and soon allowed the International Atomic Energy Agency in for a visit. The U.N. inspectors saw a series of chambers built into the side of a mountain and connected by tunnels with thick walls and blast-proof doors. Some of the bunkers were protected by as much 200 to 300 feet of mountain.

The underground plant — not yet fully operational — is relatively small, with space for about 3,000 centrifuges, compared with the tens of thousands planned for Natanz. But analysts say it’s big enough to process the enriched uranium necessary for at least one nuclear weapon a year, should Iran decide to build them.

Iran started enriching uranium in the Fordow plant in January. A report by U.N. inspectors last week confirmed that the plant is making a purer form of enriched uranium that can be relatively easily converted to weapons-grade fuel.

Iran has publicly defended Fordow’s unusually robust fortification, citing repeated threats by Israel to destroy the country’s nuclear program.

Western analysts think Fordow has not only the protection afforded by natural rock but also additional hardening that draws on North Korean bunker-building expertise. A report last week by the Center for Strategic and International Studies, a Washington think tank, said the facility was thought to include multiple “blast-proof doors, extensive divider walls, hardened ceilings, 20-centimeter-thick concrete walls, and double concrete ceilings with earth filled between layers.”

“Such passive defenses could have a major impact” in blunting the effect of an aerial bombardment, said the report, written by Anthony Cordesman, a former director of intelligence assessment at the Defense Department and now the holder of the center’s Arleigh A. Burke chair in strategy.

Cordesman acknowledged that reports about such fortification “are often premature, exaggerated, or report far higher construction standards than are actually executed.”

Still, current and former U.S. officials acknowledge that Fordow’s fortifications far exceed those of other facilities encountered in other conflicts, including the al-Taji bunkers that shielded Iraqi leader Saddam Hussein’s command posts on the eve of the 2003 Iraq invasion.

Non-military means

The United States has an array of weapons and tactics to use against such a target — including, U.S. officials say, sabotage and other covert measures that could disable Iran’s centrifuges without resorting to a military attack.

If bombs were to be used, the Pentagon has an array of conventional bunker-busting weapons. They include the 5,000-pound BLU-122, capable of penetrating more than 20 feet of concrete or 100 feet of earth before detonating, as well as the far more powerful Massive Ordnance Penetrator, a 30,000-pound titan that can be delivered by the country’s largest strategic bombers. Although the weapon’s precise capabilities are classified, the MOP is estimated to be capable of boring through up to 200 feet of dirt and rock before exploding.

The Pentagon is investing tens of millions of dollars to enhance the MOP’s explosive punch and concrete-piercing capabilities. Some note that the weapon’s performance is partly dependent on geology, particularly the type and density of the rock through which it passes.

It’s impossible to know exactly what the impact of a bomb would be against such a difficult target. Certainly, U.S. warplanes would set back Iran’s nuclear efforts, said Michael Eisenstadt, a former military adviser to the State Department and the Pentagon.

But for how long?

“You never really know until you do it,” said Eisenstadt, director of military and security studies for the Washington Institute for Near East Policy. “We may be so close to the outer performance limits of the current technology that it becomes a roll of the dice.”

Ultimately, the ability to knock out the Fordow facility does not depend on whether a bomb penetrates the cavern where centrifuges operate, several analysts said.

“There are good outcomes short of destroying” the centrifuge hall, Cordesman said in an interview. Strikes against more accessible targets — from tunnel entrances and air shafts to power and water systems — can effectively knock the plant out of action. Repeated strikes would make Iran fearful of attempting to repair the damage, he added.

Other analysts stressed the particular vulnerability of centrifuges, machines that spin at supersonic speeds to purify uranium gas into the enriched form usable in nuclear applications. Almost anything that upsets the delicately balanced machines — from shock waves and debris to power disruptions — can render them useless, said one former Pentagon official who also requested anonymity to discuss potential Iranian targets.

“If you can target the one piece of critical equipment instead of the whole thing, isn’t that just as good?” the official said. “Even by reducing the entrances to rubble, you’ve effectively entombed the site.”

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