Iran's nuclear sites are not impregnable, experts say

The Washington Post -- Western spy agencies for years have kept watch on a craggy northwest Iran peak that houses one of the world's most unusual nuclear sites. The facility, known as Fordow, is built into mountain bunkers meant to withstand 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 of 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 have cautioned that a U.S. attack -- or one by its closest Mideast 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 remains far more vulnerable than generally portrayed, current and former military and intelligence analysts said. 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, likely setting back Iran's program by years, officials said.

The weapons' capabilities are likely to factor into discussions with a stream of Israelis 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.

White House officials are worried that Israel may launch a preemptive strike against Iran with little or no warning, a move that U.S. officials argue would be do little to stop Iran's nuclear ambitions and may, in fact, deepen Iran's determination to become a nuclear state.

In arguing their case, U.S. officials acknowledged some uncertainty over whether even the Pentagon's newest bunker-buster weapon -- the Massive Ordnance Penetrator, or MOP -- 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 likely render the plant unusable by collapsing tunnels and irreparably damaging 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 agreed to discuss sensitive U.S. strike capability details on condition of anonymity. "The question is, how many turns do you get at the apple?"

U.S. officials have raised the necessity of multiple sorties as they warn Israel against a unilateral strike against Iran's nuclear installations, the officials said. While Israel is capable of launching its own bunker-buster bombs against Fordow, it lacks both the United States' more advanced munitions and the capability of waging a sustained bombing campaign over days and weeks, U.S. officials and analysts said.

The U.S.-Israeli rift over the urgency of stopping Iran's nuclear progress stems in part from some Israeli officials' belief that their window for successfully attacking Iran's nuclear installations is rapidly closing, as Tehran moves key assets into bunkers. Mr. 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, the "zone of immunity," if it exists at all, is still years away. The Obama administration, while not ruling out a future strike, regards military action as a last resort, preferring to allow more time for changing Iran's behavior by economic and political pressure.

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

Fordow is in the barren hills of northwestern Iran just outside Qom, the ancient city that is the spiritual home of the 1979 revolutionary movement. U.S. intelligence officials believe that 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 Barack Obama, flanked by other world leaders, publicly exposed the partially built facility and demanded that Iran specify its intentions.

Iran acknowledged that it was building a second uranium-enrichment plant and soon allowed in the International Atomic Energy Agency 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 bunkers were protected by as much 200 to 300 feet of mountain.

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.

Western analysts believe that Fordow has not only protection afforded by natural rock, but also additional hardening that draws on North Korean bunker-building expertise. A report last week by Washington's Center for Strategic and International Studies, or CSIS, said Fordow was believed 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 impact of an aerial bombardment, said the report, written by Anthony Cordesman, a former Defense Department director of intelligence assessment and now holder of CSIS's Arleigh A. Burke Chair in strategy.

Against such a target, the United States 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 MOP, a 30,000-pound titan that can be delivered by the nation's largest strategic bombers.

While the MOP's precise capabilities are classified, it is estimated to be capable of boring through as much as 200 feet of dirt and rock before exploding.

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

"There are good outcomes short of destroying" the centrifuge hall, Mr. 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 will also make Iran fearful of attempting to repair the damage, he added.

Other analysts stressed the 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 delicately balanced machines, from shock waves and debris to power disruptions, can render them useless, said a former Pentagon official who also requested anonymity to discuss potential Iranian targets.