The U.S. is working on a new class of deep-penetration bombs for potential use against underground bunkers and weapons complexes. Among the lead designers is Boeing.

As U.S. concern about the nuclear ambitions of North Korea and Iran have grown, weapons designers have been working on bombs capable of destroying those countries’ underground nuclear sites.

Some nuclear facilities in both countries are believed to be buried deep, so U.S. designers — including some from Boeing — are developing a new class of bombs for plowing through hundreds of feet of earth and concrete before detonating.

The latest of these weapons is the MOP — short for Massive Ordnance Penetrator — built by Boeing’s Advanced Systems unit in St. Louis. The 20-foot-long bomb that weighs 30,000 pounds — much heavier than the 21,000-pound MOAB, or Massive Ordnance Air Burst bomb, unveiled in the prelude to the Iraq war. The MOAB was designed by the Air Force Research Lab and is built at the McAlester Army Ammunition Plant in Oklahoma.

The MOP is an unusual bomb in that more than 80 percent of its 15 tons is in its casing, while it carries only 5,300 pounds of explosive payload.

The MOP was successfully tested earlier this month at White Sands, N.M. A Boeing handout last week made clear the weapon’s likely targets:

“The weapon’s effectiveness against hard and deeply buried targets allows the warfighter to hold adversaries’ most highly valued military facilities at risk, especially those protecting weapons of mass destruction,” said Bob McClurg, Boeing Advanced Systems MOP program manager.

At Wired magazine’s defense blog, Danger Room, David Hambling says that the MOP has much more penetrating power than military’s best current non-nuclear option, the 5,000-pound BLU-113, which can penetrate 22 feet of concrete:

MOP will go a lot deeper — 200 feet of 5,000 psi concrete. MOP pulls it off by not being all that explosive — less than 20% by weight, compared to almost 90% for the MOAB. That’s because bunker-busting bombs need very thick casings to survive the effects of impact.

The U.S. also has nuclear “bunker busting” weapons, in particular the B61-11, which was developed in the 1990s.

The recent White Sands MOP test was static — that is, the bomb was placed inside a tunnel and detonated. Testing later this year is to include dropping the bomb from a B-52 bomber onto an underground complex. Assuming it performs as expected, the government will decide whether to order and stockpile some of the bombs.

The composition of the MOP casing has not been released, nor has the type of explosive it uses. Hambling says it’s possible that the explosive could be “a thermobaric mixture like that used in the BLU-118 ‘cave buster.’ This type of explosive produces a sustained blast which travels much more effectively through underground complexes, as the blast reportedly “flows” around corners. To say that over two and a half tons of it would do a tremendous amount of damage in a confined space is something of an understatement.”

The U.S. already has nuclear “bunker busting” weapons, in particular the B61-11, which was developed in the 1990s.

The MOP is being developed in part because there are at least two major problems with nuclear weapons:

• A nuclear attack on Iran or North Korea would carry a heavy political and human price because radioactive fallout would spread hundreds of miles from the impact site. Hundreds of thousands of people would be exposed to potentially fatal levels of radiation. This animation, developed by the Union of Concerned Scientists and based in part on an Air Force study, says non-lethal, but significant fallout from a bomb dropped on Iran would drift over Afghanistan, Pakistan and India — all at least nominal U.S. allies.

• No U.S. nuclear weapon are known to possess deep-penetration capability because their casings are too light.

In his blog, Noah Schactman, now editor of Danger Room, cites a Union of Concerned Scientists assessment of the likely effect of a B61-11 nuclear weapon, whose explosive force can be varied depending on need:

For a penetration depth of three meters and a yield of 0.3 kilotons, the B61-11 could destroy a target buried under roughly 15 meters [= 50 feet] of hard rock or concrete. For the same penetration depth and the maximum yield of 340 kilotons, the destruction depth would be roughly 70 meters [ =210 feet ] for a hardened target.

If these estimates are correct — and since they’ve never had a real-world test they are somewhat speculative — a high-yield B61-11 explosion likely would have destructive force on a deeply buried target similar to that of a MOP, which uses conventional explosives with a small fraction of the B61-11’s nuclear power.

Boeing’s MOP is by no means the only or most sophisticated deep-penetration weapon being studied. The concept for it is the simplest, though, and thus may prove the easiest to develop quickly.

Other concepts under study include:

Lockheed Martin’s Kinetic Energy Cavity Penetrator Weapon, which would rely on a phenomenon called “terradynamic cavitation” to drastically reduce drag as the bomb rips through earth and concrete to its target.

The Deep Digger, would sport a 7-barrel rapid-fire cannon in its nosecone that in theory would pulverize earth and concrete in its path as it burrows into the earth. Its developers speculate that a synchronized array of 20 Deep Diggers could produce an artificial earthquake that could collapse tunnels and other underground facilities. The Digger is a joint project from several weapons developers, including designer ARTEC.

The size of Boeing’s MOP presents a delivery challenge. There are only two planes in service that could deliver the bomb against significant air defense, the Northrop Grumman B-2 stealth bomber, which would have to be modified, and Boeing’s aging B-52.

Potential targets for MOP or other deep penetrators would include the Iranian underground centrifuge cascade at Natanz and the heavy-water reactor complex under construction at Arak.

The Natanz centrifuges produce enriched uranium that contains more of the fissionable isotope U-235. At low enrichment levels, the uranium can be used as fuel in nuclear-power reactors (which Iran asserts is its goal); at high enrichment levels it can be used for nuclear weapons. The Institute for Strategic and International Studies (ISIS) has some satellite imagery of early construction work at Natanz here.

A February report by the U.N. International Atomic Energy Agency confirmed that Iran is continuing to enrich uranium at Natanz in violation of U.N. resolutions.

Heavy-water reactors can more quickly generate plutonium, an element primarily useful in nuclear weapons, than the much more common light-water design used by most commercial nuclear plants. Iran maintains that the Arak reactor will be used to develop special isotopes for such purposes as radiation therapy. A recent ISIS brief on Iran’s heavy-water complex at Arak shows how construction has progressed over time..

In addition, Iran has other nuclear facilities. The World has a useful roundup of the major ones here, as well as a timeline to the continuing dispute between Western countries and Iran over the latter’s nuclear intentions.

Besides major facilities, Iran is believed to have dozens of other nuclear-related sites, but little is known about them.

Knowledge is even sparser about North Korea’s nuclear program. The isolated nation conducted a nuclear test in October 2006, though there still are questions about whether it was fully successful. In any case, North Korea is now widely believed to have perhaps a half-dozen nuclear weapons.

The authoritative has several pages of information about the North Korean nuclear program. This BBC map shows the locations of some major North Korean facilities.

The likelihood of MOP, or any of the other advanced bunker-busters, being used against Iran or North Korea is impossible to guess.

An attack on North Korea’s nuclear facilities seems least likely. If North Korea does possess even a handful of nuclear weapons it might be able to retaliate against South Korea and Japan. And even if the North has no nukes, or lost them in a U.S. strike, it is judged capable of inflicting hundreds of thousands of casualties in the Seoul area with conventional artillery and rockets.

In addition, North Korea has at least tentatively agreed to scrap its nuclear program in continuing talks with the United States, South Korea, Japan, China and Russia.

Iran, on the other hand, continues to assert its right to develop a nuclear capability — peaceful, it insists — despite United Nations insistance that it halt uranium enrichment.

It is unclear how close Iran is to having enough nuclear material to make a bomb, but ABC News reported recently that,

Iran has more than tripled its ability to produce enriched uranium in the last three months, adding some 1,000 centrifuges which are used to separate radioactive particles from the raw material.

The development means Iran could have enough material for a nuclear bomb by 2009, sources familiar with the dramatic upgrade tell ABC News.

If true, this will add new urgency to efforts to convince Iran to halt its nuclear program — and, most likely, to weapons programs like MOP.

Tom Brown is associate editor of and can be reached at