FAS Public Interest Report - Fire In The Hole

by Michael Levi

Editor's Note: The following is adapted from Strategic Security Project Director Michael Levi's working paper, "Fire in the Hole: Nuclear and Non-Nuclear Options for Counterproliferation". The complete paper can be found here. For an earlier FAS take on the issue, see Robert Nelson's "Low-Yield Earth-Penetrating Nuclear Weapons" in the Public Interest Report, 54:1, Jan./Feb. 2001.

Introduction and Summary

At the end of the Gulf War, many military thinkers began to argue that precision guided munitions had made tactical nuclear weapons obsolete. Television images of bombs threaded through chimney pipes made graphic the revolutionary advance in guidance technology. With weapons able to explode within meters of their targets, the massive destructive radii of nuclear weapons were apparently unnecessary.

That argument didn't last long before the counterattack. Reacting to America's military revolution, Iraq and others began to build underground, where their facilities became easier to conceal and much harder to destroy. The proliferation of chemical and biological weapons, often built in what looked like typical industrial facilities, presented new challenges in finding targets and avoiding collateral damage during attacks. These requirements were identified by nuclear weapons designers as potential missions for new nuclear weapons.

In March 2002, portions of the U.S. Nuclear Posture Review leaked to the public revealed a renewed interest in developing a range of specialized nuclear weapons. The review identified capabilities shortfalls in attacking hardened and deeply buried targets, and facilities containing weapons of mass destruction, and suggested that nuclear weapons might have unique capabilities to address these threats. It revealed a new program to build a Robust Nuclear Earth Penetrator, a modified nuclear weapon designed to destroy deeply buried targets. But it went further, noting "Nuclear weapons could be employed against targets able to withstand non-nuclear attack, (for example, deep underground bunkers or bio-weapon facilities)."

Ultimately, however, the posture review was noncommittal, leaving open a crucial question: Do the military advantages gained by development of new nuclear weapons offset the massive political liabilities (domestic and international) of their development or use? To help answer that question, we explore the military abilities development of new nuclear weapons might deliver, and compare them with what might be obtained by aggressive pursuit of non-nuclear capabilities. [See the working paper for this discussion.]

With a toolkit of potential weapons, we turn to case studies of three potential targets. We evaluate the potential of nuclear and non-nuclear weapons in attacking a shallow but hardened bunker containing biological weapons, a chemical weapons production plant tunneled several stories underground, and a nuclear facility tunneled beneath hundreds of meters of granite.

Case 1 - The Tarhunah Chemical Weapons Complex

In a recent article in Jane's Intelligence Review, Geoffrey Forden described the Tarhunah Chemical Weapons Complex:

"In the mid-1990s, the USA alleged that Libya had constructed an underground nerve-agent production plant, buried under at least 18m of earth, 60km southeast of Tripoli. The main difficulty with attacking this facility would not be its depth, which appears well within the reach of even sub-kiloton weapons, but uncertainty about its underground location."

Forden argues that a five kiloton ground-penetrating nuclear weapon could be used to destroy the facility. He notes one caveat:

"Other geologic formations in the area could significantly reduce the effectiveness of such a nuclear weapon. For instance, deep crevasses, if they lay between the explosion and the underground facility, would effectively neutralize the destructive power of the bomb."

Another factor weighing against use of nuclear weapons for destruction of this facility would be the fallout produced. The precise nature of the fallout would depend on whether the weapon was detonated inside the facility or in the surrounding earth, but to be conservative, military planners would have to assume the latter. Based on our calculations, this would result in one-hundred percent lethality over approximately fifteen square kilometers. Though this zone would not reach Tripoli, fallout concerns would require medical monitoring for civilians up to twenty kilometers downwind from the blast. If American troops were in this area, they would have to halt operations or take the risk of being exposed to fallout. Troops could not enter the immediate facility area to inspect damage or collect intelligence, even with protective gear.

Many non-nuclear approaches might also be used to destroy or neutralize the complex:

A single earth penetrating conventional bomb could reach the facility if the target's location was precisely known. If the facility was operating, seismic sensing methods might be used to detect the locations of active machinery. An earth penetrating missile the length of the current GBU-28 penetrator, modified to impact the earth at twice the GBU-28's current impact speed, could penetrate the eighteen meter cover of soft rock and reinforced concrete and destroy the facility using conventional explosives.
If the facility is not able to be precisely localized (and this seems to be the likely case), several penetrator missiles used simultaneously could mimic the area effect of a small nuclear weapon. Extending the small diameter bomb concept to missiles the length of the GBU-28 would allow up to twenty-four penetrating missiles to be delivered simultaneously; several would be expected to penetrate the facility. Alternatively, multiple sorties could be used to cover the entire suspected facility area.
If it were determined that no available bombs could penetrate the facility, cruise missiles could be used to temporarily block its entrances. This would not, however, keep personnel and equipment out of the facility an extended period.
A no-personnel or no-vehicle zone could be established around the facility. A range of American intelligence assets would be trained on a designated area surrounding the complex, and any attempt to move material to or from the facility would be stopped. While the facility itself might continue to produce weapons, those weapons could not be removed and used on the battlefield.
If the facility were operating, conventional electromagnetic pulse weapons might be applied to destroy or disable equipment inside. Because the pulse can easily travel down a bunker's power and ventilation ducts, equipment inside would be vulnerable to attack. Such a weapon could be delivered by cruise missile.

In each case of applying conventional weapons, collateral damage due to chemical dispersal would be minimal outside the facility. Inside, chemical agents would be dispersed, but U.S. troops inspecting the area could mitigate the dangers from these by wearing protective gear.

Case 2-Iraqi Surface Bunker Containing Anthrax

Iraq is suspected of retaining stockpiles of weaponized anthrax and is known to use hardened bunkers extensively. Here, we consider a hypothetical cut-and-cover bunker built with 5-meter-thick walls and a roof of reinforced concrete, buried under an additional 5 meters of earth. The facility, 5 kilometers south of Baghdad, covers an area measuring 400 square meters and is 20 meters high. Built during the absence of United Nations weapons inspections, the bunker's existence became known to American intelligence through satellite imagery captured during its construction. It is believed to contain several tons of anthrax in storage barrels, though, in the absence of a continuing ground presence, this cannot be confirmed.

Early in a campaign against Iraq, military planners ask whether it would be possible to destroy the bunker's contents. A review of available penetrating weapons shows that conventional weapons can easily breach the facility, but military and political leaders are concerned that an attack would simply spread anthrax about the countryside. They ask for a review of options that would minimize collateral damage, and are presented with the following:

If it were developed, a 20-ton penetrating nuclear weapon, detonated at the floor of the facility, could incinerate the bunker's contents, preventing the dispersal of anthrax. It would, however, spread nuclear fallout. Deaths from acute radiation poisoning would be expected as far as 1 kilometer downwind. People nearer than 4 kilometers downwind would, if they were not evacuated quickly, receive a radiation dose greater than that received by a nuclear worker over a single year.
If the stress of bomb impact caused the nuclear weapon to malfunction, the conventional explosives might detonate, but with no nuclear yield and, although unlikely, anthrax could be dispersed from the bunker without being neutralized. Alternatively, the nuclear bomb might detonate, but at its "natural" yield of 10 kilotons, in which case radioactive fallout would then kill people as far downwind as 30 kilometers, perhaps including many in Baghdad.
A penetrating bomb carrying a fragmenting warhead and incendiary materials could be used. The fragmenting warhead would break the anthrax out of exposed containers, and the heat from the incendiary materials would neutralize the anthrax. If containers were heavily shielded, they would not break open and, while the anthrax would not be destroyed, neither would it be released. The bunker would remain intact.
A penetrating bomb carrying submunitions and neutralizing chemicals could be used. The submunitions would spread throughout the bunker and break the anthrax out of its containers, even if it were stored behind barriers, but the neutralizing chemicals would render the anthrax inert. The bunker would probably remain intact, but it could be breached if it had been poorly constructed.
A watch could be placed on the facility using satellite imagery coupled with armed unmanned aerial vehicles. Anything attempting to enter or leave the bunker would be destroyed, making the anthrax inside unusable.

Case 3-North Korean Nuclear Weapons Complex at Kumchangri

In a recent Congressional Research Service report, Larry Niksch describes the North Korean Kumchangri underground complex, built in the side of a mountain:

"U.S. intelligence agencies reportedly became aware of the Kumchangri underground facility in the second half of 1996. The Defense Intelligence Agency (DIA) reportedly prepared a classified report at the end of 1997, which concluded that the facility, located about 25 miles north of Yongbyon [50 kilometers north of Pyongyang], "possibly could be a nuclear weapons-related facility by 2003." The report stated that: "The function of this site has not been determined, but it could be intended as a nuclear production and/or storage site. . . ." The Clinton Administration responded to the disclosure by pressuring North Korea to allow the United States access to the Kumchangri facility. An agreement was reached on March 16, 1999, providing for multiple inspections of the site in return for at least 500,000 tons of new U.S. food aid for North Korea. . . . Administration officials declared that no evidence of nuclear activity was found. However, previous reports indicated that North Korea had removed equipment from the facility."

Had the United States or North Korea rejected a diplomatic solution, and had the United States concluded that the facility was being used to build nuclear weapons, what military choices would have been available for destruction or neutralization of the facility? The depth of the facility is not publicly known, but given it is tunneled into the side of a mountain, the main facility could quite possibly be deeper than 200 meters, putting it out of the range of even megaton-sized, earth-penetrating nuclear weapons. Even if the facility were only 150 meters underground, a 1-megaton penetrating nuclear weapon would be required to destroy it, and the resulting nuclear fallout would have enormous consequences:

If the wind were blowing southwest, residents of Pyongyang, 80 miles away, would have to be evacuated within hours of detonation to prevent the death of more than 50 percent from radiation poisoning.
If the wind were blowing north or northwest, residents of several large cities in China would have to be evacuated within hours of detonation to avoid numerous radiation deaths.
If the wind were blowing south, residents of several large cities in South Korea would have to be evacuated within hours of detonation to avoid numerous radiation deaths, and U.S. troops stationed in the DMZ would have to be evacuated.

These consequences would almost certainly deter any U.S. leader from launching such an attack. Instead, military planners might seek to disable, rather than to destroy, the facility. The following options might be considered:

A nuclear weapon might be used to deposit radioactive contamination at the entrances to the complex and thus to isolate the facility. However, a weapon small enough not to entail fallout problems would be unlikely to keep workers out of the facility for more than a period of weeks, especially since workers would be exposed only when outside the facility-once inside, the surrounding rock would shield them from radiation.
Cruise missiles could be used to collapse entrances to the bunker. The entrances might be reopened quickly, and as with radiological area denial, the effect would likely be brief.
Thermobaric weapons could be used to send high-pressure shock waves down the tunnels, possibly destroying equipment inside the facility.

Again, these options are unlikely to be satisfactory. If a military solution were still desired, the information-umbrella-type approach could be applied. The United States, possibly together with allies, would declare that no North Korean vehicles would be allowed to come near the facility, and would use land mines and train surveillance assets in the Kumchangri area to monitor this curfew. Any vehicle attempting to enter or leave the facility would be destroyed.

There are non-nuclear solutions to most military problems for which nuclear weapons are hyped. Still, we find that excessive constraints on current non-nuclear programs for addressing HDBT and WMD problems could lead to otherwise avoidable shortfalls in important capabilities. We recommend that planners:

Invest in Intelligence: Since adversaries can dig deeply and can hide WMD activities in industrial settings, simple pursuit of powerful weapons will never solve the HTBT and Agent Defeat problems. The ability to locate and characterize threat facilities is the foundation of any efforts in this area, and must be our first priority.
Use the Flexibility Provided by Air Superiority: The original driver for enemies' pursuit of HDBT and WMD capabilities is American airpower dominance. Yet constraining requirements such as the ability for all weapons to be deliverable by tactical fighter jet, or for target destruction to be accomplished in a single aircraft pass, are driven by the belief that air superiority will not exist. If enemies' actions are driven by our air superiority, we should use that same advantage in countering these actions.
Focus on Biological over Chemical Agents: Our current approach seeks a single weapon that can neutralize both chemical and biological agents. But while biological agents are much more strategically important targets, chemical agents are much harder to destroy. Our present approach leads to shortfalls in our ability to neutralize biological agents because the requirement that the same technology be able to destroy chemical agents unnecessarily constrains it.
Evaluate Weapons in a Battlefield Context While conventional weapons are tested in war-games and vetted with military chiefs, nuclear weapons are judged mainly by their political implications. The taboo against the use of nuclear weapons perversely shields these weapons from the same scrutiny during their development that all other weapons receive. If civilian leaders decide to consider pursuit of new nuclear weapons, uniformed military must confront these weapons concepts with the same scrutiny they apply to other weapons systems.