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Mini-Nuclear Weapons and the U.S. Nuclear Posture Review

B-2 bomber dropping a B61-11 bomb during test.

By Charles D. Ferguson

Leaked portions of the new U.S. Nuclear Posture Review (NPR)[1] and recent press reports[2] have raised alarm that the United States may develop mini-nuclear weapons. Administration officials, however, have tried to downplay this concern and have pointed out that they are only launching a feasibility study, which begins this month.[3] Congress would have to authorize any new nuclear weapon system before actual construction could begin.

The NPR and Earth-Penetrating Weapons Options

Under the heading of "Advanced Concepts Initiative," the NPR calls for "improved earth penetrating weapons (EPWs) to counter the increased use by potential adversaries of hardened and deeply buried facilities."[1] Three EPW options will reportedly be examined in a Department of Defense (DOD) and Department of Energy (DOE) feasibility study over the next two to three years. The study's estimated cost is $45 million.[3] These options are:

• conventional weapons,
• modifications to existing nuclear weapons, and
• development of new nuclear weapons.

Conventional weapons are the least controversial option from a political viewpoint, but they are the least likely to destroy hardened and deeply buried targets, according to advocates of mini-nuclear weapons. Nonetheless, the U.S. military arsenal already has very powerful and precise conventional weapons, which could demolish hardened targets buried approximately 50 feet below the surface. During the Gulf War, for example, the U.S. Air Force used the 4,000-pound GBU-28 bomb to destroy a bunker north of Baghdad protected by more than 30 feet of earth, concrete, and steel. The GBU-37 guided bomb, an improved version of the GBU-28, might be able to obliterate silo-based inter-continental ballistic missiles. Such hardened targets were previously believed to only be vulnerable to nuclear attack.[4] Improvements in this class of conventional weapons are almost certain to follow. But these weapons would still be unlikely to destroy very deeply buried targets because even the most explosive conventional weapon would not have a large enough explosive yield. Moreover, fundamental physical limitations on the strengths of warhead, bomb, or missile components constrain how deeply warheads can be driven into the ground, as discussed below.

The Physics Constraints on Nuclear Weapons as Bunker Busters

In contrast to the conventional weapons, the second option--modification of existing nuclear weapons--is politically the most controversial, but it is most likely to accomplish the military mission of destroying very deeply buried targets. The extant nuclear weapons in the U.S. arsenal typically have large explosive yields in the range of 100 to 475 kilotons (kT).[5] In comparison, the Nagasaki bomb had an equivalent yield of 22 kT, which is small relative to the current warheads' explosive energy. (A kT is equivalent with respect to blast damage to 1,000 tons of conventional explosives, or 500 of the 4,000-pound GBU-28 bombs.) All but one of the current U.S. nuclear weapons, if used as bunker busters, would undoubtedly result in massive amounts of radioactive fallout, potentially leading to huge civilian losses.

The one possible exception in the current nuclear arsenal worth exploring is the B61 bomb, which has a dial-a-yield between 0.3 to 170 kT. At the lower yields, weapons designers thought that this bomb could be modified to be a bunker buster that would minimize fallout. Critics cried foul because they said that the modification, called the B61-11, violated the U.S. policy of not developing new nuclear weapons. DOE and the weapons designers countered that the nuclear explosive itself is not new; only the packaging around the explosive was changed by placing the old warhead inside a hardened steel casing to increase the penetrating capability. But this capability is limited. Tests showed that the B61-11 can only penetrate about 20 feet into dry earth when dropped from 40,000 feet. Not only would such a bomb be questionable for destroying very deeply buried bunkers, but it would produce tremendous lethal radioactive fallout even with its low explosive yields.

Faced with the difficulties posed by the first and second options, some weapons scientists have argued for a third option: the development of a whole new class of nuclear weapons.[6][7] They would aim for two principal properties:

• low explosive yields to minimize radioactive fallout, and
• missile and bomb casings with deep-penetrating capability.

However, according to Dr. Robert Nelson, a physicist at Princeton University, "The goal of a benign earth-penetrating nuclear weapon is physically impossible."[8] His recent report shows that "EPWs cannot penetrate deeply enough to contain the nuclear explosion and will necessarily produce an especially intense and deadly radioactive fallout." Moreover, "A missile made of the hardest steels cannot survive the severe ground impact stresses at velocities greater than about [one kilometer per second] without destroying itself." Further, "This limits the maximum possible penetration depth into reinforced concrete to about four times the missile length -- approximately 12 meters for a missile 3 meters long." Even low-yield earth-penetrating nuclear weapons would excavate substantial craters, "throwing out a large amount of radioactive dirt and debris." His dose calculations indicate that "A one kiloton earth-penetrating ‘mini-nuke' used in a typical third-world urban environment [such as Baghdad] would spread a lethal dose of radioactive fallout over several square kilometers, resulting in tens of thousands of civilian fatalities."

The severe adverse consequences of nuclear weapons as earth-penetrators argue against their use. Weapons designers should instead shift development to conventional weapons that can render hardened and deeply buried bunkers inoperable. Though such weapons would be unlikely to destroy the bunkers, highly accurate conventional weapons could be used to seal up the entrances to these bunkers. As long as weapons of mass destruction (WMD) arsenals remain trapped inside closed bunkers, these arsenals pose no threat.

The NPR is Only the Latest Story in the Mini-Nuclear Weapons Saga

The administration has emphasized that planning for the development of EPWs is not new to the NPR. In certain respects this is true, but the story is more complicated.

The Gulf War re-sparked concern that Saddam Hussein or some other tyrannical leader could hide WMD, missile factories, or command and control facilities in hardened and deeply buried bunkers, which would be impervious to attack. A seminal 1991 paper by Thomas Dowler and Joseph Howard of the Los Alamos National Laboratory took as its axiom that "The existing U.S. nuclear arsenal had no deterrent effect on Saddam, and is unlikely to deter a future tyrant."[7] Other analysts have countered that during the Gulf War, the U.S. conveyed credible deterrent threats to Saddam, which contributed to stopping him from using WMD. To the contrary, Steve Fetter has argued that "There is evidence that Iraq was unable, rather than unwilling, to use its chemical weapons."[9]

From their vantage point, Dowler and Howard argued for "the development of new nuclear weapons of very low yields, with destructive power proportional to the risks we will face in the new world environment." They described three classes of nuclear weapons to fulfill this mission:

• "micronukes" with an explosive yield of about 10 tons or equivalent to 20,000 pounds of high explosives;
• "mininukes" with an explosive yield of about 100 tons;
• "tinynukes" with an explosive yield of about 1,000 tons or one kiloton.

On the political front in the early 1990s, the trend was away from development of new nuclear weapons. On September 27, 1991, President George H. W. Bush, as part of his reductions in U.S. nuclear forces, declared that the U.S. would eliminate all of its ground-based tactical nuclear weapons -- a major class of battlefield nuclear weapons. On October 2, 1992, he approved a moratorium on U.S. nuclear testing. In 1993, Rep. Elizabeth Furse (D-OR) and Rep. John Spratt (D-SC) successfully attached in the FY 1994 defense authorization bill a provision that prohibits the national laboratories from research and development leading to a precision, low-yield nuclear weapon. In particular, this legislation defined mini-nuclear weapons as those with explosive yields of five kilotons or less.

In 2000, Senators John Warner (R-VA) and Wayne Allard (R-CO) took a major step toward reversing this political trend by introducing an amendment in the FY 2001 defense authorization bill that called for a DOD and DOE study on "the defeat of hardened and deeply buried targets."[10] Although this legislation passed, it did not explicitly override the Furse-Spratt provision. In October 2001, DOD transmitted the "Report to Congress on the Defeat of Hard and Deeply Buried Targets."[11]

Scientists Speak Out

In parallel to this report to Congress, influential national laboratory scientists and analysts tried to make the case for useable nuclear weapons. In June 2000, for example, Stephen Younger, then-Associate Laboratory Director for Nuclear Weapons at Los Alamos National Laboratory, challenged "conventional thinking on nuclear issues." In addition to making a case for lower yield nuclear weapons, he argued that "it might be desirable to retain a small number of higher-yield nuclear weapons" to destroy "very hard targets."[6] A year later, C. Paul Robinson, the president and director of Sandia National Laboratory, advanced similar arguments.[12] In opposition, prominent arms control scientists and scientific organizations have spoken out against the possible development of mini-nuclear weapons, citing the likelihood of massive collateral damage and the adverse political costs of crossing the nuclear threshold if the U.S. would ever use these weapons.[13]

As the recently launched DOD and DOE feasibility study proceeds and as security concerns over rogue states expands or contracts, the political and scientific battles over whether mini-nuclear weapons are useable or necessary will continue.

Sources:

[1] Excerpts from the Nuclear Posture Review are available at: http://www.globalsecurity.org/wmd/library/policy/dod/npr.htm; J. D. Crouch, Assistant Secretary of Defense for International Security Policy, Special Briefing on the Nuclear Posture Review, Department of Defense, January 9, 2002, http://www.defenselink.mil/news/Jan2002/t01092002_t0109npr.html.

[2] Philipp C. Bleek, "Report Says U.S. Studying New Nuclear Capabilities," Arms Control Today 32 (January/February 2002), http://www.armscontrol.org/act/2002_01-02/tredreportjanfeb02.asp; William M. Arkin, "Secret Plan Outlines the Unthinkable," Los Angeles Times, March 10, 2002, http://www.latimes.com/news/opinion/la-op-arkinmar10.story; David E. Sanger, "Bush Finds That Ambiguity is Part of Nuclear Deterrence," New York Times, March 18, 2002.

[3] Philipp C. Bleek, "Energy Department to Study Modifying Nuclear Weapons," Arms Control Today 32 (April 2002), http://www.armscontrol.org/act/2002_04/nucapril02.asp.

[4] Robert W. Nelson, "Low-Yield Earth-Penetrating Nuclear Weapons," FAS Public Interest Report, January/February 2001, http://www.fas.org/faspir/2001/v54n1/weapons.htm.

[5] NRDC Nuclear Notebook, "U.S. Nuclear Forces 2001," The Bulletin of the Atomic Scientists 57 (March/April 2001), http://www.thebulletin.org/issues/nukenotes/ma01nukenote.html

[6] Stephen M. Younger, "Nuclear Weapons in the Twenty-First Century," Los Alamos National Laboratory Report, LAUR-00-2850, June 27, 2000, http://www.fas.org/nuke/guide/usa/doctrine/doe/younger.htm.

[7] Thomas W. Dowler and Joseph S. Howard II, "Countering the Threat of the Well-Armed Tyrant: A Modest Proposal for Small Nuclear Weapons," Strategic Review (Fall 1991). Two commentaries on this article are William M. Arkin and Robert S. Norris, "Tiny Nukes for Mini-Minds," The Bulletin of the Atomic Scientists (April 1992), http://www.bullatomsci.org/issues/1992/a92/a92.arkin.html and William M. Arkin, "Nuclear Junkies: Those Lovable Little Bombs," The Bulletin of the Atomic Scientists (January 1993), http://www.bullatomsci.org/issues/1993/ja93/ja93Arkin.html.

[8] Robert W. Nelson, "Low-Yield Earth-Penetrating Nuclear Weapons," February 2002, to be published in Science & Global Security.

[9] Steve Fetter, "Limiting the Role of Nuclear Weapons," Chapter 3 in The Nuclear Turning Point, Harold A. Feiveson, editor, http://www.puaf.umd.edu/faculty/papers/fetter/Brook_chap_3.pdf.

[10] "Report on the Defeat of Hardened and Deeply Buried Targets," Sec. 1018, S. 2549, National Defense Authorization Act for Fiscal Year 2001 (The full text can be found at http://thomas.loc.gov); Leona C. Bull, "Congress proposing ‘mini' nuclear weapons," Aerotech News and Review, September 29, 2000, http://www.aerotechnews.com/starc/2000/092900/Mini_Nukes.html.

[11] "Report to Congress on the Defeat of Hard and Deeply Buried Targets," July 2001; submitted in October 2001 by the Secretary of Defense in Conjunction with the Secretary of Energy, in response to Section 1044 of the Floyd D. Spence National Defense Authorization Act for Fiscal Year 2001, P.L. 106-398, http://www.nukewatch.org/nwd/HiRes_Report_to_Congress_on_the_Defeat.pdf.

[12] C. Paul Robinson, President and Director, Sandia National Laboratories, "Pursuing a New Nuclear Weapons Policy for the 21st Century," March 22, 2001, http://www.sandia.gov/media/whitepaper/2001-04-Robinson.htm.

[13] "Scientists: ‘Clean' Nuclear Weapon Isn't," Federation of American Scientists' press release, April 16, 2001, http://www.fas.org/press/010416-mininuc.htm; "Scientists Urge President to Limit Nuclear Missions," Union of Concerned Scientists' Press Release on the Nuclear Posture Review, March 13, 2002; Sidney Drell, Raymond Jeanloz, and Bob Peurifoy, "Bunkers, Bombs, Radiation," Los Angeles Times, March 17, 2002; Henry C. Kelly and Michael A. Levi, "Nix the Mini-Nukes," The Christian Science Monitor, March 28, 2002, http://www.csmonitor.com/2002/0328/p09s01-coop.htm.

For comments on Mini-Nuclear Weapons and the Nuclear Posture Review, contact:

Dr. Charles D. Ferguson, Scientist-in-Residence
Washington, D.C. CNS Office
Telephone: (202) 478-3426
E-mail: charles.ferguson@miis.edu

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