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World Court Project:

The Lawfulness of "Low-Yield," Earth-Penetrating Nuclear Weapons

John Burroughs1

January 20, 2003



In its July 1996 advisory opinion, the International Court of Justice (ICJ) formally concluded that "the threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict."2   In its analysis, the ICJ stated that "the use of such weapons in fact seems scarcely reconcilable with respect for … requirements" of humanitarian law including that prohibiting methods and means of warfare "which would preclude any distinction between civilian and military targets."3

There is now discussion in U.S. government circles of the desirability of developing "low-yield," earth-penetrating nuclear weapons to reach targets like underground bunkers. It is sometimes suggested that such weapons would limit "collateral damage" to civilians and civilian property to an acceptable level.

Would use of such nuclear weapons be compatible with the requirements of international law set forth by the ICJ? The ICJ itself stated:

The Court would observe that none of the States advocating the legality of the use of nuclear weapons under certain circumstances, including the "clean" use of smaller, low yield, tactical nuclear weapons, has indicated what, supposing such limited use were feasible, would be the precise circumstances justifying such use; nor whether such limited use would not tend to escalate into the all-out use of high yield nuclear weapons. This being so, the Court does not consider that it has a sufficient basis for a determination on the validity of this view.4

This memorandum reaches the conclusion that "low-yield," earth-penetrating nuclear weapons now deployed or under consideration by the United States do not meet the criteria of lawfulness identified by the ICJ.


Low-yield nuclear weapons are not new. "Low-yield" is often used to refer to explosive devices with an explosive output of less than five kilotons TNT equivalent. (For comparison, the bombs with which the United States devastated the cities of Hiroshima and Nagasaki had yields in the range of 12 to 15 kilotons.) From 1961 to 1991, the United States deployed an artillery shell with a yield as low as 10 metric tons (.01 kiloton).5  Nor are earth-penetrating nuclear weapons altogether new. In 1996, the United States introduced into its arsenal a bomb (the B-61-11) with a variable yield, reportedly from 300 tons to 300 kilotons, and an earth-penetrating capability of a few meters.6

Since the collapse of the Soviet Union and the experience of the Gulf War against an Iraq armed with chemical and biological weapons, there has been strong interest in the U.S. nuclear establishment and, to a lesser extent, the U.S. military in developing nuclear weapons capable of successfully attacking underground targets with acceptable "collateral damage."7  To give a recent example, in the spring of 2000 the Washington Post quoted a former senior Pentagon official "involved in government military and intelligence research" who cited the need for a "deep penetrator that could hold at risk a rogue state’s deeply buried weapons or Saddam Hussein’s bunker without torching Baghdad."8   The 2002 U.S. Defense Department Nuclear Posture Review, according to the New York Times, "cites the need to improve ‘earth-penetrating weapons’ that could be used to destroy underground installations and hardened bunkers" and calls for such weapons both with lower yields to lessen nuclear fallout and larger yields to attack deeply buried targets.9 

The pressure has begun to yield results in Congress. In 2002 Congress authorized research, with $15 million slated for 2003, into the feasibility and cost of developing a "robust nuclear earth penetrator."10  The nuclear weapons laboratories are expected to study modifications to strengthen casings on existing warheads with yields substantially higher than five kilotons. Prior to commencement of the study, the administration must report on anticipated use policy, military requirements, and potential targets as well as the ability of conventional weapons to accomplish the same missions. Congress also provided for a National Academy of Sciences study assessing the short- and long-term effects on civilian populations of the robust nuclear earth penetrator versus use of conventional weapons or nuclear weapons exploded above ground. Two years earlier, Congress required, but did not specifically fund, a "study relating to the defeat of hardened and deeply buried targets."11

To date, Congress has not overturned a 1994 law banning research "which leads to the production … of a low-yield [less than five kiloton] nuclear weapon."12  The United States also apparently still has a declared policy of not producing "new" nuclear weapons.13   However, those restrictions are under assault and in any event are far from airtight. For example, the nuclear weapons laboratories have assisted with "concept studies" on chemical and biological "Agent Defeat" and "Hard and Deeply Buried Target Defeat" which they claim do not violate the ban on low-yield nuclear weapons development.14  Also, the B-61-11 earth penetrator deployed in 1996 was described as a modified, not a new, weapon.15

Legal analysis

Discrimination: The ICJ described the following principle of humanitarian law as "cardinal," "fundamental," and "intransgressible": "States must never make civilians the object of attack and must consequently never use weapons that are incapable of distinguishing between civilian and military targets."16  The use of the term "never" is significant: under humanitarian law, in no circumstance, including reprisal against a prior nuclear, chemical or biological attack, may a state use inherently indiscriminate weapons.

That raises the simple and central question: can any of the "low-yield" or earth- penetrating weapons now deployed (e.g., the B-61-11) or under consideration limit effects on civilians such that its use could meet the requirement of discrimination? As noted above, aspects of that question are under investigation by the U.S. government and the National Academy of Sciences. But according to well qualified specialists, the answer is already known: No. Indeed, earth-penetrating weapons create more radioactive fallout than devices that explode above ground.

Robert W. Nelson, a physicist with the Princeton Program on Science and Global Security and the American Federation of Scientists, in a 2002 article concluded based on analysis of physical constraints that earth-penetrating warheads

cannot penetrate deeply enough to contain the nuclear explosion and will necessarily produce an especially intense and deadly radioactive fallout. … A one kiloton earth-penetrating ‘mininuke’ used in a typical third-world urban environment would spread a lethal dose of radioactive fallout over several square kilometers, resulting in tens of thousands of civilian fatalities.17

Similarly, Sidney Drell, a physicist at Stanford University’s Linear Accelerator Center, Raymond Jeanloz, a geophysicist at the University of California at Berkeley, and Bob Peurifoy, a weapons designer and former vice president at Sandia National Laboratory, all of whom have consulted extensively with the U.S. government on technical issues of nuclear weapons, wrote in a March 17, 2002 commentary in the Los Angeles Times:

Low-yield nuclear weapons have limited effectiveness against buried targets and they would disperse significant amounts of radioactivity…. Taking into account realistic limits on material strengths, 50 feet is about the maximum depth a warhead can dig and maintain its integrity in dry, hard soil, the likely locations for buried targets. Even a 1 kiloton warhead – 1/20th the yield that destroyed Hiroshima – detonated at a depth of 20 feet would eject about 1 million cubic feet or radioactive debris from a crater about the size of ground zero at the World Trade Center.18

Necessity and proportionality: The ICJ unanimously concluded that a "threat or use of force by means of nuclear weapons that is contrary to Article 2, paragraph 4 of the United Nations Charter and that fails to meet all the requirements of Article 51, is unlawful."19  In its analysis, the ICJ explained that lawful self-defense under Article 51 must meet the requirements of necessity and proportionality.20

Necessity limits the use of force to that required to achieve a legitimate military objective. Assuming that the United States was using force in individual or collective self-defense, or that force was being used under Security Council authorization or directive, the use of any particular method or means of combat must still be necessary. Accordingly, if a weapon is available which causes less damage and suffering than a nuclear weapon to carry out a legitimate military mission, use of a nuclear weapon is barred. There are credible reports that the United States now has or is rapidly developing such capabilities. Robert W. Nelson states that the United States "already has a number of conventional weapons capable of destroying hardened targets buried within approximately 50 feet of the surface."21

Proportionality forbids the use of measures in response to an attack that are excessive in relation to the scope of the attack and, more controversially, to the requirements of repelling the attack and ending the conflict on favorable terms. Whether a measure is disproportionate includes consideration of effects on the environment and the prospects for nuclear escalation.22  The use of the phrase "weapons of mass destruction" has unfortunately created the impression of an equivalence among nuclear, chemical, and biological weapons. However, the reality is that in general nuclear weapons are orders of magnitude more destructive than chemical or biological arms. Accordingly, absent extraordinary circumstances (e.g., a biological attack on an urban area that contrary to most predictions causes tens or hundreds of thousands of casualties), a nuclear use would be disproportionate to a chemical or biological attack or threatened attack, and certainly to an attack by conventional means.

Negative security assurances: The ICJ unanimously concluded that a "threat or use of nuclear weapons should also be compatible with … specific obligations under treaties and other undertakings which expressly deal with nuclear weapons."23  The emphasized phrase appears to refer in part to "negative security assurances" provided by states acknowledged as nuclear weapon states by the Nuclear Nonproliferation Treaty, including the United States, to non-nuclear weapon states parties to that treaty.24  The ICJ thus appears to regard those assurances, which were reaffirmed in connection with the indefinite extension of the NPT in 1995, as legally binding. The U.S. declaration made in 1995 provides:

The United States reaffirms that it will not use nuclear weapons against non-nuclear-weapon [NPT parties] except in the case of an invasion or any other attack on the United States, its territories, its armed forces or other troops, its allies, or on a State toward which it has a security commitment, carried out or sustained by such a non-nuclear weapon State in association or alliance with a nuclear-weapon State.

The United States is therefore under an obligation not use to "low-yield" or earth- penetrating or any kind of nuclear weapon against NPT-compliant non-nuclear weapon states. That includes in circumstances of acquisition of chemical or biological weapons or their threat or use. The point is important given the prevalent but mistaken assumption of equivalence among nuclear, chemical and biological weapons. If chemical or biological weapons are used, the doctrine of reprisal could come into play, but it remains subject to the requirements of necessity, proportionality, and discrimination.



Other legal requirements of lawful use of any weapon, including a nuclear weapon, were identified by the International Court of Justice. They include the ban on infliction of unnecessary suffering, the requirement that damage to the environment not be disproportionate, and respect for the rights of neutral states. Taken together with the requirements of discrimination, necessity, and proportionality and the awesomely destructive nature of nuclear weapons, they form the basis for the ICJ’s conclusion of the general illegality of threat or use of nuclear weapons.

The Court did not have before it comprehensive and reliable information regarding all types of nuclear weapons and their effects. Its statement that it could not form a view regarding the "feasibility" or lawfulness of "clean" uses of "low-yield" nuclear weapons in certain circumstances is thus understandable. However, since the 1995 hearings, more attention has been paid to such scenarios. The weight of evidence is that there are no "clean" uses, and in particular that earth-penetrating warheads, whether or not "low-yield," would cause significant radioactive fallout and civilian casualties. Accordingly, the ICJ’s conclusion of general illegality applies to threat or use of such weapons.

The ICJ’s last conclusion is that "in view of the current state of international law, and of the elements of fact at its disposal, the Court cannot conclude definitively whether the threat or use of nuclear weapons would be lawful or unlawful in an extreme circumstance of self-defence, in which the very survival of a State would be at risk."25  Given, among other things, the overwhelming non-nuclear military power of the United States, a situation in which use of a "low-yield," earth-penetrating nuclear weapon might be considered would not involve a threat to the survival of either the United States itself or of a state which the United States is defending. Moreover, since 1996 the facts coming to light about such weapons point strongly towards the unlawfulness of use of nuclear weapons in all circumstances.


1 Executive Director, Lawyers’ Committee on Nuclear Policy. Author, The Legality of Threat or Use of Nuclear Weapons: A Guide to the Historic Opinion of the International Court of Justice (Piscataway, NJ: Transaction Publishers, 1998). Co-editor and contributing author, Nicole Deller, Arjun Makhijani, and John Burroughs, eds., Rule of Power or Rule of Law? An Assessment of U.S. Policies and Actions Regarding Security-Related Treaties (New York: Apex Press, 2003).

2 Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, ICJ Reports, 1996, p. 226, para. 105(2)E. All subsequent references to the opinion simply give the paragraph number.

3 Para. 95

4 Para. 94.

5 Robert W. Nelson, "Low-Yield Earth-Penetrating Nuclear Weapons," 10 Science and Global Security (2002, no. 1) 1, at n. 14, p. 19 [hereinafter "Nelson"].

6 Greg Mello, "New bomb, no mission," Bulletin of the Atomic Scientists (May/June 1997) 28 [hereinafter "Mello"]; Nelson at 4.

7 Andrew M. Lichterman, "Looking for New Ways to Use Nuclear Weapons: U.S. Counterproliferation Programs, Weapons Effects Research, and ‘Mini-Nuke’ Development," Western States Legal Foundation Information Bulletin (Winter 2001) at 1-4, online at www.wslfweb.org [hereinafter "Lichterman"].

8 Walter Pincus, "Senate Bill Requires Study of New Nuclear Weapon," Washington Post, June 12, 2000, p. A2, cited in Lichterman at 6.

9 Michael R. Gordon, "U.S. Nuclear Plan Sees New Targets and New Weapons," New York Times, March 10, 2002.

10 Christine Kucia, "Congress Approves Nuclear ‘Bunker Buster’ Research," Arms Control Today (December 2002).

11 Lichterman at 6.

12 Lichterman at 4.

13 Lichterman at 3 and n. 15.

14 Lichterman at 6.

15 Mello; Lichterman at 4.

16 Paras. 78, 79 (emphasis added).

17 Nelson at 1.

18 Sidney Drell, Raymond Jeanloz, and Bob Peurifoy, "Bunkers, Bombs, Radiation," Commentary, Los Angeles Times, March 17, 2002.

19 Para. 105(2)C.

20 Para. 41.

21 Robert W. Nelson, "Low-Yield Earth-Penetrating Nuclear Weapons," F.A.S. Public Interest Report (January/February 2001) 1, at 3.

22 Paras. 30, 43.

23 Para. 105(2)D; emphasis added.

24 See George Bunn, "The Legal Status of U.S. Negative Security Assurances to Non-Nuclear Weapon States," The Nonproliferation Review (Spring-Summer 1997).

25 Para. 105(2)E.




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