A Strategic Choice: New Bunker Busters Versus Nonproliferation

Sidney Drell, James Goodby, Raymond Jeanloz, and Robert Peurifoy

The United States has repeatedly emphasized the importance of international cooperation in the effort to slow down and, as possible, counter the spread of weapons of mass destruction. In particular, it has specified that strengthening the nuclear Nonproliferation Treaty (NPT) is vital to this effort.

Secretary of State Colin Powell stressed the U.S. commitment to bolster the treaty and its efforts to counter the spread of nuclear technology to other nations in his testimony before the Senate Foreign Relations Committee last July: “The committee members know that the NPT is the centerpiece of the global nuclear nonproliferation regime. It plays a critical role in efforts to prevent the spread of nuclear weapons including to terrorists and states that support them. The NPT’s value depends upon all parties honoring their obligations. The United States places great importance on fulfilling its NPT undertakings.”

The joint declaration that Presidents George W. Bush and Vladimir Putin issued in Moscow on May 24 of last year affirmed that “the United States and Russia will also seek broad international support for a strategy of proactive non-proliferation, including by implementing and bolstering the Treaty on the Non-Proliferation of Nuclear Weapons and the conventions on the prohibition of chemical and biological weapons.” President Bush reiterated the importance of international cooperation in preventing the spread of nuclear weapons and other weapons of mass destruction in his letter issuing the new National Security Strategy on September 17, 2002, as well as in the National Strategy to Combat Weapons of Mass Destruction that was published three months later.

There is good reason for the United States to support the NPT and promote its objectives. Today, 57 years after atomic bombs destroyed Hiroshima and Nagasaki, just eight nations—many fewer than predicted originally—are believed to possess deployed nuclear weapons. All but four countries in the world (India, Israel, Pakistan, and North Korea, which has recently withdrawn) are formally committed to the NPT, which first entered into force in 1970. A number of nations that had started down the road to nuclear weapons have abandoned them. These include Argentina and Brazil, which mutually shut down their advancing programs; South Africa, which destroyed its initial force; and Belarus, Ukraine, and Kazakhstan, which returned all of their nuclear weapons to Russia after the collapse of the Soviet Union. There is now a norm of nonpossession to which most nations adhere.

But the nonproliferation regime is fragile, and it currently faces severe challenges from Iran, Iraq, and particularly North Korea. Unfortunately, in recent months it has also been challenged by the United States. Statements by the administration, including the portions of the Nuclear Posture Review leaked in March 2002, suggest that the United States needs new, low-yield—and presumably “more useable”—nuclear weapons to destroy hardened and deeply buried targets. The world’s only superpower would send a negative signal to the non-nuclear states if it felt the need to develop new types of nuclear weapons.

Such an initiative would further undermine the NPT if it led to a resumption of nuclear explosive testing in order to deploy new weapons designs. In 1995, many of the world’s non-nuclear nations made it clear that their continued adherence to the NPT was contingent on the cessation of all nuclear-yield testing. Although it has adhered to a self-imposed moratorium on such tests for more than a decade, the United States has refused to ratify the Comprehensive Test Ban Treaty (CTBT), thereby forgoing the opportunity to strengthen the NPT regime. A decision to resume testing to build low-yield nuclear weapons could deal the regime a fatal blow while providing the United States with a capability of questionable military value.

Utility of Bunker Busters

The Bush administration has said it may need to build a new class of low-yield, earth-penetrating nuclear weapons—sometimes called “bunker busters”—because of its concern about whether the U.S. military can destroy the growing number of hard and deeply buried facilities being built in a number of countries. Citing recent government studies, the Nuclear Posture Review states that more than 70 countries now have such underground facilities for military purposes. These include more than 1,000 known or suspected strategic targets, which are used for storing weapons of mass destruction, protecting senior leaders, and executing top-echelon command and control functions. Among the underground targets of most concern are very hardened structures built at depths of 1,000 feet or so with reinforced concrete capable of withstanding up to 1,000 atmospheres overpressure.

Destroying such targets requires knowing exactly where they are and then precisely delivering a warhead that can penetrate into the earth without damage before detonating. The warhead must also have a sufficiently large explosive yield to transmit a strong shock. These challenges are recognized in the NPR as follows: “New capabilities must be developed to defeat emerging threats such as hard and deeply buried targets (HDBT), and to define and attack mobile and relocatable targets, to defeat chemical and biological agents and to improve accuracy and limit collateral damage.” The NPR emphasizes further that “need may arise to modify, upgrade, or replace portions of the extant nuclear force or develop concepts for follow-on nuclear weapons better suited to the nation’s needs. It is unlikely that a reduced version of the Cold War nuclear arsenal will be precisely the nuclear force that the United States will require in 2012 and beyond.”

The United States has already designed and tested a variety of low-yield nuclear devices that could be adapted for delivery in structurally strengthened warheads for destroying underground targets at shallow depths. Recently it adapted a high-yield weapon—the B61-11 bomb, with yields that exceed a hundred kilotons—in this manner. A key technical challenge is to develop the means to deliver such a bomb intact to depths of 10-20 feet before detonation. Detonation at such depths increases, by a factor of 10 to 20 relative to a surface burst, the energy of the explosion that is delivered into the ground instead of into the atmosphere. The warhead therefore hits the target—a hardened, buried bunker or tunnel—with a much stronger shock than an identical warhead that is detonated on or above the surface.

Taking into account realistic limits on material strengths, about 50 feet is the maximum depth to which a warhead dropped from the air into dry rock soil could maintain its integrity until detonated. This is true even with impact at supersonic speeds. For the shock to reach down to 1,000 feet with enough strength to destroy a hard target in dry rock, the warhead would require a yield significantly larger than 100 kilotons. Accuracy is also crucial. A major challenge for destroying hardened underground targets is the need to improve significantly our ability to locate, identify, and characterize such targets. The payoff of accuracy in target location and delivery of a weapon is significant. It is also important to find any vulnerable points such as tunnel entrances or air ducts.

Given these technical facts, how can the United States hold HDBTs at risk? The most important steps are gaining better intelligence for accurate target characterization and location; improving precision of delivery of warheads; further hardening warheads so they can penetrate the earth to a depth of at least 20-30 feet, instead of just a few feet, as is possible now; and establishing control of the area around localized underground targets using conventional forces and tactics.

But the Nuclear Posture Review, and a number of members of the defense establishment, have suggested that the United States develop a new class of hardened, low-yield nuclear weapons. The implication is that, if their resulting collateral damage can be substantially reduced by lowering the explosive power of the warhead, nuclear weapons would be more politically palatable and therefore more “useable” for attacking deeply buried targets in tactical missions—even in or near urban settings, which can be the preferred locales for such targets.

Consider, however, the radioactive contamination from a one-kiloton warhead, detonated at a depth of 20-50 feet. This is, approximately, just 1/13 the yield that destroyed Hiroshima, yet it would eject more than 1 million cubic feet of radioactive debris from a crater about the size of ground zero at the World Trade Center—bigger than a football field. Indeed, the Hiroshima bomb was detonated at an altitude of close to 1,900 feet in order to minimize radioactive fallout by not digging any crater. A weapon intended to destroy hard, buried targets is therefore going to produce a lot of dangerous radioactive fallout. Of course, a nuclear weapon with a yield capable of destroying a target 1,000 feet underground—a yield well over 100 kilotons—would dig a much larger crater and create a substantially larger amount of radioactive debris.

We emphasize this point because recent reports, columns, and quotes in the media call for the United States to develop new, low-yield nuclear weapons for use against hard, deeply buried targets because they would produce less collateral damage. But even a one-kiloton earth penetrator would be quite devastating in a city, and against really deep targets, yields in the hundreds of kilotons would be required. In the past, the United States has developed, tested, and deployed nuclear warheads with a full range of yields, from small fractions of kilotons up to many megatons. We can make further improvements in their delivery—both in accuracy and earth penetration—that would be significant. But as we have seen, even at the low-yield end of the repertoire, there will be major collateral damage because the blast will eject radioactive debris. Burrowing a few tens of feet into the earth will increase the damaging effects of the shock, but a large proportion of the fallout will still enter the atmosphere and be spread by wind.

Ratifying the CTBT

A further problem would arise if the need to develop “new capabilities…to defeat emerging threats,” as is called for in the Nuclear Posture Review, led the United States to resume underground nuclear explosive tests. As explained earlier, the United States has already designed and tested nuclear devices with a broad range of yields. Building better earth-penetrating nuclear weapons does not require resumed nuclear testing, as has been suggested by bunker-buster advocates who oppose the CTBT. It requires precise delivery with deeper penetration on accurately located targets. A resumption of underground nuclear explosive testing would have minimal technical benefits, but a major, harmful impact on the nonproliferation regime.

Many nations signed on to the indefinite extension of the NPT in 1995 on the explicit condition that the nuclear powers would cease all nuclear-yield testing. This situation presented the United States and the other nuclear powers with a strong political and strategic incentive to formalize the moratorium on testing by ratifying and working to bring into force the CTBT. It is obviously one of the critical cornerstones of the NPT, which, as Secretary Powell said in his Senate testimony, “is the centerpiece of the global nonproliferation regime.”

A U.S. decision to resume testing to produce new nuclear weapons would therefore dramatically undermine the NPT. Conversely, a U.S. decision to ratify the already signed CTBT and lead the effort to bring the treaty into force would be an effective way of strengthening the NPT and, through it, worldwide nonproliferation and counterproliferation efforts. Bringing the treaty into force would have the added technical advantage of allowing for the full implementation of the international monitoring system intended to verify compliance with the CTBT. Implementation would add challenge inspection protocols that would further strengthen the verification regime and increase its transparency.

Many U.S. allies in NATO, including the United Kingdom, Germany, and France, have signed and ratified the CTBT, as have Japan and Russia. Others, including China, have indicated they will work to bring the treaty into force once the United States has ratified it. As of March 2003, 166 nations have signed the CTBT and 97 have ratified it, including 31 of the 44 “nuclear-capable states” that must ratify the treaty for it to enter into force. But the U.S. Senate refused to consent to ratification of the treaty when it came up for a vote in 1999, and the Bush administration has refused to reopen the question. The administration continues for the time being, however, to honor a moratorium on all testing that President George H. W. Bush established in 1992.

Why is the United States reluctant? In addition to the dubious need to develop “concepts for follow-on nuclear weapons better suited to the nation’s needs,” including nuclear earth penetrators against HDBTs, opponents of the CTBT have asked, “How can we be sure that many years ahead, we will not need to resume yield testing in order to rebuild the stockpile?” The answer is that total certainty never can be achieved. But it is possible to ensure that there is a strong program in place with the necessary support of competent engineers and scientists, who would sound a warning bell should a serious, unforeseen problem arise.

With the enhanced, multifaceted, science-based program of stockpile stewardship established during the past seven years, the United States can have confidence in its ability to understand the character of the stockpile and the way in which special bomb materials age. As a result of the stockpile surveillance program, a number of flaws have been reported and dealt with appropriately. The flaws thus far uncovered within the nuclear devices themselves are related to design oversights. That is, the flaws, or their precursors, were present when the weapons were put into the stockpile. In comparison, unexpected flaws due to the unknown effects of aging thus far appear to be minimal.

The United States can be assured that the CTBT is consistent with the ability to retain high confidence in the reliability of its existing nuclear force for decades. This conclusion has been demonstrated convincingly since 1995. Specifically, a number of detailed technical analyses by independent scientists working with colleagues from the weapons community, including leaders involved in creating our current nuclear arsenal, reached this finding. It was that determination that led the United States to negotiate the CTBT and sign it in 1996.

Most recently, in August 2002 the National Academy of Sciences published a comprehensive study on technical issues related to the CTBT. The study group, which included retired directors of weapons labs, bomb designers, and technical and scientific experts, concluded that the United States can maintain confidence in its enduring stockpile under a ban on all nuclear-yield testing, provided it has a well-supported, science-based stewardship and maintenance program, together with a capability to remanufacture warheads as needed. The study group also verified that the United States could monitor compliance by other CTBT signatories to standards consistent with its national security.

Two years earlier, a similar detailed analysis led by General John M. Shalikashvili, former chairman of the Joint Chiefs of Staff, was conducted with government cooperation and authorization. It reached the same conclusion and affirmed that the CTBT “is a very important part of global non-proliferation efforts and is compatible with keeping a safe, reliable U.S. nuclear deterrent.” General Shalikashvili further added, “I believe that an objective and thorough net assessment shows convincingly that U.S. interests, as well as those of friends and allies, will be served by the Treaty’s entry into force.”

Strengthening the NPT

Although it raises the question of the need for new nuclear weapons, the Nuclear Posture Review does not discuss nonproliferation efforts, nor does it discuss the potential impact of its initiatives on the strategic policy and weapon-acquisition decisions of other nations. This is curious because their nuclear weapons decisions are apt to have greater impact on the United States than ours will have on them. Rather than developing nuclear devices for new tactical missions, the focus of the U.S. nuclear weapons program should continue to be maintaining a credible strategic deterrent and preventing the spread of nuclear weapons to more countries. A weakening or collapse of the worldwide cooperative effort to counter nuclear proliferation would hurt U.S. interests more than any gains from testing and building new low-yield nuclear weapons would help.

The nonproliferation regime is clearly under stress, and a significant weakness is the apparent failure of its verification provisions. Events since the end of the Cold War have made clear the urgent need for nations to join forces in an effort to strengthen the nonproliferation regime. In 1991, after Desert Storm, the international community was surprised to find that Iraq, a signatory of the NPT, was well on the way to a nuclear capability. Similarly, in the 1990s the United States learned of the North Korean program to produce plutonium for a nuclear weapon, and it recently confronted Pyongyang over its attempts to enrich natural uranium.

Recognizing the limitation of its verification abilities under current arrangements, the International Atomic Energy Agency (IAEA) in Vienna has been engaged in ongoing negotiations to strengthen the NPT’s compliance provisions. Until the early 1990s, the IAEA was not used to discover and frustrate secret nuclear weapons programs because its member states had not agreed to that goal. Rather, the IAEA was used to see that agreed safeguards were applied to installations declared to be engaged in peaceful uses of nuclear materials. Thus, the agency did not go beyond the inspections of installations declared as nuclear by the inspected states, such as Iraq. This deficiency can be fixed by giving the IAEA the power to inspect suspect sites that are not reported by member nations as nuclear installations. A protocol proposed by the IAEA—and supported by the Bush administration and others—would correct this situation. A diplomatic campaign should be mounted to secure its ratification.

Greater care also needs to be taken with export controls. Under the NPT, nuclear-weapon states were encouraged to provide the non-nuclear states all that they needed to reap the peaceful benefits and uses of nuclear energy. That was the basic deal that caused non-nuclear weapons states to accept the limitations of the NPT. The sovereign rights of buyers or sellers of exports relevant to nuclear facilities were limited by an understanding among supplier countries that, in effect, prohibited the transfer of technology useful for fabricating a nuclear weapon. But dual-use technology always presented a problem. It is now up to the nuclear suppliers to agree to and police even stronger restrictions on the sale or transfer of items that could be used for weapons production by non-nuclear countries. Unless these types of transactions can be stopped, the whole nonproliferation effort will be seriously undermined.

These are tough problems and require more international cooperation than has been mustered to date. Rather than moving to develop new nuclear weapons, the United States should push to strengthen the nonproliferation regime through example and through stronger compliance measures directed at those who flout its basic purposes.

Sidney Drell is professor emeritus of physics at Stanford University and a senior fellow at the Hoover Institution. James Goodby was an adviser to President Clinton on the CTBT and is diplomat-in-residence at Stanford University. Raymond Jeanloz is a professor of Earth and planetary science at the University of California at Berkeley. Robert Peurifoy was a vice president of Sandia National Laboratories.