The US withdrawal from the 1987 Intermediate-Range Nuclear Forces (INF) Treaty due to Russian violations has raised doubts as to the effectiveness of future arms control efforts, especially in light of their seeming inability to manage the spread of intentional disinformation (NATO Press Release 2019). Consequently, an effective treaty will hinge on successful verification measures that can confirm participants’ compliance with the treaty terms. Should future arms control also include China, such measures will be key, given China’s record of disregard for international norms in areas such as trade and intellectual property rights (Rosenbaum 2019).
For the Biden administration to succeed in developing a new arms control framework after extending the New Strategic Arms Reduction Treaty (New START) in February 2021, it will need to convince Congress that the United States can verify Russian compliance. Congress has been unable to approve for ratification an arms control treaty since 2010, despite the abundance of such low-hanging fruit as three Nuclear-Weapon-Free-Zone treaties submitted for advice and consent—and the Comprehensive Test Ban Treaty, which merely confirms what is already US policy. A partisan—bordering on ideological—distrust of agreements interpreted as restricting the United States’ ability to ensure a qualitative and quantitative advantage over its adversaries has taken root. This dynamic, combined with incidents of cheating, have undermined global arms control.
Part of the problem is due to an inherent feature of any arms control agreement: the need for transparency on the one hand (in order to assure all parties that the agreement is being adhered to), versus security on the other (to make sure that not too much sensitive information is given away). Both sides want greater insight, but giving away too much broad information can undermine one’s own security.
But a solution may be at hand. New verification technologies with greater control over specifically what information is collected can help find a balance in this trade-off. By getting verification right in such an agreement, physics may be able to overcome both international and domestic politics.
Parity is a central tenet of any arms control arrangement, with delivery systems serving as a proxy for a given states’ strike capability in a potential nuclear war. Consequently, verification has primarily focused on delivery vehicles—such as ballistic missiles and strategic bomber aircraft, and their associated launchers—which can be tallied up by inspectors. These features will remain important, but as reductions in the total numbers of weapons continue, achieving confidence in accounting for non-deployed nuclear warheads or tracking mobile assets may be difficult via traditional methods such as satellite photography (sometimes known as “national technical means”)—and on-site visits may not be as palatable in the future as they were in the past. For example, any new potential agreement with China or North Korea may need to rely on other verification methods to retain confidence that each state is abiding by weapons limits or disarmament, because on-site inspections would be new and uncomfortable territory for them.
Furthermore, all states with nuclear weapons are modernizing their arsenals—which may lead to enhanced capabilities or even entirely new weapon systems. As further reductions are made, verification will need to step beyond counting delivery vehicles and address more challenging verification matters.
Three key areas in need of verification
There are three key areas that need to be addressed in order to verify traditional arms control agreements and take steps towards nuclear disarmament. The first category is limits on deployed strategic warheads. The same types of systems counted under New START—intercontinental ballistic missiles, submarine-launched ballistic missiles, and strategic bombers—will likely need to be addressed. But in addition, it will also be necessary to have a means of verifying that new destabilizing technologies, such as hypersonic weapons, are not paired with nuclear warheads; verifying such a thing would reduce ambiguities between the great powers. Finally, including weapons like non-deployed nuclear warheads or mobile assets under any treaty would also increase confidence.
The second category—non-strategic weapons—could be limited for the first time. US Secretary of State Antony Blinken has said that the United States will pursue “arms control that addresses all nuclear weapons” (emphasis mine)—presumably meaning non-strategic weapons as well (Blinken 2021). This is a long overdue step, because so-called battlefield, or tactical, nuclear weapons have evolved to the point where each one can have a yield greater than the weapon that destroyed Hiroshima, and the belief that they are sub-strategic increases the likelihood of their use. The use of a non-strategic nuclear weapons is probably the most likely path that would lead to a full-blown US-Russia nuclear war. The numerical disparity in this area between Russia (at 1,900 non-strategic nuclear weapons) and the United States (230 B61 non-strategic nuclear bombs) will be the greatest political challenge (Kristensen and Korda 2021). Moscow has been reticent to discuss these weapons, however, because it views these weapons as a means to balance perceived American and Chinese conventional superiority.
Finally, the third category—obstacles to direct warhead verification—should be discussed. This challenge has not been addressed by a treaty yet because foreign governments would need to inspect highly classified nuclear warheads. James Acton, a nuclear physicist at the Carnegie Endowment for International Peace, compared this type of verification to Bilbo’s riddle to Gollum in The Hobbit: “What have I got in my pocket?” — a state has a concealed object that it claims is a nuclear weapon. Inspectors need to prove it without revealing the object’s design, which would equate to disclosing classified intelligence (Finney and Acton 2014). New techniques are being developed to address this challenge but have only been proven in principle. So far, they have not been tested in the field.
Getting these three areas right will be critical to the success of any future agreement. Let us now look in detail at each of these three areas in turn, as well as at recent technological and political developments that make progress in each area difficult. We will then discuss the technologies in existence—or still under development—that could provide a base upon which states involved in discussions about arms control could conduct joint research to suitably adjust the technology to meet their security needs. This article will then conclude by identifying the missing ingredient to achieving new objectives in arms control and disarmament: the lack of lab-to-lab engagement. The world’s top scientists must be free to collaborate on technical matters to prepare for the day when there is political room for a deal.
Traditional arms control and verification
Historically, treaties between the United States and Russia primarily focused on the verified dismantlement of delivery systems, as previously referred to. For its part, New START contains a strict verification regime that has succeeded for a decade, including provisions governing the use of satellites and other so-called “national technical means” of verification; the goal was to prohibit measures of concealment which would make it more difficult to gather data on each side’s forces and activities. Other strong verification measures include: an extensive, continually updated database that identifies the numbers, types, and locations of items limited by the treaty; on-site inspections to confirm that the limits on reentry vehicles and non-deployed launchers are being adhered to, count the number of nuclear weapons attached to deployed heavy bombers, and confirm the conversions or eliminations of weapons system and facilities; and a Bilateral Consultative Commission where questions of compliance or implementation may be raised.
Under this treaty, both sides rely heavily on national technical means to collect information about the numbers and locations of the other’s strategic forces. Now, however, commercial satellites and private actors can play a larger role in verifying the terms of a treaty, which will have its pros and cons, as government control over what had been considered sensitive national security information is being eroded (Samson 2022). Not only is it more difficult for global foreign policy decision-makers to set the narrative by disclosing only the information that serves their interests, but they can be forced to confront inconvenient truths that will cause embarrassment.
Private actors have already proven their savvy by identifying three Chinese missile silo locations under construction over the summer of 2021 (Kristensen and Korda, 2021b). The further proliferation of commercial satellites will play a role in efforts to verify treaty compliance, whether a government wants them to or not. While governments may be nervous about the erosion of their control, there are, however, scenarios in which these open-source analysts can be useful—for instance, if silo-based intercontinental ballistic missiles are unilaterally or jointly reduced or eliminated under a new treaty. Let’s look at the pros and cons a little deeper.
Pros: Governments are often loathe to release information collected via intelligence sources because it could reveal those sources. If the government is unable to cite its source, then it is easier for an adversary to dismiss whatever evidence that is presented as disinformation or fabricated. Private actors using commercial satellites can freely cite their sources. In this way, it may be easier to correct cheating if, for example, an agreed-upon verification mechanism does not provide enough evidence of one state’s wrongdoing. Many states lack large-scale intel agencies. Private actors, working together, can provide a cost-efficient substitute and provide imagery analysis previously available to only a few governments. This will have the effect of deterring cheating and increasing confidence should evidence come from sources other than the intelligence apparatus of an interested party.
Cons: Disclosure of information could generate public pressure for the government to take action in cases it might have otherwise tolerated or sought to resolve through extensive talks. Disinformation campaigns are not limited to governments and seemingly legitimate satellite imagery could shift public debate about whether an adversary or partner country is in compliance with its treaty obligations, with potentially disastrous consequences for global trust.
What about hypersonic weapons? The primary challenge to including hypersonic weapons in future arms control talks is simply that there is no political will. While China, Russia, and the United States partake in an arms race without any restrictions, they also have a comparative advantage over the rest of the world. Also, all three countries have stated that they will not mate hypersonic weapons with nuclear warheads. If hypersonic weapons were included, verification would not be any more difficult than the mechanisms negotiated under New START. In fact, New START allowed US experts to inspect Russia’s new Avangard hypersonic missile system in November 2019 (Tass 2019). New START does not apply to hypersonic weapons under the treaty’s definition of strategic weapons because they do not fly on a ballistic trajectory for more than 50 percent of their flight, but Article V of the treaty allows the parties to consider restricting new arms through the treaty’s Bilateral Consultative Commission.
Politically, this may be more challenging because many countries, including Pakistan and North Korea, are making claims about supposed hypersonic capabilities, and promoting a dubious narrative about how much these weapons could ensure superiority. Use of neutron detectors could determine if a nuclear warhead is affixed to any missile, whether hypersonic or not, and there remain options for keeping the delivery vehicles stored separately from nuclear warheads.
Non-strategic nuclear weapons
These weapons were often small enough to be deployed with troops in the field or at forward bases. However, there is no clear dividing line, so this report chooses to define nonstrategic nuclear weapons as any nuclear weapons not covered by strategic arms control treaties.
Where to start? Both sides could start by providing the other with a full accounting of their numbers of nonstrategic nuclear weapons and the status of deployment—i.e., deployed, stored, or awaiting dismantlement. Verification could be assured using the same tools as those agreed upon under New START, including on-site inspections and neutron detectors to determine if a nuclear warhead is affixed to a delivery system. However, the parties must deal with the challenges that led to the collapse of the INF treaty and the United States’ recent response.
Lessons From INF. Intermediate-range missiles were banned under the now dissolved 1987 INF Treaty. The Russians violated the Treaty by deploying the Novator 9M729 missile system (NATO designation SSC-8). Officially, the 9M729 has a range of 480 kilometers, which is not in violation of the treaty. The United States documented that the 9M729 has been tested up to ranges of 2000 kilometers, which is in violation of the treaty. The evidence to support these accusations is not in the public domain and Russia categorically denies violating the INF Treaty. Every other NATO government, however, after seeing the classified reporting, found it irrefutable. Although the treaty ultimately collapsed, it was for years the most successful arms control agreement, due in part to its verification regime. All INF missiles were eliminated by May 1991, within the three-year limit, and a further 10 years of on-site inspection techniques to assure compliance included baseline data inspections; inspections of closed-out facilities; short-notice inspections of declared sites and inspections to observe eliminations of the missile systems; and continuous monitoring operations at the portal and perimeters of a former missile production facility in each country to confirm that production of prohibited missiles had ceased.
From May 2001 until the treaty’s collapse in August 2019, compliance was verified by national technical means, primarily satellites. This mechanism was able to detect Russian noncompliance and make a convincing case to NATO allies that this was the case (Congressional Research Service 2019). Resurrecting the on-site verification provisions may be necessary for future agreements and possibly include a dispute mechanism with greater inspection powers than the Special Verification Commission of the INF treaty.
To increase the signaling and decision-making time in a crisis, the United States and Russia could store non-strategic nuclear weapons at declared storage sites separate from facilities basing the delivery vehicles. Mating the weapon with the delivery vehicle would take time and could be monitored by other national technical means. Confidence in verifying numerical limits, if they are indeed ever set, might include simple technologies such as through a motion-detection, tamper-proof detection system known as a “buddy tag” (Glaser and Kutt 2020). Such a system would require attaching physical tags to potentially missiles launchers or warheads, which would also get around issues related to on-site inspections at highly sensitive locations.
Warhead dismantlement: Can physics hack politics?
Current verification techniques have allowed for reductions in deployed nuclear forces because both sides can more easily verify limits on the number of delivery vehicles, such as missiles and aircraft. Despite these reductions, there is still the difficult challenge of having foreign governments inspect highly classified nuclear warheads. Ultimately, direct verification of warheads will be needed to facilitate warhead dismantlement, eliminate the possibility of rapid rearmament, and reduce the risk that so-called “loose nukes” fall into the hands of terrorists.
Dual-use weapons—meaning those able to carry both nuclear and conventional warheads—will continue to pose a challenge to verification regimes. For example, verifying the 1987 INF treaty included methods such as the use of radiation detection devices to assure that a Soviet SS-20 missile—limited by the treaty—was not contained in the canister of missiles not limited by the treaty, such as the SS-25ICBM. At the moment, radiation detection techniques have only sought to detect the presence of plutonium; they would be inadequate for uranium devices. New instruments under development could provide a more complete picture, particularly if more weapons and states are included in future treaties.
To address the gap in instruments able to detect both the presence of plutonium and uranium, passive gamma-ray detection techniques are under consideration (Lepowsky, Jeon and Glaser 2021). Not only does this technology avoid providing highly classified information about the warheads themselves by simply confirming the absence of fissile materials, but it solves the problem of the number of possible warheads at a site exceeding the number of declared warheads. In this way, concerns about cheating can be quickly addressed and a more complete verification regime assured. Yet, this only touches on the potential for radiation measurement.
Without limits or barriers, however, both gamma and neutron radiation detection technologies could reveal too much sensitive information about a warhead. The challenge for warhead dismantlement verification techniques is two-fold: one, prove that a warhead is indeed a warhead; and two, prove that it has been dismantled without revealing highly sensitive information about the warhead design. Countries do not want to reveal design information that could be used to reverse-engineer the weapon.
Fortunately, there is a great deal of research underway that could provide answers.
There are two measurement approaches for warhead confirmation. The general approach, known as the attribute approach, seeks to measure an attribute and confirm it is a warhead with previously agreed-upon properties. Possible attributes might include: the presence of radiation or plutonium, the plutonium mass, or the uranium enrichment level (Spears 2001). A threshold value for that attribute must be established to ensure confidence of measurement, but such an approach is attractive because it does not require the retention of any classified data.
The template approach seeks to confirm the identity of a given, specific warhead—in a sense, fingerprinting the weapon as what its owner claims it is—without determining the properties that characterize the item.
Among the potential techniques for doing this are the use of information barriers (attribute), zero-knowledge protocols (template), and physical cryptography (template). I will address each of these approaches, and their overlap, in turn:
The information barrier. Broadly speaking, data would pass from a detector through what is known as an information barrier—meaning that some sensitive information is hidden or removed—and the results are displayed in a pass/fail manner. The barriers that could be erected range from software or hardware data manipulation to systems based upon neural networks, genetic algorithms, and pattern recognition (Aitkenhead, Owen, and Chambers 2012). There remain potential opportunities for tampering, however, including software hacks or hidden switches in the electronics. Furthermore, the conditions themselves may need to be somewhat loosely defined so that they will not, by their very presence, suggest information about the warhead design. In order to overcome the lack of trust in the ability of electronics to protect sensitive information, some researchers have proposed using vintage 1970s technology that cannot be hacked (Kutt and Glaser 2019). For example, the central processing units used in Apple IIe computers are so simple that there are few opportunities for the other party to implement backdoors or hidden switches on the hardware level.
Zero-knowledge protocols. Under this technique, sensitive data is never measured, so nothing needs to be hidden by a barrier. The inspector would seek a similar pass/fail determination but would be specifically looking to see if the measurement matches a given template. The problem with the template approach is the need for a “trusted” template in order to make comparison measurements. One solution is to select the template from one of the deployed weapons. Clearly, there are means to overcome this challenge, but it is up to the negotiators to resolve.
A potential method to employ the zero-knowledge protocol is to interrogate two or more items with energetic neutrons and measure the neutron transmission and emission counts to see if there is a match. As described by researchers at Princeton University, “the inspector uses detectors that are preloaded by the host with the negative of the contained warhead’s radiograph” (Glaser, et al 2014). The inspector simply seeks to determine if an object is as declared without viewing any sensitive information. Researchers at the Massachusetts Institute of Technology have proposed an epithermal zero-knowledge component verification. By passing an epithermal beam through a hollow plutonium pit and combining the measurement of this object with its reciprocal, the resulting image will cancel out as long as the pit is authentic (Danagoulian 2018).
Physical Cryptography. While many proposals are similar to the techniques described above because of the need for a reference or template, physical cryptography uses a physical secret key instead of software information barriers. The key in one such proposal is simply a thin foil. Essentially, by passing x-rays through a warhead and then onto a foil—the composition of the foil is only known to the weapon owner—the foil will light up and match a certain pattern that can be identified as a warhead in comparison to a template (Engel and Danagoulian 2019).
The Missing Ingredient
The key ingredient that is currently missing is lab-to-lab engagement between the United States and Russia. There can be little hope of success without joint scientific collaboration to develop these technologies and techniques, even when a treaty is not imminent. A perfect example is the lab-to-lab engagement between Rocky Flats, Lawrence Livermore National Laboratory, and the Siberian Chemical Combine located at the closed Russian city of Seversk under the mutual reciprocal inspections agreement. Signed in 1994, US and Russian technical experts engaged in exchange visits to figure out how to inspect fissile materials removed from dismantled nuclear weapons. Sometimes there are breakthroughs where the end goal is not clear. For instance, a simple form of singles counting to determine if the neutron emitting material was indeed plutonium was developed in this time. While no agreement was reached, the technology research and discussions were relied upon to develop the verification regimes for more structured negotiations between the United States and Russia for years to come (Spears 2001).
Rose Gottemoeller, the former chief US negotiator for New START, has warned that “interdisciplinary science diplomacy is languishing” (Gottemoeller 2020). Successful technological development of these verification methods is vital, but the two sides need to get there together. A revitalization of joint US-Russia collaboration on technical matters is desperately needed, and ideally, such collaboration could be extended to other countries too.
Verification is an essential element of effective arms control and its perceived shortcomings are most often cited as objections to reducing numbers of weapons. This should not be seen as an obstacle to further agreements but rather a challenge to be met in making any such agreements as strong as possible. The success and ultimate conclusion of the INF Treaty provides object lessons in the importance of effective verification. Negotiating such a regime is likely to present one of the greatest challenges in future negotiations, but scientific progress and creative thinking will open further possibilities to build the necessary confidence.
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