In October, the Wall Street Journal reported that the United States and Russia were nearing an arms control agreement that would freeze the number of nuclear warheads on each side and extend the New Strategic Arms Reduction Treaty (New START) for a year.

Although the report proved to be incorrect in its conclusion that Russia and the United States were about to reach their first nuclear arms control agreement in more than a decade, the simple fact that the United States and Russia were formally negotiating an extension to New START was grounds for optimism. What was most remarkable was that the sides were discussing a verifiable freeze on the total number of nuclear warheads in their arsenals. This has never been done before. In fact, neither side has ever told the other how many nuclear warheads it possesses.

Nuclear arms control agreements up to now have only directly limited the nuclear warhead delivery vehicles (land-based missiles, submarine-based missiles, and aircraft) and not the warheads themselves. New START, for example, limits the number of strategic nuclear warheads deployed by each side to 1,550 and the number of deployed delivery vehicles to 700. It entered into force in February 2011 and will expire in February 2021 unless both sides agree to extend its duration by up to five years. New START does not, however, cover thousands of non-deployed or non-strategic nuclear warheads on both sides. That is, a warhead is considered a warhead under the treaty only if it is deployed—even though each side has thousands that are not deployed.

October was a long time ago. Since then, negotiations led by the Trump administration have collapsed, and Joe Biden has become the president-elect. All indicators suggest he intends to extend New START without any new conditions.

But that does not rule out the possibility of an agreement to freeze or reduce warheads in follow-on talks. If such an agreement were reached—one that allowed both sides to know their total nuclear warhead inventories, and to verify a cap or reduction to those inventories—it would be a political, technical, and diplomatic achievement unprecedented in the history of nuclear arms control.

The incoming Biden administration is likely to pursue a world with fewer nuclear weapons and perhaps sketch the first steps on a road to zero. Verifying warhead inventories and their eventual dismantlement is essential to this objective. Overcoming the political and technical challenges of warhead arms control is a daunting task that will require expanded and sustained effort.

Fortunately, scientists and strategic planners in several nations possessing nuclear weapons have been investigating the practical requirements of nuclear warhead arms control for more than 25 years, and much progress has been made. Several candidate technologies and procedural approaches for verifying the storage and dismantlement of nuclear warheads exist. However, these potential solutions need further technical development to be successfully used in the context of a future international treaty. To make nuclear warhead arms control a reality, President-elect Biden’s national security team will need to launch a strategic verification initiative and coordinate the cooperation of the US interagency community and key international partners.

The challenges of warhead arms control. It remains profoundly in the security interests of the United States, Russia, and the rest of the world to aggressively develop a safe and reliable means to verify a potential nuclear weapons freeze and additional reductions to global nuclear arsenals. First, because of their immense destructiveness, the effects of nuclear weapons cannot be limited to the nations that use them in a potential conflict. The use of even a handful of nuclear weapons would cause worldwide human suffering. The risk of nuclear war remains unacceptably high and has been joined by a nexus of global threats, such as pandemics, climate change, and terrorism, against which nuclear weapons provide no defense.

Second, the United States and Russia, as members of the 1970 Nuclear Non-Proliferation Treaty, are obligated along with all nations possessing nuclear weapons (except India, Pakistan, Israel, and North Korea, who are not party to the treaty) to work toward the eventual elimination of nuclear weapons.

Third, 50 nations have already declared their willingness to make the possession of nuclear weapons illegal under international law. They did so by ratifying the 2017 Treaty on the Prohibition of Nuclear Weapons. The treaty will enter into force in January 2021. None of the states possessing nuclear weapons has joined the treaty. However, as more countries join on and become bound by its terms, the more powerful their argument will be that their millions of citizens are being unjustifiably endangered by the tiny minority of states that claim the right to possess and use nuclear weapons.

Thankfully, the number of nuclear warheads in the world has declined significantly since the Cold War, down from a peak of approximately 70,300 in 1986 to an estimated 13,410 in early 2020. Nearly all of these reductions in nuclear warheads have been unverified—they were declared to have taken place by the countries that built them, but never confirmed by any international authority.

There is reasonable confidence that the Russian reductions have indeed taken place because US officials have been able, by means of national intelligence, arms control inspections, and other cooperative agreements, to verify the dismantlement of thousands of missiles, submarines, and bomber aircraft that once carried the larger arsenals of warheads. The outside world has also been able to observe with satellite and other imagery the movement of highly secure convoys of trains and trucks taking surplus nuclear warheads to national storage sites and dismantlement factories. Additionally, Russia sold the United States hundreds of tons of highly enriched uranium that came from retired and dismantled nuclear warheads. Finally, these same intelligence capabilities enable US officials to see that Russia has not added to its stockpile of fissile materials for the construction of new warheads.

But there is still a good deal of uncertainty regarding Russia’s total inventory of nuclear warheads and whether it plans to manufacture more. Nor are US officials sure how many nuclear warheads are possessed by China, India, France, the United Kingdom, Pakistan, Israel, or North Korea—and these countries, in turn, cannot independently verify the United States’ declarations of its warhead inventories.

There is a primary reason for the difficulty of verifying nuclear warheads: The designs of these items are considered military secrets by nations possessing them. Understandably, these design secrets must be protected, and no nation possessing nuclear warheads will allow a competitor or potential adversary to directly observe the dismantlement of their nuclear warheads or to know all the details of how and when they transport these sensitive items. Therefore, the challenge is to develop the means for the international community to have very high confidence that when a nation declares that it has a certain number of warheads or that it has dismantled some number of them during a certain time period that it is telling the truth.

It has long been recognized that this can only be done through a program of cooperative, international technology development. All parties to a potential agreement must believe that verification of compliance is possible. This situation is challenging, but not unprecedented. The Nuclear Non-Proliferation Treaty has a safeguards system of reporting, inspections, and monitoring to which 191 nations have agreed. The purpose of the safeguards system is to provide confidence that member states meet their obligations under the treaty. Internationally approved verification equipment is in use at hundreds of nuclear energy facilities worldwide to assure that no materials or technology from these facilities are diverted for nuclear weapons purposes. Another example of a verification system is the International Monitoring System for the Comprehensive Nuclear Test Ban Treaty. This cooperatively developed system can detect nuclear explosions anywhere on Earth.

A bilateral or multilateral system for verifying a nuclear warhead freeze or future dismantlement of nuclear warheads would share some similarities with the nuclear safeguards system and the International Monitoring System. However, its technical elements would need to account for the much more classified, sensitive, and hazardous nature of nuclear weapons versus civilian nuclear materials or detecting test explosions. Verifying nuclear warheads and their eventual dismantlement requires nations possessing those warheads to share information essential to proving compliance with a treaty while keeping secret information on the design of the weapons and anything that could compromise their safety and security.

Candidate approaches for warhead arms control. A fundamental element of any future treaty to freeze or reduce stockpiles of nuclear warheads would be declarations by the treaty partners. These declarations could include the numbers and locations of nuclear warheads subject to the treaty. The provisions of the treaty would then define how those declarations are to be reciprocally confirmed. For example, the United States and Russia could declare that they have a certain number of nuclear warheads stored among several military bases and that this number will not increase for 10 years. Alternatively, they could sign a treaty that identifies sets of warheads possessed by each side that will be dismantled over a specified time period.

The first step in confirming these declarations is to prove that the items declared to be nuclear warheads are in fact nuclear warheads. Deployed warheads sit atop missiles on mobile launchers, in silos, or aboard submarines. New START, the bilateral treaty between the United States and Russia, has established accepted rules for counting and periodically confirming the number of deployed strategic warheads on each side.

Non-deployed nuclear bombs and warheads for both strategic and non-strategic nuclear weapon systems are typically stored in containers near their carrier aircraft or missiles or at central national storage sites. For missile warheads, storage containers can be heavy steel cylindrical or rectangular containers ranging in size from a typical 55-gallon drum to that of a small automobile. Nuclear bombs are stored encased in the aero shells that deliver them from the aircraft to their targets or removed and placed in containers similar to those for missile warheads. It is these sets of warheads—those not atop strategic missiles, those stored near bomber bases, and all non-strategic nuclear warheads—that currently lack verification.

The process of confirming that an item in a container is a nuclear warhead has been called “authentication.” For more than 20 years researchers from several nations have been experimenting with methods to confirm the presence of a nuclear warhead or warhead component inside a container without revealing sensitive or classified information about the design of that warhead or component. Several technical approaches hold promise.

One approach known as “attribute matching” establishes authenticity of an inspected item by demonstrating that the item possesses the intrinsic and unclassified characteristics of a nuclear weapon. All nuclear warheads contain fissile isotopes of plutonium or uranium that emit either neutron or gamma radiation. Some of this radiation will escape from the warhead container and is “passively” detectable by instruments outside of it. The detectable radiation provides an energy “signature,” confirming the type of the radioactive materials contained within. The signature can also indicate the quantity and age of nuclear material present.

Measurable radiation can also be induced from a warhead container without opening it by “active interrogation” of the container with a beam of neutrons. Both passive and active analysis techniques could confirm several key attributes of a containerized nuclear warhead such as: the presence of weapon-grade plutonium; the presence of a threshold mass of plutonium; the age of plutonium; the presence of weapons-grade highly enriched uranium; the presence of a threshold mass of highly enriched uranium; and the presence of high explosives.

Once these attributes were confirmed, parties to a treaty might accept that a container holds an actual nuclear warhead. That container can then be tagged, sealed, and tracked to a storage or dismantlement facility.

Another approach to verify the identity of specific types of warheads is a process known as “template matching.” Template measurements do not confirm that an inspected item possesses certain attributes or characteristics. Instead, the template approach seeks to generate a unique “fingerprint” of the inspected item and then compare this signature against a recorded “template” of a similar fingerprint generated from measurements of an item known to be a real warhead. The template contains information on mechanical, thermal, electrical, acoustical, and nuclear properties of that specific warhead type. Like attribute systems, template systems can be passive or active.

The fissile materials in nuclear weapons emit gamma rays with spectral distributions characteristic of the isotopes contained in the weapon materials. Because gamma rays are scattered and absorbed by intervening materials (casing, containers, etc.) the gamma-ray distribution is also affected by non-emitting materials. The resulting spectra are sufficiently distinctive to identify items by comparing a measured spectrum with the template for the declared type of warhead.

A fundamental challenge of using the template approach is ensuring that the template was produced using an authentic weapon or weapon component. The inspecting party might not trust that the inspected party has created an accurate template to match its warheads against.

Another fundamental challenge for both attribute and template systems for warhead authentication is the need for software and hardware “information barriers.” These are features of the inspection system that prevent the unintended disclosure of the sensitive and classified details of warhead designs or the introduction of false information as a means to spoof the inspection system such that it would authenticate an item that was not a real warhead. Several approaches to this challenge are in development that employ cryptographic and information security techniques. Much work still needs to be done to compare candidate techniques for this challenge and demonstrate them in a real-world context.

Once an item has been authenticated to be a nuclear warhead, technologies and procedures are needed to provide “chain-of-custody” information regarding the disposition of that warhead to the treaty partners. In the case of a future agreement confirming the storage of nuclear warheads or their eventual dismantlement, chain-of-custody technologies would ensure that declared warhead inventories are maintained and that nuclear components from dismantled nuclear warheads are not re-assembled.

Tags that can uniquely identify items and seals on containers or storage bunkers that provide evidence of tampering are fundamental and well-established means of maintaining chain-of-custody. Remote monitoring systems that can transmit trusted information about the status and security of stored nuclear warheads have also been demonstrated. These systems often include monitored locks on doors and portals and surveillance sensors such as cameras, motion detectors, and physical barriers. While this is a more mature field of arms control verification than warhead authentication, chain-of-custody approaches for the specific purpose of nuclear warhead verification and storage monitoring require further development.

Many of these candidate warhead monitoring and verification technologies are well beyond the prototype stage. Some are available for final testing at actual nuclear warhead storage or dismantlement facilities. Still, the various stages of the nuclear warhead life-cycle are complex and hazardous. The final hurdle of getting treaty partners to accept and have confidence in the effectiveness, safety, and security of cooperatively-developed verification systems, such that they will allow them to be used on their own warheads, will yet take years to achieve.

An outstanding public resource and research tool for understanding the technical and operational challenges of warhead arms control has been created by the Washington, D.C.-based Nuclear Threat Initiative, which implements an international partnership on nuclear arms control verification supported by the US State Department. The Nuclear Threat Initiative’s research tool and partnership program provide a blueprint for the remaining technical and organizational challenges of nuclear warhead arms control as well as the foundation of a strategic plan to overcome these challenges. It could serve as a vital public and international interface to an expanded US warhead verification initiative.

What would a US-led nuclear warhead verification initiative look like? To address the capability shortcoming, the Biden administration could at last implement a key recommendation of the 2010 US Nuclear Posture Review. It can launch a national program on “expanded work on verification technologies and the development of transparency measures” for nuclear arms control and nonproliferation. Such a program would include a major technology development effort for verifying nuclear warhead storage and dismantlement.

This program would support the broad range of US nuclear security objectives.

For example, as arsenals decrease in size, it becomes more important to know that a potential adversary is actually retiring and dismantling warheads and not creating a clandestine reserve. It is also vital for all nations to know that the process of warhead transportation, storage and dismantlement is occurring in a safe and secure manner so that personnel are not harmed and dangerous materials do not fall into the hands of criminals or terrorists.

An expanded US nuclear warhead verification initiative would require the direct attention of the offices of the Secretaries of Defense, State, and Energy. It should be funded as a core aspect of the nation’s nuclear infrastructure modernization plan, and thus implemented jointly by the National Nuclear Security Administration (NNSA) and the Defense Department, with guidance from the State Department, intelligence community, and National Academy of Sciences. Such a program is affordable at a fraction of the cost of other nuclear infrastructure modernization tasks, such as building new warheads, submarines, or missiles, and is needed to maintain nuclear security in an evolving security environment.

The NNSA Office of Defense Nuclear Nonproliferation and its Office of Nonproliferation and Arms Control implement the majority of the US verification research and development activities. Historically, an additional program run by the Defense Threat Reduction Agency, Basic and Applied Sciences Directorate supports verification technology as well. The State Department’s Bureau of Arms Control, Verification and Compliance, Office of Verification and Transparency Technologies also participates in the interagency community identifying technological solutions for verification and compliance issues for arms control challenges. These three agencies co-chair the Nonproliferation and Arms Control Technology Working Group, an interagency coordinating body that guides the research and development of new verification and transparency technologies and assets. The White House and National Security Council would provide direction and coordination to an arms control verification initiative through this working group.

A robust strategic nuclear verification initiative to meet nonproliferation, verification, and monitoring technology needs should receive annual funding on the order of $250 million to $300 million. It is very important to set the precedent of creating this line item within the nuclear infrastructure modernization plan that enjoys strong bipartisan support in Congress. The missions of nonproliferation and arms control verification have too long been considered part of the “soft power” tools of the diplomatic and arms control communities. As such, responsibility for them has been scattered across mid- to low level departments within several different federal agencies without strong national champions in the interagency bureaucracy or the Congress. The Biden administration must consider nuclear verification capabilities as potent elements of “smart power” tools offering unique advantages in a rapidly evolving nuclear security environment. It is critical that they are recognized in this perspective and afforded a corresponding degree of attention and resources.

Consistent with their nuclear security mission, the US national laboratories can also contribute to the formulation of a strategic national nuclear verification program. Los Alamos, Sandia, and Lawrence Livermore labs, along with the Nevada National Security Site, stand out in this regard. They have the skills and experience to successfully develop the needed technology and procedures and should be assigned a major role in program implementation. They operate unique and essential facilities that are certified to safely and securely handle nuclear weapons and their components for real-world verification testing. Some construction or refurbishment of appropriate research and development and testing facilities for nonproliferation, verification, and monitoring technology at the national labs and other sites will be required. The wider national laboratory network can also participate in government-funded university research consortiums to advance work on warhead arms control.

In addition to providing a path forward for verifying a potential freeze on nuclear warhead inventories and further reductions to those inventories in the form of a follow-on treaty to New START, deployment of verification and monitoring systems can actually reduce tensions and build confidence between nations, leading to improved international stability and reduced nuclear risks. Joint development of these capabilities can establish valuable international partnerships and commercial relationships.

The Biden administration is likely to reaffirm the vision that President Obama outlined in his 2009 address in Prague, declaring that seeking a world without nuclear weapons was profoundly within the security interests of the United States. The rest of the world cannot verify the next nuclear warhead arms control agreement or take significant strides toward a world without nuclear weapons until it develops and reaches agreement on technologies and procedures for cooperatively verifying compliance with such new accords. A strategic nuclear verification initiative that includes developing the capabilities for making nuclear warheads items of account in a future treaty is essential for progress towards a safer, more resilient, and sustainable global community. It is in America’s interest to lead the way.