“Trust, but verify” is a mantra and a meme within the nuclear disarmament and non proliferation community. Verification is a point of contention in the debate between those who support nuclear weapons and those who seek their total elimination. Some experts have argued that the debate about verification as a purely technical and politically impartial process in an effort to undermine the feasibility of nuclear disarmament itself, instead of an honest scientific and technical challenge. 

Efforts to prevent the spread of nuclear weapons since their invention have necessitated the development and evolution of mechanisms to prevent the transfer of nuclear materials into weapons programmes, and when it comes to ensuring the destruction of nuclear weapons so they can never be used again.

Two  global nuclear weapons treaties, the Nuclear Non Proliferation Treaty (NPT) and the Treaty on the Prohibition of Nuclear Weapons (TPNW), share the same fundamental nonproliferation verification mechanisms, safeguards agreements with the International Atomic Energy Agency.  While the TPNW is the only global nuclear disarmament treaty that includes a framework to verify the physical elimination of nuclear weapons and the dismantlement of nuclear weapon programmes, in article IV of that treaty.

Verifying non proliferation

Both the NPT and the TPNW require the negotiation of Comprehensive Safeguards Agreements with the International Atomic Energy Agency “with a view to preventing diversion of nuclear energy from peaceful uses to nuclear weapons or other nuclear explosive devices.” (NPT, Article III, 1). The TPNW however, builds on the NPT agreement by mandating that states who do not have nuclear arsenals must “maintain its International Atomic Energy Agency safeguards obligations in force [for them] at the time of entry into force of this Treaty” (TPNW, Article III, 1). The difference between the NPT and the TPNW is that the NPT seeks only to verify non proliferation, whereas the TPNW includes both a reinforcement of the non proliferation verification regime and also is for the disarmament of nuclear arsenals. 

Verifying disarmament

The TPNW was negotiated to allow for nuclear-armed states to either fully dismantle their arsenals and then join the treaty or to join the treaty and disarm within ten years. There are nine nuclear arsenals in the world, and they are not the same, and this process fosters the ability to verify the destruction of each nuclear weapon and the systems in place to develop and maintain them, based on its own individual design. The TPNW recognises that ‘one size does not fit all’ when it comes to nuclear disarmament verification, and has built in flexibility to address this issue. 

For states that choose to destroy their nuclear arsenals before joining the TPNW, article 4 of the treaty requires that “Such a State Party shall conclude a safeguards agreement with the International Atomic Energy Agency sufficient to provide credible assurance of the non-diversion of declared nuclear material from peaceful nuclear activities and of the absence of undeclared nuclear material or activities in that State Party as a whole.”

For those that choose to join the treaty before eliminating their arsenals, they must “immediately remove them from operational status, and destroy them as soon as possible but not later than a deadline to be determined by the first meeting of States Parties, in accordance with a legally binding, time-bound plan for the verified and irreversible elimination of that State Party’s nuclear-weapon programme, including the elimination or irreversible conversion of all nuclear-weapons-related facilities.” The first meeting of States Parties established a ten-year deadline for the destruction of arsenals and related infrastructure.

A publication by UNIDIR (2022) “Verifying Disarmament in the Treaty on the Prohibition of Nuclear Weapons”released in the year after the TPNW entered into force explores in detail the ways and means by which the treaty supports the verified elimination of nuclear arsenals and their related infrastructure. The publication argues that the decision to join the TPNW made by a nuclear-armed state would require an internal political transformation, and be visible to the international community, and the disarming state would seek active cooperation with the international community. Precedent shows that this active cooperation then leads to coordinated removal of fissile materials, “on-site inspections, the elimination of missiles and aircraft, the shutdown of fissile material production facilities, and ambitious declassification efforts.”.  

Other treaties

Before the TPNW, only the bilateral SALT and START agreements between the Russian Federation and the US contained verification procedures to ensure the dismantlement of nuclear weapons. The new START treaty is currently in effect (until 4 February 2026) and verification mechanisms in the treaty include a database of treaty-required information, transparency measures, a commitment not to interfere with national technical means of verification, the exchange of telemetric information, the conduct of on-site inspection activities, and the operation of a Bilateral Consultative Commission (BCC).

Ongoing efforts to strengthen international verification of nuclear disarmament

In 2019, the UN General Assembly established a  Group of Governmental Experts to consider the role of verification in advancing nuclear disarmament, which met several times and delivered a final report to the UN General Assembly in 2023. The report concluded, inter alia,  that any nuclear disarmament verification “regime will always be dependent on a specific treaty, agreement or arrangement, with all its parties having an equal right to participate in the process of verification in accordance with the provisions of such a treaty, agreement or arrangement.” and that work on nuclear disarmament verification “is not an end in itself and is not a prerequisite to progress on nuclear disarmament.”

In the UN General Assembly First Committee in 2024, the “Group of Scientific and Technical Experts on Nuclear Disarmament Verification” resolution was adopted which will develop another report by the UN Secretary General to be submitted to the 2025 UNGA, “containing possible options for the establishment of a Group of Scientific and Technical Experts on Nuclear Disarmament Verification within the United Nations”.

For over a decade, the US government and the Nuclear Threat Initiative have facilitated an International Partnership for Nuclear Disarmament Verification (IPNDV). The group includes 30 partner countries and the European Union, and holds regular meetings and events  to identify and develop solutions to the technical and procedural challenges associated with effectively verifying nuclear disarmament.

These broad efforts demonstrate a willingness to engage on nuclear disarmament verification and to find working solutions so that when the political choice to dismantle an arsenal is made, there is sufficient technical capacity to do so verifiably. 

How can you be sure zero means zero?

A number of countries have had nuclear weapons programmes in the past, and South Africa had a nuclear arsenal.  The nuclear weapons programmes of South Africa, Iraq and Libya are three examples in which the international community is confident that no nuclear weapons capabilities remain. Yet, each of these verification efforts took place in differing circumstances and required different tools and approaches.  Robert E. Kelley, a participant in all three denuclearization efforts, goes into detail about this in a piece for SIPRI. 

In all three of the cases, the International Atomic Energy Agency successfully verified that no nuclear materials remained for weapons purposes and returned the countries to ordinary IAEA safeguards, as mandated for states without nuclear weapons under the NPT, and for all states under the TPNW. 

The safeguards regime under the IAEA, especially when combined with the Additional Protocol (which allows for deeper on-site investigation) is the globally agreed mechanism to prevent nuclear weapons programmes. It has some limits (for example, it does not include a mandate to consider nuclear weapon proliferation in the context of missiles designed to carry them or the overlap with other WMD activities such as chemical weapons). 

It is notoriously difficult to develop a clandestine nuclear arsenal - and with the continued advancement of open source technologies (including satellite imagery) - it is becoming increasingly more difficult.

People have signed chemical weapons treaty but chemical weapons get used- why would it be different with nuclear weapons? 

Chemical weapons and nuclear weapons are both weapons of mass destruction, but they are not the same.

The Chemical Weapons Convention (CWC) prohibits chemical weapons, but there are some dual-use chemicals (like rubbing alcohol) which are banned as chemical weapons but can be used for peaceful purposes under the CWC. Most countries have infrastructure that involves chemicals, and the verification of the Chemical Weapons Convention involves dedicated personnel to make sure that activities involving Scheduled chemicals and unscheduled discrete organic chemicals at facilities and plant sites declared by States Parties are for purposes not prohibited under the CWC.  

Nuclear weapons are prohibited under the Treaty on the Prohibition of Nuclear Weapons, but that does not limit the right of States to access and use nuclear technology. However, decades of experience through the Nuclear Non Proliferation Treaty have demonstrated how difficult is is to build a secret nuclear programme - the development of nuclear explosive devices and nuclear weapons is significantly more challenging and requires complex technological know how that cannot be easily hidden from existing nonproliferation verification mechanisms, nor open source imaging.