The discipline of nuclear forensic science (often referred to as ‘nuclear forensics’) can be defined as the examination of nuclear and other radioactive material, or other evidence contaminated with radionuclides, in the context of legal proceedings. It can contribute to both the prevention of and response to an event involving the illicit trafficking of nuclear or other radioactive materials. Nuclear forensics is a maturing and developing discipline, with many states increasing their capabilities for the conduct of a nuclear forensic examination, supported by technical assistance from the International Atomic Energy Agency (IAEA). The chapter explains where nuclear forensics fits in the bigger picture of nuclear security—charting its history, demonstrating how it works, and discussing the direction in which it is being developed.

The importance of arms control agreements and commitments was underlined early in 2022 by a joint statement by the leaders of the five permanent members of the United Nations Security Council (the P5) on ‘Preventing nuclear war and avoiding arms races’. However, the full-scale invasion by one of these nuclear weapon states—Russia—of a neighbouring non-nuclear weapon state—Ukraine—led to significant setbacks in bilateral and multilateral engagement on nuclear arms control throughout the rest of the year.

 The war presented unprecedented nuclear safety, security and safeguards challenges for the International Atomic Energy Agency (IAEA), the Ukrainian authorities and the personnel of nuclear installations in Ukraine (see  section  V). Never before had operating nuclear power plants been attacked by shelling or missile strikes by state militaries, nor occupied by military forces. The IAEA undertook multiple missions of technical experts to Ukraine in 2022, and subsequently established a permanent presence at all four nuclear power plants there. The IAEA also put forward a conceptual framework—the ‘seven indispensable pillars of nuclear safety and security’—for addressing threats to nuclear installations in wartime.

The Russia–Ukraine war continued to have a negative impact on bilateral and multilateral engagement on nuclear arms control in 2023. While modest positive steps were made elsewhere, including between the USA and China, overall the war diminished opportunities to break the long-standing deadlock in nuclear arms control and reverse the worrisome trend of nuclear-armed states developing and deploying new weapon systems (see  chapter  7).

 Russia’s continued targeting of critical infrastructure in Ukraine added to the nuclear safety, security and safeguards challenges in 2023 (see  section  IV). Frequent disturbances to the Ukrainian electricity grid caused by such attacks placed strain on Ukrainian nuclear power plants, while the destruction of the Kakhovka Dam in June threatened the supply of cooling water to Zaporizhzhia Nuclear Power Plant (ZNPP) and required a stopgap solution. The International Atomic Energy Agency (IAEA) maintained a consistent presence in Ukraine throughout 2023. Building on its ‘seven indispensable pillars of nuclear safety and security’, the IAEA also formulated and began to apply five concrete principles for application at ZNPP.

The fields of nuclear disarmament, arms control and non-proliferation are replete with challenges. Worrisome trends in nuclear forces continued in 2024, with the development, deployment and testing of new weapon systems by nuclear-armed states (see chapter 6). In the words of the United Nations secretary-general, António Guterres, the world was ‘heading in the wrong direction entirely’.1 Disarmament appeared more elusive than at any point since the end of the cold war. This was especially so as strategic dialogue between the possessors of the world’s two largest nuclear arsenals—the Russian Federation and the United States—had effectively ceased. Their bilateral relations were at ‘their historically lowest point’, a direct result of Russia’s ongoing war since its full-scale invasion of Ukraine in February 2022.2 Tensions persisted too among the five permanent members of the UN Security Council—China, France, Russia, the United Kingdom and the USA (the P5)—which also are the five nuclear weapon states (NWS) recognized under the 1968 Treaty on the Non-Proliferation of Nuclear Weapons (NPT).

There exist elements apart from uranium and plutonium that could potentially be used to construct the core of a nuclear explosive device. These belong to the so-called minor actinides (MAs), which exist innonnegligible amounts in spent nuclear fuel (SNF) and are in nearly all cases not covered by international safeguards. Future reprocessing of SNF could result in significant separation of these elements, potentially leading to new proliferation concerns. In this work, a methodology for a transparent assessment of the barriers against proliferation of MAs has been developed and applied to the case of neptunium, americium, and curium separated from spent fuel from pressurized water reactors. In this methodology, openly available data and Monte Carlo simulations have been used to assess the barriers posed by a number of parameters relevant to the production of a nuclear explosive device from SNF. The evaluation shows that the properties of neptunium present low barriers to proliferation and that it should be discussed within the context of future nonproliferation treaties and possibly be placed under international safeguards. The properties of americium and curium present higher barriers to proliferation, meaning that these elements require less focus in the nonproliferation context.

Production rates of fissile materials are often used to independently assess the number of nuclear warheads a state may possess. One key constraint of a plutonium-based nuclear weapons program is the availability of natural uranium, where a shortage of uranium will constrain plutonium production in the fuel cycle. Recycling of the reprocessed uranium can be used to mitigate such a shortage. Furthermore, since military reactors operate in short cycles to ensure that the plutonium is weapon-grade, it may be possible to operate them using slightly depleted uranium, provided that there are sufficient reactivity margins. Using slightly depleted or recycled uranium, the plutonium production can increase by a factor 2–5 as compared to a once-through scenario, for the same input of natural uranium. For future assessments of a state’s plutonium production, a uranium constraint should only be considered if there is clear evidence that no nuclear fuel cycle involving uranium recycling is implemented, or if evidence exists that the recycling is insufficient to mitigate the constraint.

This work investigates fuel properties of submarine reactor fuel from a non-proliferation and safeguards perspective in light of the deal involving Australia, the United Kingdom and United States known as AUKUS. This study investigates the isotopic composition of the spent fuel at the end of intended reactor life. The fuel in the proposed AUKUS submarine is modelled after a Virginia Class fast attack submarine, discussed as an option for Australia.

The vast majority of civil experience with plutonium production is with fuel starting at low enrichments for shorter burnups. The AUKUS fuel at the start of irradiation campaign is assumed to contain uranium enriched to between 93% and 97,3%. It is burned at high power for about 33 years before retirement. Because the fuel is mostly uranium-235 initially, there are very few thermal captures leading to production of plutonium-239. In the submarine, the majority of non-fissile captures lead to the production of uranium-236 with other capture chains that do not lead to the production of weapons grade plutonium or weapons usable uranium.

This study concludes that the final isotopic composition of the AUKUS spent fuel is no longer VHEU, but a low grade of HEU diluted largely by uranium-236 instead of uranium-238. Several kilograms of plutonium are produced but it is composed of several different plutonium isotopes with a large fraction of plutonium-238. There is little likelihood that spent AUKUS fuel will be reprocessed by any of the countries involved. But if it were reprocessed, the resulting uranium and plutonium will have very unusual isotope compositions. Resulting materials would be subject to safeguards but would not, in fact, be well-suited as fissile material for weapons purposes.

This paper examines the underlying reasons why states have chosen to pursue a particular fissile material route in their nuclear weapons development. This analysis is conducted using case studies that describe historical developments and events, alongside an evaluation of their impact on the choice of fissile material route. Key areas of interest include the role of uranium resources, the visibility or covert nature of nuclear weapons activities, international relations, military delivery systems, and measures imposed by the export controls and nuclear safeguards regimes. The results of the work show that although uranium resources play a role, they impact the route in only one case. Insight into whether or not the nuclear weapon program is conducted openly or covertly does not seem to have impacted the selected fissile material route much, and the same can be said about the military delivery systems. In contrast, international relations, both in terms of government-to-government relations and proliferation networks, appear as far more important. The impact of export control and safeguards measures is shown to depend heavily on the context of international relations and the unique circumstances of each case.