Doing Research Today for the Arms Control of the Future

As is emphasized time and again, arms control is in a deep crisis. The bilateral arms control architecture between the USA and Russia (and previously the Soviet Union) has been in decline for years. Russia's full-scale invasion of Ukraine has further exacerbated this crisis, while its nuclear threats have brought the fear of nuclear weapons use back to the forefront of many people's minds. In the current geopolitical situation, new arms control agreements seem anything but likely and disarmament even less so.

Despite this bleak situation, it is important to both talk about and research new possibilities for verification. Verification, i. e. measures to check compliance with treaties, is an elementary component of arms control. It not only prevents treaty violations but strengthens the mutual trust between treaty participants. Without concrete, tested, and transparent verification procedures, there is no guarantee that treaties are actually effective, thus lowering incentives to sign treaties in the first place. If there is to be any chance at all of overcoming the current arms control crisis, research into verification for the arms control agreements of the future must be carried out now.

This is the mission of the Research Group Science for Nuclear Diplomacy. Established in 2024 as part of the Cluster for Natural and Technical Science Arms Control Research (CNTR), the group is headed by Prof. Malte Göttsche and is based at PRIF and TU Darmstadt.

A central research area of the group is nuclear archaeology. The underlying idea is to develop verification procedures that do not focus on the nuclear bombs themselves, but on the nuclear materials needed to produce them. Nuclear warheads are not particularly large and can be hidden anywhere. It is almost impossible to inspect a country in such a way that you can be sure that there are no nuclear warheads present. What can be observed, however, is the production of the materials needed to make nuclear weapons. Yet despite ongoing discussions of fissile materials in disarmament treaties – such as the proposed Fissile Material Cut-off Treaty (FMCT) – they have never been effectively regulated.

To be able to produce nuclear weapons at all, one needs plutonium or highly enriched uranium – substances that do not naturally occur in this form. In order to obtain weapons-grade uranium, the natural uranium must be “enriched”, usually in a centrifuge, separating the desired isotope from other isotopes. Plutonium, in turn, is produced as a by-product in nuclear power plants. In order to obtain weapons-grade material, it would then have to be separated from other by-products.

These production processes leave behind traces in nuclear reactors and centrifuges, traces that can then be uncovered by nuclear archaeology. Just as traces found during excavations are used in classical archaeology to draw conclusions about past human life, nuclear archaeology uses traces to reconstruct what nuclear materials were produced or used. On this basis, it is possible to conclude whether the treaty compliance information provided by a state is accurate or deceptive.

However, operating enrichment plants alone is not enough to justify suspicion of non-compliance. Nuclear reactors and enriched uranium also have a number of civilian uses, not only for the generation of electricity but also for research and medical purposes. Enrichment facilities and nuclear power plants therefore have a ‘dual-use’ character; they can be used for peaceful, civilian purposes, but can also be used for military purposes, in this case for the production of nuclear weapons. Therefore, the problem is that such facilities cannot simply be banned because their peaceful use is permitted.

With such problems in mind, projects in the Research Group Science for Nuclear Diplomacy deal with how procedures can be created that can distinguish peaceful uses from uses for the production of nuclear weapons.

Fabian Unruh researches the composition of spent fuel rods. Commercial nuclear reactors use fuel rods made from natural or slightly enriched uranium. Once the desired energy has been generated, what remains are spent fuel rods. These still consist of around 90 percent uranium, but also a few percent each of plutonium and fission products. Fission products can be made up of many different elements in all sorts of configurations. The composition is very diverse and varies depending on a number of factors, such as the type of reactor, how long the fuel has been in the reactor and how much time has passed since it was removed, and thus the amount of radioactive decay. The composition of spent fuel rods thus forms a kind of “signature” of a particular reactor and its mode of operation. Depending on how the reactor is operated, the amount of plutonium produced also changes. By measuring the fission products, conclusions can be drawn about how much plutonium was produced. This value can then be compared with the information provided by the reactor operator to determine whether more plutonium was produced than the declared quantity.

In the case of a real inspection, such measurements would be carried out by isolating the elements in the spent fuel rods and “sorting” them using mass spectrometry, which allows us to determine composition according to different isotopes. Another important line of inquiry for the research group is the use of computational methods to simulate the operation of reactors. These are statistical simulations intended to show the probable results of certain parameters during reactor operation. However, because these are not deterministic calculations they cannot be easily reversed, making it difficult to attribute compositions to particular causes. The inference from a composition of isotopes to the operation of the reactor is therefore a more complex problem for which there are different approaches that are also being researched by the group. In addition to measurements and computer simulations, another component of nuclear archaeology is working with archives in which records of reactor operations can be studied.

Lukas Rademacher’s project is dedicated to the solid components of the reactor. These can be tubes or graphite moderators, for example. A moderator is a component of the reactor that helps the neutrons released during nuclear fission to trigger the next fission reaction. In addition to graphite, water is often used as a moderator. However, reactor operation leaves fewer traces in water than in graphite, making it less suitable as an object for observation. In the long term, solid components react with the neutrons released during nuclear fission, which are absorbed or trigger reactions. The type and extent of the reactions depends on what exactly happens in the reactor. Similar to the case of fuel rods, a “signature” of the production processes can also be determined from observations of solid components.

Enrichment plants can also be analyzed in a similar way. However, because they have less pronounced signatures, this is much more difficult. Fuel cycles represent another starting point for nuclear archaeology, meaning that not only a single plant is considered but the entire nuclear program of a state, including reactors, sources of natural uranium such as mines, and the intermediate plants that produce uranium from the products of the mine. In this way, it is possible to investigate, for example, how much plutonium or highly enriched uranium a country could produce in a certain period of time with the resources at its disposal.

The members of the research group are all working on individual aspects of possible new verification methods. The idea is that at some point a common picture will emerge from these many individual inquiries. In practice, such methods could then be combined to develop a conclusive and robust verification program capable of engendering trust in the contracting parties and compliance with the treaties, thereby providing the technical prerequisites for future arms control. (ewa)