Policy Memo | June 2025

Countering Nuclear Terrorism: The Strategic Value of the U.S. – U.K. Mutual Defense Agreement

Nuclear terrorism remains one of the most urgent and complex security threats facing the United States and its allies.1 Non-state actors continue to seek access to fissile and radiological materials that could be used in a nuclear device or a radiological dispersal device (RDD).2 The U.S.–U.K. Mutual Defense Agreement (MDA) has long served as a cornerstone of bilateral cooperation on nuclear deterrence, technology transfer, and material security. In the context of modern nuclear terrorism threats, the MDA enables deep coordination that improves detection, interdiction, and response capabilities that neither country could achieve alone. This memo outlines the evolving nuclear terrorism threat landscape, examines how the MDA supports strategic countermeasures, identifies key challenges, and proposes realistic steps to enhance bilateral cooperation against nuclear terrorism further.3

Background: Nuclear Terrorism as a Persistent Threat

The threat of nuclear terrorism stems from the potential for non-state actors to acquire and use nuclear or radiological materials. Groups such as al-Qaeda and the so-called Islamic State have expressed intent to obtain weapons of mass destruction, and there have been numerous reported cases of attempted trafficking in nuclear material over the past two decades. Radiological sources used in medical and industrial applications are often poorly secured, making them potential components for an RDD or “dirty bomb.” The IAEA Code of Conduct on the Safety and Security of Radioactive Sources provides international guidance on securing high-risk radiological sources often targeted for potential use in RDDs.4

Agencies such as the National Nuclear Security Administration (NNSA), the International Atomic Energy Agency (IAEA), and the UN Security Council (via Resolution 1540) consistently emphasize the danger posed by nuclear terrorism. The IAEA’s Incident and Trafficking Database has tracked hundreds of confirmed illicit trafficking or attempted acquisition incidents involving nuclear or radiological material. U.S. national security strategies also prioritize the prevention of nuclear terrorism as a top-tier threat. The potential consequences of even a low-yield device or RDD detonated in a central urban area include mass panic, economic disruption, and severe political destabilization.5

Strategic Role of the MDA

The U.S.–U.K. Mutual Defense Agreement, first signed in 1958 and amended multiple times since, enables extensive collaboration in nuclear security and counterterrorism. The MDA facilitates the exchange of sensitive nuclear information, joint development of technologies, and cooperation in naval nuclear propulsion, radiological threat detection, and nuclear forensics. It also allows for collaborative research and development (R&D) on protective measures against nuclear and radiological threats.

Through the MDA, both countries leverage each other’s expertise, infrastructure, and intelligence to prevent, detect, and respond to nuclear terrorism more effectively. For example, joint R&D efforts have advanced detection systems capable of identifying shielded radiological material. Shared intelligence and operational coordination improve interdiction efforts and reduce the likelihood of material trafficking going undetected. The MDA also enhances naval security through cooperation on reactor safety and dual-use technologies that contribute to broader nuclear threat reduction.

Bilateral efforts allow for agility and depth that would be difficult to replicate through multilateral or unilateral channels. The high level of trust and integration between U.S. and U.K. agencies ensures more rapid and secure exchange of information and capabilities. The MDA thus forms a unique backbone of resilience against emerging nuclear terror threats.6

Gaps and Challenges

Despite its strengths, the MDA faces several challenges that could limit its effectiveness in countering nuclear terrorism.

• Transparency and Oversight: The classified nature of the MDA has raised concerns about oversight and public accountability. Limited disclosure can generate criticism and undermine trust in the agreement’s alignment with nonproliferation norms.

• Interoperability and Coordination: While the U.S. and U.K. maintain strong bilateral ties, there remain gaps in interagency coordination, particularly in responding to unconventional nuclear terror scenarios like dirty bomb attacks in civilian areas.8

• Limited Inclusion of Third Parties: The bilateral structure of the MDA limits engagement with other close allies or partners who may contribute valuable capabilities to counter-nuclear terrorism efforts.9

• Potential Areas for Expansion: Given the classified nature of the MDA, it is unclear to what extent activities such as joint emergency response drills, red teaming of terrorist scenarios, and the collaborative development of disarmament verification tools are currently pursued. However, these are underutilized based on some publicly available sources and represent promising areas for expanded bilateral engagement.10

Policy Recommendations:

1. Institutionalize Bilateral Nuclear Terrorism Training and Exercises: Establish regular U.S.–U.K. joint exercises focused on radiological terrorism response, interdiction of smuggled nuclear material, and emergency management. These should include relevant defense, intelligence, and public safety agencies and build on existing NNSA and DOE training platforms.11

2. Create a Joint Counter-Nuclear Terrorism Task Force: Establish a formal bilateral working group comprising officials from NNSA, DoD, DHS, and their U.K. counterparts to enhance information-sharing and facilitate coordinated threat assessments. This body should also support the development of joint response protocols for nuclear and radiological incidents.12

3. Enhance Transparency and Public Reporting: Commit to annual unclassified reporting on MDA-related nuclear security cooperation, highlighting joint initiatives in counterterrorism and material protection. This would help reinforce the agreement’s legitimacy and alignment with nonproliferation principles without compromising sensitive details.13

Conclusion

The U.S.–U.K. Mutual Defense Agreement remains a vital tool in the fight against nuclear terrorism. As the threat landscape evolves, so must the mechanisms of cooperation. By addressing coordination challenges, enhancing transparency, and expanding joint preparedness efforts, the United States can ensure that its closest nuclear ally continues to serve as a force multiplier in securing radiological and nuclear materials worldwide. Strengthening the MDA aligns with the strategic objectives of the Department of Energy, NNSA, and broader U.S. national security priorities in an era of growing geopolitical uncertainty and persistent non-state threats.14

Footnotes:

1. National Nuclear Security Administration, FY25 Nonproliferation Program; National Nuclear Security Administration, NNSA 2024 Enterprise Blueprint; International Atomic Energy Agency, ITDB 2025 Factsheet.

2. National Nuclear Security Administration, Nonproliferation Overview.

3. Nuclear Information Service, U.S.–U.K. Mutual Defense Agreement; Nuclear Information Service, Reform Not Renewal.

4. National Nuclear Security Administration, FY25 Nonproliferation Program; Office of Nuclear Threat Science, DOE O 457.1A; National Nuclear Security Administration, Nonproliferation Overview; Carnegie Corporation of New York, Nuclear Terrorism.

5. International Atomic Energy Agency, ITDB Factsheet; National Nuclear Security Administration, FY25 Nonproliferation Program; National Nuclear Security Administration, Counterterrorism and Counterproliferation.

6. National Nuclear Security Administration, U.S.–U.K. Mutual Defense Agreement; Nuclear Information Service, U.S.–U.K. Mutual Defense Agreement; Department of Defense, Nuclear Weapons Handbook 2020; Lawrence Livermore National Laboratory, Nuclear Counterterrorism; Harrington and McCabe, Case for Cooperation.

7. Smith and Burt, Reform Not Renewal.

8. NNSA, FY25 Nonproliferation Program; DOE, Nuclear Counterterrorism.

9. Department of Defense, Nuclear Weapons Handbook 2020; Smith and Burt, Reform Not Renewal.

10. Smith and Burt, Reform Not Renewal; National Nuclear Security Administration, FY25 Nonproliferation Program.

11. National Nuclear Security Administration, FY25 Nonproliferation Program; U.S. Department of Energy, DOE O 457.1A.

12. National Nuclear Security Administration, FY25 Nonproliferation Program; National Nuclear Security Administration, NNSA 2024 Enterprise Blueprint.

13. Smith and Burt, Reform Not Renewal.

14. NNSA, FY25 Nonproliferation Program; DOE/NNSA, U.S.–U.K. Mutual Defense Agreement; DOE/NNSA, NNSA 2024 Enterprise Blueprint; IAEA, ITDB 2025 Factsheet; Ward, “Why U.S.–U.K. Nuclear Cooperation.”