The era of nuclear maritime propulsion began in 1954 with the launch of the USS Nautilus, the world’s first nuclear-powered submarine. Since then, nuclear propulsion has become a cornerstone of modern naval fleets, offering unparalleled endurance and operational flexibility.
However, the use of highly enriched uranium (HEU) in naval reactors raises significant proliferation concerns, particularly as more countries—including non-nuclear-weapon states—express interest in developing nuclear-powered vessels.
This growing trend poses challenges for global nonproliferation efforts, especially under treaties like the Nuclear Non-Proliferation Treaty (NPT) and the proposed Fissile Material Cutoff Treaty (FMCT).
The Rise of Nuclear Naval Propulsion
Nuclear propulsion offers a key advantage over conventional fossil-fuel systems: the ability to operate for extended periods without refueling. This capability allows nuclear-powered vessels to travel faster and farther, making them ideal for military operations. For example, nuclear-powered aircraft carriers can dedicate more space to aircraft and supplies instead of fuel storage.
Nuclear reactors onboard ships generate steam to produce electricity and power propulsion systems. Due to space constraints, most naval reactors use HEU, which contains a higher concentration of fissile uranium-235 compared to low-enriched uranium (LEU).
HEU’s energy density allows for smaller reactors and longer intervals between refueling. For instance, the U.S. Navy’s Virginia-class submarines use HEU cores designed to last the vessel’s entire 33-year lifespan.
However, HEU’s dual-use nature—it can also be used to build nuclear weapons—complicates its role in naval propulsion. While the NPT permits the withdrawal of fissile material from International Atomic Energy Agency (IAEA) safeguards for non-explosive military purposes, such as naval propulsion, this exemption creates a potential loophole.
Unsafeguarded HEU could theoretically be diverted for weapons purposes, undermining global nonproliferation efforts.
Global Use of HEU in Naval Propulsion
Currently, four navies are known to use HEU for their nuclear-powered vessels: the United States, Russia, the United Kingdom, and India. France, which previously used HEU, now employs LEU enriched to less than 10% uranium-235 in its newer submarines and aircraft carriers. China’s submarines are also believed to use LEU enriched to around 5%.
The International Panel on Fissile Materials (IPFM) estimates that the U.S. Navy requires approximately 2,000 kilograms of HEU annually, while Russia uses 1,000 kilograms, and the UK uses 200 kilograms. Globally, an estimated 382.5 metric tons of HEU are earmarked for naval use, enough to produce over 9,000 nuclear bombs if diverted.
Proliferation Risks and the NPT
The NPT, the cornerstone of global nuclear nonproliferation, requires non-nuclear-weapon states to place their nuclear materials under IAEA safeguards. However, Article III of the treaty allows states to withdraw fissile material from safeguards for non-explosive military purposes, such as naval propulsion.
This exemption has yet to be tested by non-nuclear-weapon states, but countries like Brazil and Argentina, which have announced nuclear propulsion programs, could challenge the NPT’s integrity if they pursue HEU-based systems.
Brazil, for example, plans to launch its first nuclear-powered submarine by 2030, while Argentina is focusing on nuclear-powered surface ships. Both countries are NPT members with advanced nuclear programs, and their use of unsafeguarded HEU could erode confidence in the treaty.
The Fissile Material Cutoff Treaty (FMCT) and Verification Challenges
The proposed FMCT aims to ban the production of fissile material for nuclear weapons. However, as currently conceived, it would not prohibit HEU production for naval propulsion. This creates a potential loophole, as states could stockpile HEU under the guise of naval use while secretly diverting it for weapons development.
To address this, the IPFM has proposed a verification regime where HEU earmarked for naval use remains under IAEA safeguards until needed. The IAEA would monitor the material at every stage, ensuring it is not diverted for weapons purposes. A similar framework has been suggested by Robert Einhorn as part of a Fissile Material Control Initiative, which would use monitoring as a confidence-building measure.
Transitioning to Low-Enriched Uranium (LEU)
One solution to the HEU dilemma is transitioning naval reactors to LEU. While HEU offers greater energy density, advancements in fuel design could make LEU a viable alternative. For example, the French navy uses a “caramel” fuel design, which embeds uranium dioxide in a zirconium alloy grid, improving energy efficiency and allowing the use of LEU.
Studies suggest that U.S. submarines could also adopt LEU-based reactors with minimal trade-offs. A 1990 MIT thesis by Thomas Ippolito Jr. and subsequent analyses by Chunyan Ma and Frank von Hippel indicate that LEU reactors could achieve core lifetimes comparable to current HEU systems, provided fuel designs are optimized.
Conclusion
The use of HEU in naval propulsion presents a significant challenge to global nonproliferation efforts. While HEU offers operational advantages, its dual-use nature and exemption from safeguards under the NPT and FMCT create potential risks.
Transitioning to LEU-based propulsion, as France has done, could mitigate these risks while maintaining naval capabilities. By adopting LEU and strengthening verification mechanisms, the international community can reduce the proliferation risks associated with naval HEU and reinforce the global nonproliferation regime.
