At Sea Over Naval HEU: Expanding Interest in Nuclear Propulsion Poses Proliferation Challenges

At Sea Over Naval HEU: Expanding Interest in Nuclear Propulsion Poses Proliferation Challenges

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Cole J. Harvey

Research Associate, The James Martin Center for Nonproliferation Studies

The era of nuclear maritime propulsion began in 1954, with the launch of the USS Nautilus, the first nuclear-powered submarine.[1] Nautilus was joined in 1958 by the first Soviet nuclear submarine, Leninsky Komsomol.[2] Over the years, the U.S. and Soviet / Russian navies expanded their nuclear fleets to include dozens of submarines, aircraft carriers, and other surface vessels. Russia even operates civilian nuclear icebreakers along its northern coast. The other three recognized nuclear-weapon states, China, France, and the United Kingdom also sent nuclear-powered ships to sea. India launched a nuclear submarine, the INS Arihant for two years of sea trials in July 2009.[3] As of 2008, there were approximately 180 such vessels, mostly in the U.S. and Russian navies.[4] Brazil and Argentina have also announced nuclear propulsion programs. As non-nuclear-weapon states that are members of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), these two countries' decisions raise the question of whether the development of nuclear propulsion technology by non-nuclear-weapon states will pose challenges for the NPT, as nuclear material must be withdrawn from safeguards in order to be used in a military propulsion program.

Drawing energy from an onboard nuclear reactor offers one major advantage over conventional fossil-fuel propulsion: the ability to operate for long stretches at a time without refueling. This enables nuclear-powered military vessels to proceed faster and farther to their mission areas without stopping at port or slowing for at-sea refueling. In the case of an aircraft carrier, space that would be required for fossil fuel storage can instead be used to store aircraft and supporting materials.

In a nuclear-powered vessel, an onboard reactor produces steam that is used to generate electricity and power the ship's propulsion system. Since the reactor must be small enough to fit in the confined space of a ship at sea, most naval reactors have relied on highly enriched uranium (HEU) for their fuel, which can generate more energy by volume than low-enriched uranium as a result of the greater density of fissile uranium-235 present in HEU.

Uranium enriched up to 20% is considered low-enriched by the International Atomic Energy Agency (IAEA), while enrichment to 20% or greater is considered to be HEU. Most civilian power reactors use uranium enriched to 3-5% levels.[5] The U.S. navy's nuclear program, formed under the direction of Admiral Hyman Rickover, favored HEU fuel over LEU, since the fuel volume can be smaller, and the vessel requires refueling less often.[6] Virginia-class attack submarines, for example, have HEU cores designed to last the lifetime of the submarine—an estimated 33 years.[7]

However, uranium enriched to about 90% (or considerably less) can also be used to form the core of a nuclear weapon. Using this material to power ships poses difficulties for an eventual worldwide agreement to ban the production of HEU and one day eliminate it. In particular, continued use of HEU for military propulsion by the nuclear-weapon states provides a possible justification for HEU propulsion in non-nuclear-weapon states. HEU-based propulsion also poses a specific challenge to the spirit if not the letter of the NPT regime, as the treaty permits fissile material to be withdrawn from IAEA safeguards for non-explosive military purposes such as naval propulsion. As a result, it is possible that HEU removed from safeguards for use in naval propulsion could be refashioned into bomb cores in a cheating or breakout scenario. Even if a state were abiding by the NPT and using the unsafeguarded material exclusively for non-explosive purposes, the withdrawal of the material from safeguards could erode confidence in the NPT regime among other states.

Inventories, Use, Enrichment Levels

Four navies are known to utilize HEU to fuel their nuclear-powered submarines and surface ships: those of the United States, Russia, the United Kingdom, and India. Though some of France's earlier generation submarines utilized HEU, its newer submarines and the aircraft carrier Charles de Gaulle utilize fuel enriched to less than 10 percent uranium-235.[8] China's submarines are believed to use approximately five-percent enriched LEU.[9] Brazil has begun a nuclear submarine program,[10] but its submarine fleet is currently powered by diesel fuel.[11]

The International Panel on Fissile Materials estimates that the U.S. Navy requires approximately 2,000 kilograms of HEU each year, the Russian navy 1,000kg, and the United Kingdom's navy 200kg.[12]

The U.S. Navy is known to operate with reactor cores fueled by very highly enriched uranium, either 97% uranium-235 produced specifically for naval reactors, or 93% uranium-235 extracted from surplus nuclear weapons. The United Kingdom uses a similar enrichment level,[16] and in fact imported HEU from the United States for naval fuel.[17] Russia and India are believed to use HEU enriched to approximately 40% uranium-235, below the level typically used in weapons.[18]

In total, the IPFM estimates that approximately 382.5 metric tons of highly enriched uranium are marked for use by the world's nuclear navies, of which 228 metric tons are fresh fuel.[19] The IAEA defines 25 kilograms of 20% enriched HEU as a significant quantity, or a bomb's worth, of uranium. In this context, therefore, the estimated total of naval HEU worldwide is equivalent to well over 9,000 nuclear bombs.[20]

Naval HEU and the Fissile Material Cutoff Treaty (FMCT)

A long-standing goal on the disarmament and nonproliferation agenda, endorsed repeatedly by the NPT Review Conferences held every five years, is the negotiation of a "non-discriminatory, multilateral and internationally and effectively verifiable treaty banning the production of fissile material for nuclear weapons or other nuclear explosive devices."[21] Such a ban is considered an essential step on the road to nuclear disarmament, and an important nonproliferation measure, but discussions on the proposed treaty have been stalled in the Geneva-based Conference on Disarmament for years.

An FMCT as currently conceived would not prohibit the production of HEU for military propulsion purposes or, indeed, for any purpose other than nuclear explosives. As a result, states could continue to produce and stockpile HEU earmarked for naval propulsion. Such a stockpile, if not properly safeguarded, could be secretly diverted for weapons development, or overtly used to construct nuclear weapons in a crisis. As such, if states continue to rely on HEU for their naval reactors under an FMCT, a verification system must be devised to ensure that HEU used for naval fuel is not diverted to weapons purposes.

The International Panel on Fissile Materials (IPFM) has advanced a basic model for such a verification regime, which seeks to strike the necessary balance between effective verification and the preservation of classified national security information. Under the proposal, fissile material produced or earmarked for use in naval propulsion would be kept under IAEA safeguards until needed by a state. The state would then notify the IAEA of its decision to remove a precise quantity of HEU for use in a specified ship or ships. Once the HEU had been fabricated into fuel and placed in an unshielded container, the IAEA would measure radiation emissions from the container to verify that the amount of HEU in the fuel matched the amount that had been withdrawn from safeguards.[22] Lastly, the IAEA would monitor the spent fuel removed from the reactor.[23] Following this model, the IAEA would be able to verify at every stage of the process that no HEU had been diverted from naval propulsion, all without inspectors requiring access to sensitive design information.

Robert Einhorn proposed a similar framework as part of a Fissile Material Control Initiative, intended as a voluntary international effort to reduce the dangers of the theft or diversion of existing fissile material stocks.[24] Under the initiative, monitoring of HEU set aside for naval use would be a confidence-building measure rather than an essential provision of a treaty, and so a less rigorous standard of verification could be employed.

Converting the world's nuclear navies to utilize low-enriched uranium would eliminate the need for a verification regime related to naval HEU, even if the exception continued to formally exist in the FMCT itself. Ultimately, ensuring that nuclear naval propulsion is driven by LEU exclusively would be requisite to a global ban on HEU for all purposes.

Naval HEU and the Treaty on the Non-Proliferation of Nuclear Weapons (NPT)

The NPT is the cornerstone of the international nuclear nonproliferation architecture. It requires the five treaty-recognized nuclear-weapon states—China, France, Russia, the United Kingdom and the United States—to negotiate towards nuclear disarmament, while committing its other 184 state parties not to acquire nuclear weapons. As part of this commitment, non-nuclear-weapon states are obliged to accept monitoring of their nuclear facilities and activities by the IAEA. Like the proposed FMCT, however, the NPT creates a loophole for military non-explosive uses of fissile material.

Article III of the treaty, which lays out states' IAEA safeguards obligations, states:

Each non-nuclear-weapon State party to the Treaty undertakes to accept safeguards, as set forth in agreement to be negotiated and concluded with the International Atomic Energy Agency…for the exclusive purpose of verification of the fulfillment of its obligations assumed under this Treaty with a view to preventing diversion of nuclear energy from peaceful uses to nuclear weapons or other nuclear explosive devices.

Each state is obligated to accept IAEA safeguards to ensure that no fissile material is utilized for weapons purposes. However, military propulsion is exempt from safeguards as a result of the "exclusive purpose" clause. States that choose to do so may remove fissile material from IAEA safeguards for military non-explosive purposes. In order to do so, states need only declare to the IAEA "that during the period of non-application of safeguards the nuclear material will not be used for the production of nuclear weapons or other nuclear explosive devices."[25] Consequently, it is legally possible for a state to withdraw highly enriched uranium from IAEA safeguards under the military propulsion clause, without ever proving to the IAEA that it in fact used the HEU for propulsion purposes.

To date, the IAEA has not had to deal with fissile material being withdrawn from safeguards for military propulsion by a non-nuclear-weapon state. Of the states fielding nuclear navies, five are recognized nuclear-weapon states (where unsafeguarded fissile material is not a proliferation concern), and the sixth is a nuclear-weapon possessing state that is not a party to the NPT (India). However, Brazil and Argentina have each announced programs to develop nuclear-propelled submarines or surface vessels. Both countries are relatively recent signatories to the NPT,[26] with fairly advanced nuclear programs and histories of covert nuclear weapons programs. Their pursuit of military nuclear propulsion and planned use of unsafeguarded fissile material will pose a challenge for the NPT, as state parties must be able to maintain confidence that unsafeguarded fissile material will not be put to use in a weapons program. Some time remains for policy to catch up with on-the-ground facts. Brazil announced its nuclear submarine program in 2008, expecting its first submarine to be operational by 2020.[27] Argentina announced its program in June 2010, and plans to emphasize surface ships rather than submarines.[28]

Converting Naval Reactors to LEU

What policy options exist for addressing the problems posed by the use of HEU in military propulsion? As discussed, U.S., British, and Soviet/Russian submarine designers have favored HEU for their reactor cores, since more energy can be extracted from a smaller load of fuel. This enables the reactor to fit into the small confines of a submarine, and allows the submarine to operate at long intervals—perhaps even over the lifetime of the vessel—without the need for refueling.

Since current U.S. submarine designs utilize weapons-grade HEU to pack the maximum amount of uranium-235 into the smallest volume, the Director of the Office of Naval Nuclear Propulsion wrote in a 1995 report to Congress that a shift to 20% enriched uranium could only be achieved by either a) maintaining the size of the reactor and therefore decreasing the amount of uranium-235 available for fission, or b) increasing the size of the reactor. The report stated that the smaller quantities of uranium-235 present in the first case would require more frequent refueling, reducing the life of a load of fuel from 33 years for a Virgina-class submarine to 7.5 years. A Nimitz-class aircraft carrier's reactor would last only 10.4 years instead of 45.[29] Refueling a nuclear submarine is a major interruption in the vessel's operational life; in the U.S. case refueling takes the submarine out of service for approximately two years.[30] However, the 1995 study assumed no changes to fuel design that could extract more energy from LEU fuel and thus postpone refueling.[31]

French nuclear vessels are able to extract more energy from LEU than are U.S. vessels by taking advantage of a more LEU-efficient fuel design. The French navy uses a uranium-dioxide composite embedded in a zirconium alloy grid, an arrangement known as "caramel" fuel. Caramel fuel increases the efficiency of the burn-up of uranium-235 so that lower enrichment levels and/or smaller reactor volumes can be employed with a greater energy yield. Studies have shown that a design in which small spheres of uranium dioxide are embedded in a zirconium matrix can boost the efficiency of the fission reaction even further.[32]

Using this information, and basing their findings on a 1990 MIT nuclear engineering thesis by Thomas Ippolito Jr., Chunyan Ma and Frank von Hippel estimate that a submarine reactor utilizing 20-percent enriched caramel fuel could have a core life of 33 years at a 130 megawatt output with a height of 1.7 meters and a diameter of 1.4 meters. The size of the reactor compartment of Virginia-class submarines is classified, but such a reactor could easily fit in the similarly-sized predecessor to the Virginia, the Los Angeles-class submarine.[33] Ma and von Hippel's analysis suggests that the U.S. Navy could retrofit its fleet to achieve the flexibility afforded by the present HEU cores while still complying with an international norm against the use of HEU.


Highly enriched uranium has been the fuel of choice for the U.S. and Soviet/Russian submarine and aircraft carrier fleet, the British submarine force, and some French submarines. While naval planners have favored HEU for its efficiency, the continued use of HEU for naval applications poses a challenge to attempts to build a norm against both military and civil use of HEU. Continued naval HEU loopholes in the NPT and proposed FMCT could allow states to break out of the nonproliferation regime by diverting fissile material from safeguards to covert weapons programs.

Redesigning naval reactor cores and fuel assemblies to burn uranium-235 more efficiently could enable the world's navies to utilize low-enriched uranium in place of HEU, with only minor tradeoffs, if any, in reactor size and refueling times. The French navy has already adopted this course. If other navies follow the French example, their actions would contribute to the develop of a norm against the use of highly enriched uranium that would in turn reduce the likelihood that a state could utilize naval HEU production as a front for a nuclear weapons program.


[1] "History of USS Nautilus," Submarine Force Museum,
[2] "Project 627 Kit, November class," Federation of American Scientists,
[3] "Submarine Proliferation: India Current Capabilities," Nuclear Threat Initiative, February 2010,
[4] "Global Fissile Material Report 2008," International Panel on Fissile Materials, 2008, p. 76,
[5] Natural uranium contains only about .7% uranium-235, with non-fissile uranium-238 making up the remainder. Uranium enrichment is the process of progressively removing the larger, heavier uranium-238 isotope so that uranium-235 makes up an increasingly larger percentage of the batch.
[6] "Highly Enriched Uranium: Striking a Balance; Appendix D," Department of Energy, p. 143,
[7] "U.S. Navy's Virginia-Class Attack Submarine Program," Northrop-Grumman,
[8] Mary Byrd Davis, "Fabrication of uranium-based fuel," Nuclear France, 6 January 2007,
[9] "China's Nuclear Submarine Program," Nuclear Threat Initiative,
[10] Paul D. Taylor, "Why Does Brazil Need Nuclear Submarines," Proceedings Magazine, U.S. Naval Institute, June 2009,
[11] Sarah Diehl and Eduardo Fujii, "Brazil's Pursuit of a Nuclear Submarine Raises Proliferation Concerns," WMD Insights, March 2008,
[12] "Global Fissile Material Report 2008," International Panel on Fissile Materials, 2008, p. 76,
[13] Information in this table is drawn from "Global Fissile Material Report 2008," International Panel on Fissile Materials, 2008,
[14] "Nuclear-Powered Ships," World Nuclear Association, 29 September 2010,
[15] "Historical Accounting for UK Defence Highly Enriched Uranium," UK Ministry of Defence, March 2006, p. 5,
[16] Chunyan Ma and Frank von Hippel, "Ending the Production of Highly Enriched Uranium for Naval Reactors," The Nonproliferation Review, Spring 2001, pp. 89 and 92,
[17] Steven Aftergood and Frank N. von Hippel, "The U.S. Highly Enriched Uranium Declaration: Transparency Deferred but not Denied," The Nonproliferation Review, Vol. 14, No. 1, March 2007, p. 153,
[18] "Global Fissile Material Report 2008," International Panel on Fissile Materials, 2008, p. 76,
[19] "Global Fissile Material Report 2009," International Panel on Fissile Materials, 2009, p. 13,
[20] International Atomic Energy Agency, "IAEA Safeguards Glossary, 2001 Edition," 2002, p. 23, Note that the U.S., UK, and Russian navies use HEU that is enriched to a much higher level than the 20% listed by the IAEA.
[21] The Shannon Mandate agreed to by the Conference on Disarmament in March 1995, is the basis for the negotiation of an FMCT,
[22] Since uranium is not as radioactive as—for example—plutonium, a canister of HEU fuel would not spontaneously emit enough neutrons and gamma rays to permit passive scanning. Instead, the IPFM report proposes irradiating the canister with high-energy neutrons from a neutron generator, thereby inducing fission reactions in the uranium. A detector could then measure the resulting neutron and gamma ray emissions. These detectors would be equipped with an information barrier to prevent the disclosure of sensitive information. For example, a neutron count above a certain threshold would trigger a green light on the detector, but no other information would be revealed.
[23] "Global Fissile Material Report 2008," International Panel on Fissile Materials, 2008, pp. 81-84,
[24] Robert J. Einhorn, "Controlling Fissile Materials Worldwide," in Reykjavik Revisited, eds. George P. Schultz et al (Stanford, California: Hoover Institution Press, 2008), pp. 307-308.
[25] International Atomic Energy Agency, "The Structure and Contents of Agreements Between the Agency and States Required in Connection with the Treaty on the Non-Proliferation of Nuclear Weapons," June 1972, paragraph 14,
[26] Argentina joined the treaty in 1995, and Brazil joined in 1998.
[27] "Brazil Profile," Nuclear Threat Initiative, October 2009, .
[28] Matthew Cowley, "UPDATE: Argentina to Build Nuclear-Power Ships for its Navy," The Wall Street Journal, 3 June 2010,
[29] "Report on the Use of Low Enriched Uranium in Naval Nuclear Propulsion," Director, Office of Naval Nuclear Propulsion. Quoted in Chunyan Ma and Frank von Hippel, "Ending the Production of Highly Enriched Uranium for Naval Reactors," The Nonproliferation Review, Spring 2001, p. 93.
[30] Richard Fabrizio, "Shipyard reinvents itself for future," Portsmouth Herald, 11 June 2000.
[31] Chunyan Ma and Frank von Hippel, "Ending the Production of Highly Enriched Uranium for Naval Reactors," The Nonproliferation Review, Spring 2001, p. 95,
[32] Chunyan Ma and Frank von Hippel, "Ending the Production of Highly Enriched Uranium for Naval Reactors," The Nonproliferation Review, Spring 2001, p. 95,
[33] Chunyan Ma and Frank von Hippel, "Ending the Production of Highly Enriched Uranium for Naval Reactors," The Nonproliferation Review, Spring 2001, p. 96,

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