Ban Production of Highly Enriched Uranium

by Frank von Hippel, senior research physicist, emeritus professor and cofounder of the Program on Science and Global Security at Princeton University’s Woodrow Wilson School of Public and International Affairs

Editor's Note: This essay is featured in our new report, "10 Big Nuclear Ideas for the Next President" (pdf).

The continued production of highly enriched uranium (HEU) for any purpose poses a significant threat to international security. Nations that want to acquire nuclear weapons could seek to do so under the cover of HEU production for civilian research or naval propulsion. While it is essential to strengthen ongoing efforts to secure existing stocks, the next US administration also should make it a priority to ban the production of HEU worldwide. Such a ban would greatly reduce the risks of nuclear terrorism and the proliferation of nuclear weapons to new states.

To support a global ban, the United States would need to take some steps on its own. Although Washington long ago halted HEU production for nuclear weapons and is on a path to phase out HEU use in research reactors, the next administration will need to tackle the challenge of converting US naval propulsion reactors. This is essential to the success of banning all HEU production worldwide.

Highly enriched uranium — typically containing more than 90% of the chain-reacting isotope U-235 — and plutonium are the two “fissile” materials used to make nuclear weapons. But unlike plutonium, HEU is used as a research reactor fuel around the world, and is stockpiled for use in naval reactors in four nuclear weapons nations, including the United States. This is of particular concern to nuclear security and nonproliferation experts because, compared to plutonium, the HEU path is by far the easier to follow for a state seeking a weapon or for a terrorist seeking an improvised nuclear device.

As Luis Alvarez, a senior participant in the Manhattan project, explained:

Most people seem unaware that if separated U-235 is at hand it’s a trivial job to set off a nuclear explosion, whereas if only plutonium is available, making it explode is the most difficult technical job I know.1

The United States ended all production of HEU in 1992 after downsizing its nuclear arsenal at the end of the Cold War. Its remaining stocks of surplus HEU are sufficient to supply the US Navy and other needs for another 50 years. Russia and China also ended their production of HEU with the end of the Cold War but Russia, which has the world’s largest stockpile of HEU, recently resumed production for civilian purposes — perhaps because it had unused uranium enrichment capacity.

Military HEU: US Production Halted, No Need to Resume

Unless states are conducting nuclear tests or increasing nuclear-weapon stockpiles, nuclear weapons can be remade using HEU from retired weapons.

Since the end of the Cold War, the global stock of operational nuclear warheads has declined from about 65,000 to about 10,000, with all but about 1,000 warheads located in Russia or the United States. This has freed a huge amount of Russian and US HEU for other uses. Much of this excess HEU was blended down to low enriched uranium (LEU) for use in power-reactor fuel. The United States has reserved material to fuel its naval reactors until 2060 however, and more excess HEU is available from the weapons stocks if needed.2 Neither the United States nor Russia has any need to resume HEU production for 50 years.

The surplus of HEU and plutonium created by the end of the Cold War led to a broad agreement among the states that are members of the Nuclear Nonproliferation Treaty, or NPT, (all members of the United Nations other than India, Israel, North Korea and Pakistan) to support a Fissile Material Cutoff Treaty (FMCT) that bans the production of HEU and plutonium for nuclear weapons. If and when the surplus stocks are eliminated or placed under IAEA safeguards, this would make the Russian and US warhead reductions irreversible and cap the buildups of China, India and Pakistan. Unfortunately, because of the consensus requirements of the UN’s Conference on Disarmament and objections by Pakistan, negotiations have yet to begin.

If all consumptive uses of HEU were shifted to LEU, a basis would be created to broaden the FMCT to include a ban on the production of HEU for all purposes. (A strong case can also be made to extend the production ban further to include the separation of plutonium for all purposes, but that is another story.) To achieve a global HEU ban, the United States must address civilian as well as naval uses.

Civilian HEU: Still Need to Convert Many Reactors

During the 1960s and 1970s, the US and Soviet Atoms for Peace programs exported HEU to fuel research reactors in about 40 countries, including Iran (from the United States), Iraq (Russia and France), Libya (Russia) and North Korea (Russia). On the eve of the 1991 Gulf War, Saddam Hussein launched a crash program to turn his HEU fuel into a bomb. Fortunately, it was too late. After his defeat, the IAEA shipped Iraq’s HEU fuel to Russia.

The solution to this proliferation hazard, recognized since 1978, is to convert research reactors from HEU to LEU fuel, enriched to less than 20%. The nuclear weapons states have advised the IAEA that it is not practical to make nuclear weapons with LEU.

After 9/11, the possibility of terrorists acquiring nuclear weapons became a special concern because HEU was still present at about 130 research reactors around the world, some of them in low-security sites such as university campuses.

With the cooperation of Russia, the US Global Threat Reduction Initiative (launched by the Bush administration in 2004) cleaned HEU out of 31 countries. At home, the United States converted or decommissioned most of its own HEU-fueled research reactors. As a result of pressure from the United States, most of the global radiopharmaceutical industry is in the process of switching from HEU to LEU in research reactors that produce medical radioisotopes.

Further progress in ending the use of HEU in civilian research reactors was made by the Obama administration’s series of Nuclear Security Summits initiated in 2010. As a result of these efforts, many HEU-fueled research reactors have been converted to low-enriched uranium or have been decommissioned and cleaned out.

There are still about 100 HEU-fueled research reactors left worldwide, about half of which are in Russia, which has not made converting its reactors a priority. Outside Russia, a few research reactors are still in the conversion process; it is impractical to convert some others; and a third group requires the development of new high-uraniumdensity fuel to make their conversion possible. Most of the world’s research reactors were built before 1980, and retirement is reducing their number faster than conversion. Retirement of additional HEU-fueled reactors that are no longer needed should be encouraged. The norm that has developed against building new HEU-fueled reactors should be protected, and the remaining highpower research reactors that cannot be converted to LEU should be converted from weapon-grade uranium to as low enrichment as possible.

Naval Nuclear Propulsion: Shifting to LEU

Today the largest global user of HEU is the United States Navy, whose propulsion reactors (used in submarines and aircraft carriers) are fueled with weapon-grade HEU. The remaining US HEU stocks can supply the Navy for more than 50 years. But eventually, if the Navy continues to use HEU, it would be necessary for the United States to restart production, which would legitimize other countries doing so. As an indication of the type of mischief that could result, during the Ahmadinejad administration, Iranian officials proposed that Iran begin to produce HEU for future Iranian nuclear submarines.

Together, the United States and Russia account for more than 90% of all HEU consumed by naval reactors. The United States provides HEU to, and shares reactor technology with, the UK nuclear navy; while Russia shares design information with India. France and China, the only other countries with nuclear-powered ships and submarines, already use LEU fuel.

If it were possible to convert the US Navy to LEU fuel within the next 50 years, the United States would not have to resume HEU production and could press other nations not to do so either. The United Kingdom could also switch because it has access to US naval nuclear reactor technology. Russia too could switch relatively easily because it mostly uses less than fully enriched HEU in its naval reactors and, unlike the United States, refuels them every ten years or so. India depends on Russia for naval nuclear technology.

The US Congress has twice — in the 1990s and again in the current decade — asked the Department of Energy’s Office of Naval Reactors, which designs and troubleshoots US naval reactors, about the possibility of shifting to LEU.

In 1995, the Office’s response was that it was on the verge of designing lifetime cores for the next generation of US submarines and shifting to LEU would require either giving up this goal or designing reactors with much larger cores to accommodate roughly the same amount of U-235 in a more dilute form. In converting research reactors, the Department of Energy had found that it was possible for most reactors to increase the density of the uranium in the fuel sufficiently to compensate for the more dilute U-235 in LEU, but the Office of Naval Reactors insisted that it had already maximized the uranium density of its fuel.3

In 2014, in response to a second congressional query, the Office of Naval Reactors was somewhat more positive:

Recent work has shown that the potential exists to develop an advanced fuel system that could increase uranium loading [density] beyond what is practical today while meeting the rigorous performance requirements for naval reactors.4

Congress asked for an R&D plan and the Office of Naval Reactors submitted a proposal in July. According to this plan, the effort would cost about $1 billion and take 15 years with an additional decade required to build the production capacity required to fuel the nuclear navy.5 Admiral Caldwell, Director of the Office of Naval Reactors, noted that the project would make it possible for his office to keep together its reactor design team to design the next naval reactor.

Admiral Caldwell testified that, if the R&D effort were successful, the fuel could be used in the new US Ford-class nuclear-powered aircraft carriers.6 He also indicated, however, that US submarines would require larger reactor vessels to accommodate enough LEU for lifetime cores, a high priority for the US Navy, although other countries have refueling hatches on their nuclear submarines.

The height of a submarine reactor core is only about 10% of the diameter of US attack submarines, which are considerably smaller than US ballistic missile submarines.7 Doubling the volume of a reactor core to accommodate the lower density of U-235 in LEU would only require increasing its height and diameter by 26%. If the height were fixed, the volume could be doubled by increasing the core diameter by 40%. It therefore should be possible to design next-generation US submarines for LEU cores. Alternatively, the United States could design its next generation submarines with refueling hatches similar to those that France has on its submarines, which allow for refueling as well as complete inspections of the reactor and piping in one-and-a-half to two months.8

The Office of Naval Reactors’ July 2016 report indicated that it would take 25 to 30 years to develop the new LEU fuel and the associated production capacity. Assuming that the program starts in fiscal year 2018, this would mean that the fuel could be available sometime in the period 2043-2048.

Because of budgetary concerns, the Obama administration’s support for the LEU fuel development program was more passive than active. Thus far, a few Members of Congress have been driving the US government’s engagement with the issue of developing LEU fuel for US aircraft carriers and submarines. The administration was initially conflicted because the Republican majority in Congress insisted that the funding should come from the Department of Energy’s nonproliferation budget rather than its budget for naval reactor research and development. However, just before the 2016 Nuclear Security Summit, the White House issued a statement that:

Consistent with its national security requirements and in recognition of the nonproliferation benefits to minimizing the use of highly enriched uranium globally, the United States values investigations into the viability of using low-enriched uranium in its naval reactors.9

Development of LEU fuel for naval reactors should be a major element of the next administration’s nonproliferation program. In addition, the new administration should ask the Office of Naval Reactors to explore design concepts for next-generation submarines that would include either a large enough reactor vessel to accommodate a lifetime core or a design that would facilitate rapid mid-life refueling.

If these programs succeed, US Navy requirements for fresh HEU fuel could end by around 2040 and the United States could call for a ban on the production of HEU for any purpose. This would go a long way towards eliminating non-weapons use of HEU as one of the most serious threats to the global nonproliferation regime and a potential source of a terrorist nuclear device.

Frank von Hippel is a senior research physicist, emeritus professor and cofounder of the Program on Science and Global Security at Princeton University’s Woodrow Wilson School of Public and International Affairs.



1 Luis W. Alvarez, Alvarez: Adventures of a Physicist (New York: Basic Books, 1987), pg. #125.

2 United States, US Department of Energy, Tritium and Uranium Management Plan Through 2060 (2015), Table 8.

3 United States. US Department of Energy. Office of Naval Reactors. Report on Use of Low Enriched Uranium in Naval Nuclear Propulsion. 1995.

4 United States. US Department of Energy. Report on Low Enriched Uranium for Naval Reactor Cores. 2014.

5 United States. US Department of Energy. National Nuclear Security Administration. Research and Development Plan for Low-enriched Uranium Naval Fuel. 2016.

6 Budget Hearing – Department of Energy, National Nuclear Security Administration, Weapons and Activities and Nuclear Nonproliferation and Naval Reactors (2016) (testimony of Admiral James Frank Caldwell Jr.).

7 Based on the cavity height of the M-140 naval spent fuel transport container given in US Nuclear Regulatory Commission, Certificate of Compliance 9793, 2012.

8 France. Charles Fribourg, formerly Technical Director, Technicatome, Navires à Propulsion Nucléaire and Réacteurs Nucléaires de Propulsion Navale.

9 The White House. “Feasibility of Low Enriched Uranium Fuel in Naval Reactor Plants.” Fact Sheet, 2016.


Photo: A billet of highly enriched uranium that was recovered from scrap processed at the Y-12 National Security Complex Plant. Original and unrotated. Public Domain

Ban Production of Highly Enriched Uranium by @Princeton emeritus professor Frank von Hippel.

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