In this 2018 photo, the attack submarine Colorado (SSN 788) sits pierside at Groton, Connecticut, awaiting commissioning.

In this 2018 photo, the attack submarine Colorado (SSN 788) sits pierside at Groton, Connecticut, awaiting commissioning. U.S. Navy / Chief Petty Officer Darryl Wood

Inside the fight over alternative sub fuel

A handful of lawmakers want the Navy to research low-enriched uranium fuel to reduce nuclear weapons proliferation. But funding bans are on the horizon.

Editor's note: This story was updated at 8 p.m. July 27 with a response from the director of Naval Nuclear Propulsion.

A handful of lawmakers want the Navy to research low-enriched uranium fuel to reduce nuclear weapons proliferation. But funding bans are on the horizon.

By Lauren C. Williams and Caitlin M. Kenney

There’s been a small but persistent push to get the Navy to look at using low-enriched uranium—instead of the highly enriched variety—to power its future submarines. But Congress is poised to slam the door on more research, at least for the next year, and that could scuttle the possibility of using it on the next generation of U.S. subs. 

The push is led in the House by Rep. Bill Foster, D-Ill., a former high-energy physicist who contends that the United States would be safer if it developed warship reactors that did not require highly enriched uranium, which can be used to build nuclear weapons.

“It's a crucial thing that we will continue R&D. We know it's possible to build submarines that have low-enriched uranium—the French do it...It's possible to do this. And the question is, how do you minimize the performance trade-off and enough to convince countries that this is a better route than maintaining a large stockpile of weapons-grade uranium?” Foster said at a recent House Rules Committee hearing on the 2024 defense authorization bill.

That “performance trade-off” is why the U.S. Navy is not keen on low-enriched uranium. It’s an “inferior type of fuel” that means “basically less gas in the tank,” said Tom Shugart, a retired Navy officer who served as chief engineer aboard a nuclear-powered sub. “There are political benefits, and there are non-proliferation benefits. There is no benefit from a practical perspective.” 

Shugart, now an adjunct senior fellow at the Center for a New American Security’s Defense program, noted that the Navy’s nuclear submarine program has always used HEU. It’s a “more power-dense source of fuel” that allows subs to be built around a “more compact reactor core,” he said. It is a major reason that the newer Virginia- and planned Columbia-class submarines will run their entire service life without refueling.

The National Nuclear Security Administration concurs, saying as much in a 2020 report cited by the Congressional Research Service: “It is not practical to substitute LEU into existing naval fuel systems or to design a VIRGINIA Class Submarine (VCS) replacement [i.e., the SSN(X)] around an unproven advanced LEU fuel concept.”

​​Adm. James Caldwell, Jr., the director of Naval Nuclear Propulsion, said in a written response to the congressional inquiry earlier this year that it was impractical to switch to LEU fuel: “The U.S. Navy has developed and improved technology using highly enriched uranium (HEU) fuel over the past 70 years…with unmatched asymmetric advantages in naval warfare while upholding the highest non-proliferation standards.”

But the CRS report added that it’s not clear “whether the Navy has accurately identified the SSN(X)’s required capabilities and accurately analyzed the impact on cost.”

It also noted that the performance of submarines is just one of the factors Congress must weigh.

A ‘negative signal’

From fiscal years 2016 to 2023, the Navy got funding for naval fuel systems that use low-enriched uranium. 

Foster wants to keep that $20 million in funding. 

He was testifying in support of an NDAA amendment to fund and establish a research program for LEU reactors. And he’s not alone. Earlier this year, Reps. Don Beyer, D-Va. and Rick Larsen, D-Wash., and Sen. Jeff Merkley, D-Ore., sent a letter to the administration asking for a status update on LEU fuel research.

One congressional staffer said that moving away from HEU reactors would make it less likely that a non-nuclear country would launch a nuclear-weapons program via a loophole in the Nuclear Non-Proliferation Treaty. That loophole exempts nuclear fuel intended for non-warhead uses like naval reactors from international monitoring. 

This limits oversight and the “ability to catch the diversion of naval HEU fuel to a nuclear weapons program,” the staffer said. “We definitely don't want other countries around the world to basically start using this naval propulsion loophole to basically start nuclear developments.”

James Acton, the co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, said he wants to “reinforce” the current norm for not producing any more HEU.

“In one sense, I’m not so worried about the use of HEU in U.S. naval reactors, per se. But I do think that when the time comes that we eventually have to produce more of the material, that's when, you know, it's going to potentially send out a particularly negative signal to the rest of the world,” Acton said.

In his testimony, Foster said that signal could start with Australia, as part of its AUKUS partnership with the United Kingdom and U.S. 

“The United States, in the past, has burned a lot of political capital, stopping very advanced weapons programs and some of our allies. And those allies maintain a posture of not being nuclear states…in large part because they trust the United States and they're willing to forgo the use of high-enriched uranium, which makes them a nuclear threshold state,” the lawmaker testified. “And this has become increasingly important now with the AUKUS partnership. Because Australia is now going to be using U.S. naval propulsion reactors of whatever kind we teach them how to build.”

And just having HEU for the purpose of naval fuel means it's easier to make a nuclear weapon even if that’s not the intended purpose. 

“There are conventional weapons on these submarines. But if they use high-enriched uranium that means Australia and any country that decides to look at Australia and say, ‘me too,’ is now going to be operating reactors with huge stockpiles of high-enriched uranium,” Foster said. “More and more countries are realizing they want a standby nuclear capacity, and there is no easier way for them to get this than to say, well, we're going to do what the United States is helping Australia to do, which is to develop propulsion reactors using weapons-grade uranium.” 

Closing window

Foster’s amendment was voted down, as was a similar bill last year. Now, draft legislation in both chambers of Congress have provisions that prohibit funds for the National Nuclear Security Administration to be used for LEU research. 

The House-passed bill’s provision simply prohibits LEU fuel research funding. The Senate’s version of the 2023 authorization bill out of committee includes a provision that bans funding for LEU-based fuel system until certain questions are answered, including “whether an advanced naval nuclear fuel system based on low-enriched uranium can be produced that would not reduce vessel capability, increase expense, or reduce operational availability as a result of refueling requirements.”

This kind of indefinite ban could scuttle any chance that the next generation of U.S. Navy subs use LEU power. 

A fact sheet by the Center for Arms Control and Non-Proliferation said that the U.S. has enough HEU to last until 2060 after shutting down production in 1992, leaving a few decades to research and decide whether the Navy can switch to using LEU, or has to restart HEU production for the submarine classes that come after Virginia and Columbia.

Acton said, “For these kinds of very large procurement programs, if you're going to do something like change from HEU to LEU, I think you've got to start doing that study early and laying the groundwork early. So even though the HEU stockpile is good for decades, you know, this is actually the right time to start thinking about these long-term questions.”

CRS estimates that it would take at least 15 years of dedicated research to switch to LEU power, which makes it already too late for the Navy’s next planned attack submarine, the SSN(X), which the service wants to start buying in the mid-2030s.

Correction: An earlier version of this story stated the Navy received funds for LEU research from 2016 through 2021. Funds were provided through fiscal 2023. There was also a quote that misstated which countries use LEU fuel for submarines. The text has been corrected.