Decisions that are made in the near future will determine whether or not the US Navy will be able — decades from now — to reduce or eliminate the use of highly enriched uranium (HEU) in nuclear reactors aboard US aircraft carriers and submarines.
Since HEU can be used in nuclear explosives, reducing the production, use and inventory of this material is a primary objective of nonproliferation policy. But choices that are made in the next few years will determine, or foreclose, the possibility of phasing out the production of HEU reactor cores for decades to come.
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The dispute has recently taken on partisan form in the congressional Armed Services Committees. Broadly speaking (and with some exceptions), a transition toward low-enriched uranium (LEU) for naval reactor fuel is favored by the committees’ Democrats, while it is opposed by their Republicans.
In its report on the FY 2020 defense authorization bill, the Democratic-led House Armed Services Committee affirmed and encouraged the pursuit of LEU fuel in the interests of nuclear non-proliferation:
“The committee has been supportive of efforts to assess the feasibility of using low-enriched uranium for naval reactors as such use would not only benefit nuclear non-proliferation efforts but also maintain the research and development skills necessary to sustain innovation and expertise with regard to naval fuel . . . The committee continues to support efforts to assess the feasibility of using LEU in naval reactors to meet military requirements for aircraft carriers and submarines.”
By contrast, the Republican-led Senate Armed Services Committee recommended a “prohibition on use of funds” for research and development of advanced naval reactor fuels using low-enriched uranium, citing increased costs for developing and converting to LEU fuel along with possible reductions in reactor performance.
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This division of opinion reflects divergent views of the priority of nuclear non-proliferation as a national policy goal, and disagreement over the technical feasibility and desirability of devising new LEU fuel for naval reactors.
The technical issues were helpfully explored in a 2016 report of the JASON science advisory panel which has just been declassified and released in redacted form. See Low-Enriched Uranium (LEU) for Potential Naval Nuclear Propulsion Application (redacted), JSR-16-Task-013, November 2016.
The JASON report describes a new LEU fuel concept that has higher uranium loading to limit the increase in reactor size that would otherwise result from a change from HEU to LEU. This is crucial because “increasing core volume by more than a modest fraction could have a significant negative effect on ship design, especially for submarines.”
But even if the new fuel is successfully developed, “it would be impractical to deploy LEU reactors in already-designed submarines” which are sized for HEU cores and cannot reasonably be refueled, the JASONs said.
This means that “the first practical opportunity to deploy an LEU reactor in a submarine is in the VIRGINIA-replacement class,” which is more than a decade away. But the design of that new submarine’s propulsion compartment will be determined by or before 2030.
So if the new LEU fuel is not developed, tested and validated for use in the next few years, then HEU cores could be required for US submarines through the years 2060 and beyond, when the last VIRGINIA-replacement class sub will be launched.
“This is a key insight,” said Alan Kuperman of the University of Texas, a proponent of reactor conversion to LEU. “The Office of Naval Reactors could preempt any chance to convert subs to LEU fuel until the 2070s by simply designing the SSN(X) with a small reactor compartment that is difficult to refuel.”
(The requirements of an LEU reactor for an aircraft carrier would be easier to satisfy than those for a submarine, the JASONs said.)
The JASON report described a spectrum of other options, including converting aircraft carriers only (not submarines) to LEU. And the report suggested considering the use of uranium enriched to 25% or so, which is technically considered HEU, but far below the 93-97% enriched fuel currently used by naval reactors. Exceeding the definitional threshold of LEU (which is normally set at 20% enriched) in this way would permit greater design flexibility while still advancing non-proliferation goals, the JASONs said.
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The contrasting perspectives on this topic extend to the presidential level.
In a 2016 statement, the Obama White House said that “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.”
But in a statement this month, the Trump White House said that it “objects to the . . . inclusion of funding for development of high-density, low-enriched fuels that could replace highly enriched uranium for naval applications.”
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