by Frank N von Hippel
If the US is to end its current practice of using weapon-grade highly-enriched uranium (HEU) to fuel its nuclear submarines, then the design of the next generation of US attack submarines needs to preserve the possibility of using a larger low-enriched uranium (LEU) reactor core or of refueling the reactor, a recently released report from the JASON advisory panel indicates. While the report observes that "the transition to an all-LEU fleet could begin in the 2040s," there is opposition to LEU fuel from the US Naval Nuclear Propulsion Program, the Trump Administration and some in Congress.
The heavily redacted November 2016 analysis by the Jason group of military-technology consultants on the feasibility of developing and using low-enriched uranium (LEU) fuel in US naval propulsion reactors was released in June 2019 as a result of a Freedom of Information Act request by Steven Aftergood of the Federation of American Scientists (FAS). The US Naval Nuclear Propulsion Program (NNPP) redactions of the Jason report appear to go far beyond legitimate requirements for protecting classified design information.
Currently, US and UK naval reactors are fueled by weapon-grade highly-enriched uranium (HEU) containing 93.5% of the chain-reacting isotope, U-235. (UK reactors are based on US designs and fueled with US HEU.) In 2012, the House Armed Services Committee asked NNPP to look at the possibility of developing LEU fuel for naval reactors. LEU contains less than 20% U-235 and is considered not to be directly weapon useable. The Congressional concern was that non-weapon states interested in acquiring or developing nuclear-powered submarines (Brazil and Iran, for example) could use the US example to justify producing and stockpiling weapon-usable HEU, which would destabilize the non-proliferation regime. France and China already use LEU fuel in their submarines. Russia and India use medium-enriched uranium.
The JASON report's major conclusions are:
- NNPP is testing a fuel with higher uranium density. The process is expected to take 20-25 years, i.e. till the 2040s.
- If the fuel tests out, it will be possible to refuel with 19.75% LEU at mid-life (about 25 years) most of the ten US Ford-class aircraft carriers, the first of which was commissioned in 2017.
- If the next-generation attack submarines that the Navy expects to begin ordering in the mid-2030s to succeed the Virginia-class are designed to accommodate larger cores, then it will possible to equip them with LEU lifetime cores when the new fuel becomes available.
- It is too late to design the next-generation Columbia-class US ballistic-missile submarines to use LEU cores but, if and when they are replaced, starting around 2070, the new submarines, which have about 2.5 times the displacement of attack submarines, would be correspondingly easier to design for large LEU lifetime cores.
In its markup of the Defense Authorization Bill for Fiscal Year 2020, the House Armed Services Committee asked NNPP (see p. 494) if it could design the next attack submarine to fit a life-of-ship LEU core. It also asked about a possible alternative if the LEU core size constraint is too much of an obstacle:
"the committee directs the Administrator for Nuclear Security, in coordination with the Secretary of the Navy, to provide a report to the congressional defense committees not later than December 15, 2019, assessing the feasibility of a design of the reactor module of the Virginia-Class replacement nuclear powered attack submarine that retains the existing hull diameter but leaves sufficient space for an LEU-fueled reactor with a life of the ship core, possibly with an increased module length. If a life of the ship core is unattainable, the report should include the feasibility of a reactor design with the maximum attainable core life and a configuration that enables rapid refueling."
In its formulation of that question, the Committee raised an alternative approach to the challenge of LEU core life -- going back to mid-life refueling. Mid-life refueling was standard practice with the Los Angeles-class attack submarines, which still constitute the bulk of the US fleet of attack submarines; with the current Ohio-class ballistic missile submarines; and with all US aircraft carriers.
The JASON report accepts the nuclear navy's arguments for the economic benefits of lifetime cores for submarine reactors, which have been introduced in the Virginia-class attack submarines, 17 of which are deployed with one more launched and five under construction. The group was apparently unable to study France's rapid-refueling arrangements that have reduced the refueling times for French nuclear submarines to weeks versus the years it takes the US Navy. It also does not express concern, as at least one expert has, about possible corrosion failures of nuclear power reactor systems that would not be inspected for three to four decades. Problems with these life-of-ship systems, which are not designed for service access, can be very costly. In 2015, welding was found to be defective in a joint in the steam supply piping of three new Virginia-class submarines. Contriving a way to replace the joint took the first submarine out of service for two years. France's nuclear safety authority requires that French naval reactors be thoroughly inspected every ten years.
Five years ago, NNPP was receptive to Congressional interest in the development of LEU fuel - concluding in a 2014 study that "an advanced fuel system might enable either a higher energy naval core using HEU fuel, or allow using LEU fuel with less impact on reactor lifetime, size, and ship costs." (See also the 2016 report.)
More recently, however, NNPP has been lobbying against the LEU option with the result that, in 2018, the Trump Administration's Secretaries of the Navy and Energy wrote the Congressional Armed Services Committees:
"The replacement of highly enriched uranium with LEU would result in a reactor design that is inherently less capable, more expensive, and unlikely to support current life-of-ship submarine reactors. The LEU fuel system would affect operational availability of military assets due to necessary refueling, and would require significant new shipyard infrastructure."
As reported by Aftergood, the Trump Administration objected to funding for naval LEU fuel R&D in the Fiscal Year (FY) 2020 House Energy and Water Appropriations bill, and the Republican-led Senate Armed Services Committee, which had previously been passive on the issue, voted in its markup of the National Defense Authorization Act for a "[p]rohibition on use of funds for advanced naval nuclear fuel system based on low-enriched uranium."
For its part, the House Armed Services Committee split on the issue. It accepted the report language cited above and directed the National Nuclear Security Administration to formally create a naval LEU R&D program, but it deleted by a 33-to-24 roll-call vote $20 million included in the Chairman's mark that had been authorized for research and development on LEU fuel. The amendment to delete funds was offered by Democratic Representative Elaine Luria, a former naval-reactor engineer representing a Virginia district containing the US Navy's largest shipyard, which builds all US nuclear-powered aircraft carriers and half the modules for the Virginia-class attack submarines, and also contains Norfolk naval station, the world's largest naval base, homeport to four of seven US carrier strike groups. The full House did, however, approve $20 million in the Energy and Water Appropriations bill on June 19.
Until this year, naval LEU fuel R&D had been supported on a bipartisan basis with $5 million authorized and appropriated annually from FY16 to FY18, and $10 million in FY19 - all in bills signed into law by Presidents Obama and Trump.