The US Department of Energy’s (DOE) Argonne National Laboratory and Fermi National Accelerator Laboratory (Fermilab) have been selected to receive $3.2m in funding from the DOE Advanced Research Projects Agency-Energy (ARPA-E). The funding is part of ARPA-E’s Nuclear Energy Waste Transmutation Optimised Now (NEWTON) programme, which aims to make the reprocessing of US commercial used nuclear fuel economically viable within 30 years.
Transmutation is a process in which long-lived radioactive isotopes in used nuclear fuel are converted into shorter-lived isotopes. Technologies supported by the ARPA-E NEWTON programme, such as transmutation, would speed up the processing cycle of the US used nuclear fuel stockpile, improving safety and decreasing the capital expenditure needed for permanent long-term storage.
Studies have found that particle accelerators offer the greatest potential of any existing technology to address the challenge posed by the 90,000 tonnes of waste at operating US NPPs. Superconducting cavities – specialised components stacked together in linear particle accelerators – can efficiently propel charged particles, such as protons, to high speeds with energies near 1bn electron-volts (GeV).
These proton beams can be used to create an intense flux of neutrons from a heavy-element target (typically made of lead or bismuth) through a process called spallation, according to Michael Kelly, Argonne physicist and team leader. When directed at radioactive waste inside a nuclear reactor, these neutrons multiply and burn the used fuel, turning it into a material that decays more quickly.
However, most of today’s accelerator cavities made from pure niobium, are large and cost several hundred thousand dollars each. They must be cooled with costly central cryogenic plants that use a considerable amount of liquid helium at a temperature between 2 and 4 kelvins. When units of six to eight cavities – known as cryomodules – are strung together, they are equivalent to railroad freight cars in size.
Of the $3.2m allocated for this project, some $2.2m will be used to develop a practical approach to reduce the size and cost of superconducting linear accelerators while simultaneously improving their reliability. Kelly and his team will leverage smaller, better-performing superconducting cavities based on an emerging technology known as thin-film Nb3Sn (niobium-three-tin). This film is only 2-3 micrometres thick. The team will produce these cavities using a process called vapour diffusion.
The new niobium-three-tin cavities would require less helium for cooling and also replace the large, water heater-sized cavities with much smaller cavities, about the size of a coffee can. The physical size reduction of the accelerator cryomodules would be a factor of three to five. “We think it’s a big deal,” said Kelly. “We won’t know precisely what the size and cost reduction is until we do a lot more research and development. That’s a major part of what this R&D intends to address,” he added.
Critically, niobium-three-tin technology helps eliminate the single point of failure associated with the cryogenic plant by making it possible to replace large liquid helium refrigerators with new generation smaller plug-in cryocoolers. Typically, if the liquid helium refrigerator malfunctions, the accelerator is forced to shut down entirely. The researchers hope to avoid such forced shutdowns by eliminating the large refrigerator and replacing it with a distributed set of fault-tolerant 10-watt cryocoolers.
The accelerator must be extremely reliable, with an uptime – a period of time in which a machine is continuously functional and available – of 95% or higher to avoid interruptions in the transmutation process, according to Argonne physicist Brahim Mustapha ee working on the project.
If the accelerator stops, the spallation target that produces neutrons inside the reactor cools off. If the process restarts, the spallation target heats up again. When this stop-start sequence happens frequently, the process causes thermal and mechanical stress that can damage the target.
In a separate project, Kelly’s team has been using the Argonne Tandem Linac Accelerator System (ATLAS), a DOE Office of Science user facility at Argonne, to improve reliability through cryocooler development. The team uses artificial intelligence, machine learning and other strategies to minimise accelerator downtime due to malfunctions.
The ultimate goal for the researchers working on the ARPA-E NEWTON-funded project is to demonstrate two high-performance niobium-three-tin-coated cavities optimised for protons moving near 50% the speed of light – a crucial step toward the full high-power accelerator. They also aim to develop an end-to-end linac design and conceptual layout optimised for niobium-three-tin.
The project faces some challenges. For example, relatively complex geometries are required for medium-velocity cavities because of their hard-to-reach surfaces. These irregular shapes may hinder the deposition of smooth, uniform niobium-three-tin films, potentially allowing impurities like dirt or dust to enter the film coating and contaminate it. Small contaminants, and even geometric irregularities, could lower the performance of the superconducting cavity.
While the Argonne team is focused primarily on the niobium-three-tin linac and cavity design and demonstration, Fermilab is providing its expertise and infrastructure to perform the vapour diffusion process that underpins the niobium-three-tin technology. Grigory Eremeev and Sam Posen, both senior scientists and 2016 recipients of the DOE Early Career Award, are leading Fermilab’s efforts.
“Support from DOE made it possible to develop highly capable niobium-three-tin coating facilities at Fermilab and to develop techniques to achieve high performance in cavities with complex geometries,” said Posen. “Now we are building on that foundation, advancing coating techniques and applying them to these exciting applications.” Eremeev added that niobium-three-tin films are critical for this type of application. “While complex geometries are challenging for deposition, we’ve already seen excellent results in our collaborative work with Argonne,” he said.
Looking to the future and the construction of the full accelerator, it is critical to industrialise the technology. To help with this, the project involves collaboration with two companies. RadiaBeam will industrialise most or all of the process of building niobium-tin cavities, and RadiaSoft will conduct reliability studies for the proposed linac design.
This project is one of 11 selected in 2025 to receive $40m in ARPA-E NEWTON programme funding to develop cutting-edge technologies that enable the transmutation of used nuclear fuel.
One of the other projects awarded $7m in funding, will complement the joint Argonne-Fermilab effort. A team of researchers, led by Taek Kim, a principal nuclear engineer who manages the nuclear systems analysis group within Argonne’s Nuclear Science and Engineering division, is developing a novel transmutation system using an innovative separation method to remove waste by-products from the process.
The project, Liquid Lead Suspended Fuel Subcritical Fission Blanket for Nuclear Waste Transmutation, focuses on a new type of transmutation process. It also uses innovative separation methods – centrifugal force – to remove the waste by-products from the process. Fission is the process of splitting an atomic nucleus into two or more smaller nuclei, releasing a large amount of energy.
The project aims to transmute the entire US stockpile of minor actinides within 30 years, reducing the nuclear fuel mass by 28 times, which is roughly equivalent to shrinking a large swimming pool filled with used fuel down to the size of a small hot tub. It will also decrease radiotoxicity management time 333-fold.
“I am very excited to receive funding from the NEWTON program to advance this brand-new technology,” said Kim. “This method uses physics-based separation instead of conventional chemical separations such as PUREX, making it a separation technology that is more secure and more difficult to use for nefarious purposes.”
The proposed transmutation system uses a proton accelerator to start fission in a liquid lead setup containing tiny minor actinide particles — the heavy, radioactive elements near the bottom of the periodic table. As the minor actinide particles fission, the two new smaller nuclei are ejected from the particle and can be separated from the actinide particles by centrifugal methods in a recycling system.
Together, these projects will yield a complete accelerator-driven system for nuclear waste transmutation. By significantly reducing the mass, volume, activity and effective half-life of the existing stockpile of commercial used nuclear fuel, these and other ARPA-E NEWTON-funded projects will help shift used nuclear fuel disposal from an intergenerational issue to an intra-generational one.
