Brown combines nuclear waste disposal with mobile energy generation

It is common for nuclear engineers to be concerned with the disposition of nuclear waste. After all, the United States alone currently has about 90,000 metric tons of spent nuclear fuel stored in concrete casks on nuclear reactor sites—enough to fill 18 Olympic-sized swimming pools—and is still producing about 2,000 metric tons every year.

Multiple administrations have tried to determine safe locations for final disposition, while generations of nuclear semiotics experts have considered how to safely keep people away from those storage sites.

Pietro F. Pasqua Fellow Nick Brown, a professor in the Department of Nuclear Engineering at the University of Tennessee, Knoxville, has a different approach.

“That spent fuel is a national resource,” he said. “There’s a ton of useful material there.”

This December, Brown and his colleagues will complete a project called Matrix Engineered TRISO Compacts Enabling Advanced Reactor Fuel Cycles (MATRICY), a $3.4-million research project headed by Stony Brook University and funded by the Department of Energy’s Advanced Research Projects Agency – Energy (ARPA-E).

In 2022, the MATRICY researchers started investigating whether the heavy elements in used nuclear material could be recycled into fuel for microreactors—nuclear reactors small enough to fit in shipping containers.

“We’ve determined that this is not only possible, it’s potentially extremely attractive,” Brown said. “We’re talking about reducing the cost of energy from these reactors by four to seven times while reducing nuclear waste stockpiles by 93 percent.”

MATRICY’s timing is just as incredible as the project’s results. Earlier this year, the US government issued multiple executive orders on nuclear energy, with goals including both adding nuclear energy capacity and efficiently recycling spent nuclear fuels.

“Nuclear energy is one of the most important ultra-low-carbon sources of electricity and heat in the world,” said Brown. “In nuclear engineering, we’re always trying to ensure reactors are as safe as possible, but it’s also important to minimize the burden of nuclear waste for future generations. I think it’s incredible that this team was prioritizing this work years in advance, and have been developing concepts that the federal government is now really interested in.”

Stony Brook Synergy

Years ago, Brown had been working with Stony Brook Professor Jason Trelewicz on an ARPA-E project devoted to developing advanced materials for moderators (parts of a reactor that enable more complete fission). After seeing how well those technologies performed with high-assay, low-enriched uranium (HALEU) fuel, Brown and Trelewicz decided to propose a follow-on project focused on using recycled fuel in the same systems.

“We were very excited to extend some of our previous innovations to a new program focused on reducing nuclear waste,” Brown said.

As in their previous project, the researchers at Stony Brook developed fabrication and synthesis techniques for fuels and moderator materials while Brown led work at the University of Tennessee developing reactor designs and quantifying the technological and economic benefits.

“We have a very synergistic collaboration with Stony Brook,” said Brown. “We’re doing the computational and design work that informs their experimental work to actually fabricate the materials that would be used in these systems.”

Small Reactors and Big Elements

Brown, Trelewicz, and their colleagues on MATRICY focused on microreactors because of their flexibility and broad use case.

“The idea is you’d have a mobile reactor, transported by train or truck, that could operate either independently or as part of the grid,” Brown explained. “It can be used to power critical infrastructure or industrial sites—everything from hospitals and water treatment plants to drilling sites—in remote communities such as in Alaska.”

Like most large-scale fission reactors in the US, most proposed microreactors would be fueled with HALEU. However, since current HALEU production facilities are very limited, the US is facing a HALEU shortage.

Instead, MATRICY researchers investigated using elements heavier than uranium (transuranic elements), like plutonium. Those elements do not occur naturally, but are artificially created in reactors.

“Continuing to use those materials in reactors has been looked at before,” Brown said, “but never for microreactors.”

A Responsible, Economical Solution

During MATRICY, Brown and his graduate students Venkata (Teja) Vallabheneni, Donald Doyle, and Edan Estes-Lumpkin compared the energy output, cost, and waste production of microreactors fueled with recycled transuranic fuel versus typical HALEU-fueled microreactors.

They found that using recycled transuranic elements as fuel would result in a whopping 93 percent reduction in nuclear waste headed for final disposition. In addition to reducing the existing stockpile, adopting the transuranic elements as a new fuel stream would decrease the amount of new waste generated by reactors going forward.

The energy density of the transuranic elements is also economically favorable, with the recycled fuel producing between four and seven times more energy than an equivalent amount of classical enriched uranium.

“I think that waste is one of the most significant challenges and opportunities for us to address in nuclear energy,” said Brown. “What is unique and exciting about this project is showing that we can significantly reduce the burden of nuclear waste for future generations while simultaneously generating energy.”