New US liquid metal nuclear fuel could boost energy output of reactors

Oak Ridge National Laboratory (ORNL) has officially signed a research license agreement with Australian investment firm Out The Back Ventures to accelerate the development of a transformative nuclear fuel technology. 

“A new approach to nuclear fuel developed at Oak Ridge National Laboratory could enhance the performance of new light water nuclear reactors,” said the lab in a press release.

“Light water reactors make up most of the US operating nuclear reactors and generate power using uranium dioxide fuel pellets.”

The partnership aims to move the Uranium Dioxide Liquid Metal Suspension (ULIMES) concept from the laboratory toward commercial viability.

A new state for nuclear fuel

The breakthrough centers on a “flowing” fuel system that reimagines the internal chemistry of nuclear reactors. 

While traditional light water reactors (LWRs)—which provide the vast majority of carbon-free power in the US—rely on solid uranium dioxide (UO2) pellets, the ULIMES design uses unsintered UO2 particles suspended in a liquid metal (LM) carrier.

This suspension creates a circulating fuel system that flows through the reactor core. By switching from solid pellets to a liquid-metal suspension, the fuel can move continuously between high and low neutron flux regions. 

This results in uniform burnup and eliminates the “self-shielding” effect that typically leaves untapped energy in the center of standard fuel pellets.

Safety and efficiency gains

The ULIMES concept offers several critical advantages over conventional fuel. It provides superior cooling because the liquid metal carrier significantly increases thermal conductivity, which allows the reactor to remove heat more efficiently and lowers peak fuel temperatures.

“This design improves a reactor’s ability to cool itself, offering a major gain in efficiency, in addition to driving significant safety gains and lowered maintenance costs,” added the press release.

This design also results in reduced material damage, as fission gases do not accumulate to the same degree as in traditional solid fuels, and the liquid metal carrier remains immune to the “collision cascade” damage that typically degrades solid structures.

Furthermore, the technology allows for seamless integration with existing infrastructure since ULIMES is chemically compatible with the materials already used in today’s light water reactors. This effectively avoids the need for massive, multi-decade material qualification programs that often stall new nuclear designs.

“ULIMES bridges today’s reactors with tomorrow’s technologies, using proven materials to provide next-generation performance without next-generation construction costs,” says ORNL’s Ian Greenquist.

Moving toward the grid

The involvement of Out The Back Ventures provides the necessary capital to fund additional research into the density and viscosity required to maintain the suspension, whether through natural circulation or mechanical mixing. 

Because the design allows for a higher uranium concentration than pure liquid fuels, it offers a high degree of flexibility for various reactor sizes, from large-scale plants to small modular reactors (SMRs).

By improving cooling and energy extraction while lowering maintenance costs, ULIMES could extend the life of the current nuclear fleet while providing a high-performance foundation for the next generation of reactors.