A new approach to nuclear fuel developed at Oak Ridge National Laboratory (ORNL) could enhance the performance of new light water reactors (LWRs). The LWRs that make up most of the US nuclear fleet generate power using uranium dioxide fuel pellets. ORNL’s Uranium Dioxide Liquid Metal Suspension (ULIMES) fuel concept is designed to cool the reactor more efficiently and extract even more energy from uranium fuel, improving overall reactor performance.
The ULIMES design uses uranium dioxide fuel particles suspended in liquid metal to form a flowing fuel system that circulates fuel through a reactor core. 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. The concept is also compatible with materials already used to construct light water reactors.
“ULIMES bridges today’s reactors with tomorrow’s technologies, using proven materials to provide next-generation performance without next-generation construction costs,” said lead researcher Ian Greenquist. ORNL has signed a research licence agreement with Australian investment firm Out The Back (OTB) Ventures to fund additional ULIMES research.
OTB Ventures is providing the necessary investment to move ULIMES from a laboratory concept toward commercial viability. This partnership is part of a broader trend at ORNL to leverage Industry Engagement and Technology Transfer to accelerate the deployment of advanced energy solutions.
OTB has developed relationships with hundreds of researchers and in 2023 engaged (including campus visits) with over 30 universities and national labs in the USA, Australia, New Zealand, Singapore, UK, Switzerland, Sweden and Netherlands. The investment of time into these research communities in specific fields of deep/frontier tech research is an important aspect of how OTB identifies investment opportunities.
ULIMES (Uranium Dioxide Liquid Metal Suspension) is a “flowing” nuclear fuel concept developed by Oak Ridge National Laboratory (ORNL) that reimagines the internal chemistry of nuclear reactors. Unlike traditional solid fuel pellets, ULIMES uses unsintered uranium dioxide particles suspended in a liquid metal carrier, such as a lead-tin or bismuth-lead-tin alloy.
Because the fuel is a circulating suspension, it moves between high and low neutron flux regions. This eliminates the “self-shielding” effect where the centre of a solid pellet often goes unused, allowing for more energy to be extracted from the same amount of uranium.
Uranium dioxide is naturally a poor heat conductor. By suspending it in liquid metal, which has much higher thermal conductivity—the reactor can remove heat more efficiently, significantly lowering the peak temperature of the fuel.
The ULIMES concept provides several passive safety advantages. The improved thermal performance means the reactor is better able to cool itself, potentially reducing the need for complex, redundant safety systems. Solid fuels suffer from “collision cascade” damage and the accumulation of fission gases, which cause swelling and degradation. In ULIMES, the liquid metal carrier is immune to this structural damage, and fission gases do not accumulate in the same way.
A key advantage of ULIMES is its chemical compatibility with materials already used in current LWRs. This “bridges” today’s technology with future designs without requiring decades of new material qualification. Current studies, led by ORNL’s Ian Greenquist, are focusing on the density and viscosity required to maintain a stable suspension, whether through mechanical mixing or natural circulation.
Experiments have shown that at typical operating temperatures (400°C), candidate liquid metals do not cause significant corrosion to Zircaloy-4 cladding. The fuel’s high uranium concentration makes it suitable for various scales, from large NPPs to small modular reactors (SMRs).
