After five years of research, scientists at the US Idaho National Laboratory (INL) believe they have created the perfect recipe to fuel the world’s first critical fast-spectrum molten salt reactor.
The Molten Chloride Reactor Experiment (MCRE) at INL will test a new type of reactor that uses a mixture of molten chloride salt and uranium as fuel and coolant. The MCRE aims to evaluate the safety and physics of a molten chloride fast reactor (MCFR) that Southern Company and TerraPower plan to build.
Conceptual designs of the MCFR expanded into testing in 2016 when the US Department of Energy (DOE) awarded a five-year, $40m cost-sharing award for continued research and development of the MCFR programme. The award initiated a public-private partnership that includes TerraPower, Southern Company, Oak Ridge National Laboratory, INL, the Electric Power Research Institute and Vanderbilt University.
In December 2020, DOE selected the MCRE proposal, as a winner of the second Advanced Reactor Demonstration Program (ARDP) risk reduction pathway. Under the ARDP project, the team will construct the experimental system offsite and ship it to INL, where it will be fuelled using materials currently in storage at the laboratory.
INL’s National Reactor Innovation Centre (NRIC) is developing two reactor demonstration test beds. The Laboratory for Operations and Testing in the US (LOTUS) test bed is expected to support operations beginning in 2027/28 including the MCRE project. The MCRE experiment is scheduled to run for approximately six months at INL, after which it will be decommissioned. The aim is to provide data crucial to the development of the MCFR design.
INL says making fuel for the MCRE/MCFR involves converting uranium metal into a compound that dissolves in the molten salt. “It’s like baking a cake,” said Bill Phillips, technical lead for MCRE. The key challenge was efficiency – converting over 90% of metal uranium feedstock into usable fuel salt. “Nobody has ever made this amount of uranium chloride before,” he noted. “We had to develop the process from scratch.”
Phillips and his team began developing the process and equipment needed for salt synthesis in 2020 at the Fuels & Applied Science Building at INL’s Materials & Fuels Complex.
It wasn’t easy, said Nick Smith, MCRE project director. “We started out wasting too much of the uranium metal we have access to, and we would not be able to make enough fuel salt for the reactor to go critical,” he explained. “After years of experimentation and revision, we finally found the right process to reach the perfect yield. It takes a special kind of perseverance to keep working the problem when there is no guarantee that you will find a solution.”
The team faced a number of technical challenges, including the need to develop a custom prototype furnace and specialised equipment to ensure safety. Overcoming these hurdles was crucial. Phillips pointed out that highly enriched uranium is an extremely valuable and expensive resource. By using depleted uranium for experimentation, the team could test and refine its processes without wasting enriched material.
“When I joined the project, we were having success at making about 2-3 ounces of fuel at once,” said Jacob Yingling, research scientist and a key figure in fuel salt technology development. “We were having serious difficulties at larger scale. Now we can make more than a dozen pounds of reactor fuel in a single batch.”
Other INL projects are planned to build comprehensive molten salt reactor capabilities, including a molten salt characterisation facility and fuel salt irradiation. In 2023, Phillips synthesised and irradiated enriched uranium-fuelled molten chloride salt in the Molten Salt Research Temperature Controlled Irradiation experiment, advancing understanding of how such fuel will perform in commercial reactors.
The Molten Salt Flow Loop Test Bed, which recently became operational, allows for continuous, real-time monitoring and analysis that makes molten salt fuel research much more efficient and effective. INL says the development of uranium chloride fuel salt for MCRE marks a significant milestone that could feed the energy needs of the future.
“We’re almost at the moment where we can bake the cake,” Phillips said with a smile. This means the team has all the ingredients and knows the precise conditions and methods needed. The next goal is to demonstrate the full-scale production of enriched fuel MCRE salt and to make five batches by October 2025. MCRE should be operational by 2028, running parallel to development of LOTUS.
