Molten Chloride Reactor Experiment

Molten Chloride Reactor Experiment
Artistic rendering of MCRE
Reactor conceptMolten-salt reactor
StatusUnder construction
LocationIdaho National Laboratory
Coordinates43°35′35.2″N 112°39′16.8″W / 43.593111°N 112.654667°W / 43.593111; -112.654667
Main parameters of the reactor core
Fuel (fissile material)HEU
Fuel stateLiquid
Neutron energy spectrumFast
Primary control methodB4C control rods
Primary coolantNaCl-UCl3 circulating fuel
Outlet temperature650 °C
Reactor usage
Primary useExperimental
Power (thermal)150 kW
Operator/ownerSouthern Company

The Molten Chloride Reactor Experiment (MCRE) is an experimental molten-salt reactor under construction at Idaho National Laboratory (INL). It will generate 150 kW of heat and no electricity, and use a mixture of sodium chloride and uranium chloride as both nuclear fuel and coolant. It is intended to prototype a fast-spectrum molten-salt reactor using chloride salt, instead of the better-studied fluoride salts. Such reactors have attracted attention for their high fuel efficiency, proliferation resistance, and ability to burn nuclear waste as fuel.

The reactor is being built by a consortium led by Southern Company, TerraPower, and Core Power, and funded by the US Department of Energy. It will be located at INL's LOTUS test bed, inside the former Zero Power Physics Reactor. MCRE is expected to be the world's first fast-spectrum molten-salt reactor, and is expected to be operational in 2027. Its fuel salt is being produced by INL.

Design

MCRE will generate 150 kW of heat, and will operate between 600 and 650 °C. As a molten chloride fast reactor (MCFR), it will use molten chloride salt as both its fuel and coolant. Its fuel is a eutectic mixture of sodium chloride and uranium trichloride (67-33 mol%). The uranium will be highly-enriched (93.2% 235U). MCRE is expected to require approximately 1300 kg of fuel salt in total. The reactor also uses a flush salt, composed of a eutectic NaCl-MgCl mixture, to flush out the system during commissioning and decommissioning.

The reactor core consists of a cylindrical vessel containing the molten fuel salt. Fuel circulates upward through the reactor vessel, and enters a centrifugal pump atop the reactor. The pump circulates the salt downward through a pipe loop, where it re-enters the vessel from the bottom. The pump also contains an expansion volume to accommodate thermal expansion of the fuel salt during operation. The piping loop is linked through a drainpipe to the fuel handling system.

MCRE will use four control rods, arranged surrounding the core vessel, as its reactivity control system. Each rod contains neutron-absorbing boron carbide. To reduce neutron leakage from the core, magnesium oxide (MgO) is used as a neutron reflector. MgO bricks are stacked surrounding the core to create the reflector assembly. The core and reflector are encased with insulation, with heater panels inside. The externally-powered heaters ensure the fuel salt remains liquid during operation. The reactor lacks a primary coolant system, and therefore cannot sustain long-term high-power operation.

MCRE's primary objective is to characterize MCFR behavior, in support of a future demonstration reactor. The experiment is anticipated to run for six months before decommissioning. The data collected from MCRE is expected to guide deployment of commercial and maritime MCFRs.

History

Background

Unlike most nuclear reactors, which use solid fuel, molten-salt reactors (MSRs) use nuclear fuel dissolved within a liquid salt. The molten chloride fast reactor (MCFR) is a type of fast-spectrum MSR whose fuel is a chloride salt. Chlorine, especially 37Cl has very favorable nuclear properties for a fast reactor, including low neutron absorption. A molten-salt reactor using chloride salts results in one of the hardest neutron spectra of any fast reactor concept. Its high neutron economy can then be used either to destroy nuclear waste or as an efficient breeder reactor. As with other MSRs, MCFRs feature low-pressure operation, online refueling, and a large negative temperature coefficient of reactivity. However, chlorine is largely transparent to neutrons, requiring a large neutron reflector. It is also less chemically stable than fluoride salts, which causes issues with corrosion.

Thermal-spectrum molten-salt breeder reactors using the thorium fuel cycle use fuel salt with a very small (~2%) fissile fuel fraction. This results in high sensitivity to fission products. Thus, they require online reprocessing to prevent buildup in the fuel salt. Online reprocessing including separation of 233Pa is also frequently required for breeding, and they must use fluoride salts. In contrast, many chlorides allow significantly higher fissile fractions, which in a fast reactor results in little sensitivity to fission products. MCFRs therefore can be excellent breeders using either uranium-plutonium or thorium-uranium fuel cycles. Alternatively, an MCFR can operate in a "breed-and-burn" configuration, where depleted uranium is used as the feed material but fuel is not reprocessed.

Research on chloride salts for MSRs occurred during the Aircraft Nuclear Propulsion program, and the civilian MSR program at Oak Ridge National Laboratory. However, the graphite-moderated molten salt breeder reactor using a thorium fuel cycle was chosen instead, due to its low fissile inventory and subsequent low doubling time. As a result, relatively little experimental data has been collected for chlorides compared to fluorides. More recently, the MCFR has attracted attention due to its breed-and-burn fuel cycle. Such a configuration allows for very high fuel efficiency without requiring uranium enrichment or reprocessing, which are considered proliferation risks.

Southern Company is part of a public-private partnership with TerraPower, UK-based Core Power, the US Department of Energy (DOE), and several other collaborators aiming to commercialize the molten chloride fast reactor. In 2016, the Department of Energy awarded a $68 million cost-sharing agreement, under its Advanced Reactor Competition program, to support development of the MCFR.

Funded by the DOE under an Advanced Reactor Concepts (ARC-15) award, the consortium constructed an integral effects test (IET) system at TerraPower's facility in Everett, Washington. The IET, using chloride salt without any nuclear material, entered operation in 2023. The IET is a non-nuclear salt loop, using external heaters supplying 1 MW of heat. It was intended to inform the design of a 180 MW demonstration MCFR.

Construction

In 2020, Southern Company was selected under the ARDP's risk reduction pathway to design, construct, and operate the Molten Chloride Reactor Experiment (MCRE, pronounced "McCree"). The risk reduction pathway was created to finance projects that could be licensed and deployed within 10 to 14 years. In November 2021, Southern Company and the DOE announced a cooperative agreement to construct and operate MCRE at Idaho National Laboratory (INL). The project is supported by a five-year $170 million cost-sharing agreement under the ARDP. MCRE is expected to be the first molten chloride salt-fueled reactor.

MCRE is planned to be DOE-authorized and is sited inside the Laboratory for Operation and Testing in the United States (LOTUS) test bed. LOTUS is a test facility for advanced reactor demonstration projects, and comprises the reactor cell area of the Zero Power Physics Reactor (ZPPR), which was closed in 1992. The test bed is intended for microreactors up to 500 kW. LOTUS is capable of handling fuels that require enhanced security. The final environmental assessment and finding of no significant impact for MCRE was issued by the DOE in August 2023.

In support of MCRE, INL constructed a Molten Salt Flow Loop Test Bed. The test loop uses 4.5 kg of lithium chloride and potassium chloride mixture, plus an inert cover gas. It was built to monitor corrosion and instrument performance in hot chloride salt, and entered operation in 2025.

MCRE is expected to be the first fast-spectrum molten-salt reactor, and the first fast-spectrum circulating-fuel reactor. On October 14, 2026, Southern Company and Core Power announced the DOE's approval of cost-sharing under ARDP for the final stage of the project. According to the announcement, MCRE is expected to achieve criticality in 2027.

Fuel production

Close-up of salt crystals produced at INL for the Molten Chloride Reactor Experiment

Production of the NaCl-UCl3 fuel salt for MCRE began in 2020. The HEU for MCRE is sourced from fuel plates left over from the ZPPR. However, early attempts experienced issues with incomplete conversion. A technical breakthrough in 2024 allowed for faster production. By 2025, INL claimed it could produce one batch in a single day. In December 2025, the DOE announced that it had finished the first batch of fuel for MCRE. The reactor is expected to require 72 to 75 such batches in total. According to INL, MCRE represents the largest fuel production effort at the laboratory since the Experimental Breeder Reactor II more than 30 years prior. In 2023, a test sample of chloride salt fuel similar to MCRE was irradiated in INL's Neutron Radiography Reactor (NRAD).

References

  1. 1 2 3 4 5 6 TerraPower (December 27, 2022). MCRE Design Description in Support of External Model Development (Technical report). MCRE-ENG-PRSNT-0029. Retrieved October 18, 2026.
  2. ↑ Meadows, Kate (December 5, 2025). "Breakthrough: Idaho Lab Produces World's First Molten Salt Fuel For Nuclear Reactors". Cowboy State Daily. Retrieved October 18, 2026.
  3. ↑ "Molten Chloride Reactor Experiment". Idaho National Laboratory. May 2025. Retrieved October 18, 2026.
  4. 1 2 3 Final Environmental Assessment for the Molten Chloride Reactor Experiment (MCRE) Project (PDF) (Report). US Department of Energy. August 2025. INL/RPT-22-68976. Retrieved October 18, 2026.
  5. 1 2 3 Nasr, Fadel M.; Tano, Mauricio; Yaseen, Mahmoud; Azmy, Yousry (2026). "Transient uncertainty quantification and Global Sensitivity Analysis of the open-source Molten Chloride Reactor Experiment (MCRE) using GP-PCA surrogate models". Nuclear Engineering and Design. 453. doi:10.1016/j.nucengdes.2026.114898.
  6. 1 2 3 Phillips, Bill (July 21, 2025). Fuel Salt Synthesis for the Molten Chloride Reactor Experiment: Scale-up, Operations, and Production Update (Technical report). Idaho National Laboratory. OSTI 3343584.
  7. 1 2 Nuclear Science and Engineering Division (September 13, 2023). Development and Porting of Nuclear Reactor Models for the NRIC Virtual Test Bed in FY23 (PDF) (Technical report). Argonne National Laboratory. ANL/NSE-23/51. Retrieved October 18, 2026.
  8. 1 2 "Idaho National Lab Creates First Batch of Fuel for World's First Fast Spectrum Molten Salt Reactor Experiment". US Department of Energy. December 3, 2025. Retrieved October 18, 2026.
  9. 1 2 3 "DOE boosts molten chloride reactor demo". Nuclear Engineering International. October 16, 2026. Retrieved October 18, 2026.
  10. 1 2 3 "Idaho lab produces first-ever fuel for fast molten salt reactor experiment, opening door to maritime commercial reactor deployment". Idaho National Laboratory. December 3, 2025. Retrieved October 18, 2026.
  11. 1 2 3 4 Krepel, Jiri; Kramer, Kevin J. (2024). "TerraPower fast chloride reactor". Molten Salt Reactors and Thorium Energy (2nd ed.). Woodhead Publishing. doi:10.1016/B978-0-323-99355-5.00020-3. ISBN 978-0-323-99355-5.
  12. 1 2 3 4 Patel, Sonal C. (December 17, 2020). "DOE Picks More ARDP Winners; One or More Addvanced Nuclear Demonstrations Will Be in Washington State". POWER. Retrieved October 18, 2026.
  13. ↑ "Operations begin at US chloride salt test system". World Nuclear News. October 4, 2023. Retrieved October 18, 2026.
  14. ↑ Dalton, David (October 10, 2023). "Experiment Begins That Aims To Boost Development of MCFR Technology". NucNet. Retrieved October 18, 2026.
  15. ↑ Larson, Aaron (April 9, 2024). "Molten Salt Reactor Technology Solves Several Nuclear Industry Problems". POWER. Retrieved October 18, 2026.
  16. 1 2 "Get to know MCRE, the fast-spectrum MSR from Southern and TerraPower". Nuclear Newswire. American Nuclear Society. March 30, 2023. Retrieved October 18, 2026.
  17. 1 2 3 4 5 "Molten salt–fueled reactor experiment approved for DOE cost-shared funding". Nuclear Newswire. American Nuclear Society. October 15, 2026. Retrieved October 18, 2026.
  18. ↑ Peachey, Caroline (November 22, 2021). "Southern Company, DOE to lead demonstration of molten-salt reactor". Nuclear Engineering International. Retrieved October 18, 2026.
  19. ↑ "Zero Power Physics Reactor Facility". Idaho National Laboratory. December 2023. Retrieved October 18, 2026.
  20. ↑ "Molten salt test loop at INL means real-time data on sensors and materials". Nuclear Newswire. American Nuclear Society. April 3, 2025. Retrieved October 18, 2026.
  21. ↑ "Researchers irradiate chloride-based molten salt in first-of-a-kind experiment". Idaho National Laboratory. September 12, 2023. Retrieved October 18, 2026.