Thorium
THE FULL STORY
For the past eight decades, commercial nuclear power has predominantly used solid uranium dioxide (UO₂) pellets contained in fuel assemblies. An alternative approach—first demonstrated in concept and early experiments (ORNL, 1972)—uses fissile and fertile material dissolved in a molten salt. Molten salt reactors (MSRs) are a Generation-IV reactor class in which the fuel is carried in a high-temperature molten salt that simultaneously serves as the primary coolant.
In thorium MSR designs (TMSRs), thorium is the principal fertile feedstock; neutron capture and subsequent decay transmute thorium into fissile uranium-233 (U-233). Fuel salts are typically fluoride-based (for example, mixtures of lithium fluoride, beryllium fluoride and actinide fluorides), though chloride-based salts are considered in some designs. Uranium molten salt reactors (UMSRs) use uranium as the primary fissile/fertile material; in many MSR concepts, recycled actinides or spent fuel can also be dissolved into the salt.
Technical characteristics of liquid-fuel MSRs
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The fissile and fertile materials are dissolved or suspended in the circulating molten salt, so the entire core inventory is available to the neutron flux rather than confined to discrete solid pellets or assemblies.
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The molten salt simultaneously functions as fuel carrier and coolant, transferring heat from the core to the power conversion system.
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On-line chemical processing and continuous addition or removal of isotopes are possible while the reactor remains at power, enabling flexible fuel management strategies and improved fuel utilization compared with solid-fuel systems.
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Because there are no solid fuel rods and associated cladding assemblies, radiation and heat transfer dynamics differ from conventional light-water reactors; the circulating fuel permits continuous purification (removal of neutron poisons) and supplementation with fresh fuel.
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MSR designs can be implemented with either fluoride or chloride salts depending on tradeoffs among corrosion, neutronics, and processing chemistry.
These features give MSRs distinct operational and fuel-cycle characteristics—most notably the potential for high resource utilization, reduced long-lived waste streams, and flexible integration of thorium or recycled actinides—while also imposing specific materials, chemistry control, and safety-analysis requirements that drive reactor design and engineering.
Thorium Molten Salt Reactor Basic Schematic
Simplified view of a single-fluid liquid fuel molten salt reactor power plant (Source: redrawn from Dolan 2017)
Liquid-fuel systems offer a fundamental neutronic advantage because the fuel and fission products are continuously mixed in the circulating salt, so all isotopic constituents are exposed uniformly to the neutron flux. In contrast, solid fuel pellets contain spatially fixed isotopes: fertile material (e.g., ²³⁸U) surrounds and partially “self-shields” the limited fraction of fissile ²³⁵U, reducing the effective neutron exposure of the fissile inventory as burnup proceeds.
Solid pellets also develop radial burnup gradients and surface-alteration layers (from chemical transmutation, corrosion, or fission-product deposition) that further impede neutron access and degrade reactivity locally. Continuous online chemistry control and feed in liquid-fuel reactors therefore reduce self-shielding effects, enable more homogeneous fuel utilization, and permit dynamic management of neutron poisons and fissile inventory while the reactor remains in operation.
Solid fuel uranium oxide pellet compared to a molten salt fuel stream
The Oak Ridge National Laboratory (ORNL) was significantly involved in nuclear research, including the development of early nuclear reactors such as the ORNL Molten Salt Reactor Experiment (MSRE). This reactor was primarily a uranium-fueled reactor designed to study advanced reactor concepts. The MSRE at ORNL (1960–1969) was a uranium-fueled molten salt reactor designed to test the feasibility, safety, and efficiency of liquid-fluoride reactors, which operated successfully for over four years. The outcome provided valuable data on corrosion, radiation effects, and system behaviour.
One of the MSRE experiments conducted used a liquid fluoride salt (primarily a mixture of LiF-BeF₂-ZrF₄-UF₄) as both fuel and coolant (ORNL 1972). The reactor achieved temperatures over 600°C and demonstrated high system efficiency, safety features, and reliable operation for over 4 years (6000 hours). Data collected included corrosion behaviour, radiation damage, and system stability.
This system was later used to run a thorium flouride salt fuel.
The MSR used in the Oak Ridge Molten salt reactor (7.4 MW) commercial pilot 1969
Experimental
Foundations
Copenhagen Atomics is a Danish company developing small modular nuclear reactors, specifically focusing on molten salt reactor (MSR) technology. Their main offering is the development of advanced, safe, and scalable nuclear energy systems designed for decarbonization, especially in remote locations and for industrial applications.
Copenhagen Atomics plans to sell energy at a lower price than any other energy technology.
Simon Michaux standing in front of a water circuit test rig at the Copenhagen Atomics factory
(Image: Simon Michaux)
When Simon Michaux visited the Copenhagen Atomics factory (in development since 2014), it was apparent that the complexity of these MSR units was similar to a 3-bedroom kit home, and their plan to manufacture one of these units a day (and associated fuel) is a practical target.
The size of the Copenhagen Atomics full sized MSR unit, Pictured is a non-fission prototype test unit
(Image: Copenhagen Atomics)
The Onion Core concept design by Copenhagen Atomic (copyright Copenhagen Atomics)
Each commercial reactor is planned to be able to produce 100 MW of thermal heat, or 40 MW of electricity. This is a modular system, where many units can be fitted together to construct a range of power capacities. Each reactor is the size of a 40 foot shipping container.
1g of thorium or uranium produces 24 MWh of thermal energy. A plant producing 1 GW electricity needs 800kg of 232Th metal in salt form each year. Copenhagen Atomic plan to manufacture one of these 40 MW capacity MSR systems a day.
China selling thorium generated electricity commercially since 2022
The Shanghai Institute of Applied Physics (SINAP) of the Chinese Academy of Sciences has been granted an operating license by the PRC Ministry of Ecology and Environment for the experimental TMSR-LF1 thorium-powered molten-salt reactor (installed capacity 2 MW th), construction of which started in Wuwei city, Gansu province, in September 2018 (WNN 2023). As of August 2022 this reactor was authorized to commercially sell electricity.
The Thorium Molten Salt liquid flouride (TMSR-LF1) salt reactor operating in Circa Whuhai, China (WNN 2023)
The TMSR-LF1 will use fuel enriched to under 20% U-235, have a thorium inventory of about 50 kg and conversion ratio of about 0.1. A fertile blanket of lithium-beryllium fluoride (FLiBe) salt with 99.95% Li-7 will be used, and fuel as UF4. If the TMSR-LF1 proves successful, China plans to build a reactor with a capacity of 373 MW(th) by 2030 (WNN 2023).
Thorium MSR Fuel Cycle - FUJI-U3
The Molten Salt Reactor (MSR) concept has been developed over the last few decades by multiple international groups (Dolan 2017). The FUJI MSR system (and related accelerator technology) has been developed by Kazuo Furukawa and his research group in Japan, since the early 1980’s (Furukawa et al. 2008, 2010, and Yoshioka 2013).
The FUJI-U3 (using 233U as fissile) is mostly based on the MSBR designs developed at the Oakland Ridge Nuclear Laboratory (ORNL) with some improvements. The latest FUJI design is shown in Mitachi et al. (2007, 2008). The FUJI system has been deigned to be size-flexible from 100 MWe to 1000 MWe. Current planning has an installed capacity of 200 MWe with a thermal output of 450 MWth. This means FUJI MSR has a thermal efficiency of approximately 44%. This is higher than the LWR technology (33%) due to its high exit temperature.
Thorium Fueled MSR - Thorcon
Thorcon is a US based company developing small modular molten salt reactors (SMRs) designed for large-scale, safe, and affordable nuclear power generation. Thorcon aims to provide affordable, scalable, and safe nuclear energy that helps address climate change by replacing fossil fuels with zero-carbon electricity.
Thorcon’s reactor designs are proposed to be passively safe, walk-away safe, and operate at atmospheric pressure. Uses liquid fluoride salt as both fuel and coolant, which allows for high-temperature operation and efficient power generation.
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Large-scale capacity: Designed to produce around 500 MW (electric) per module.
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Safety: The reactor is designed with passive safety systems, meaning it can shut down without human intervention or external power in case of emergency.
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Fuel Cycle: Utilizes thorium or uranium as fuel, with high fuel efficiency and low waste generation.
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Scalability: Multiple modules can be combined to meet larger power demands.
Self-sustaining Thorium Gas-cooled Pebble Bed Reactor (STGR)
A Self-sustaining Thorium Gas-cooled Reactor (STGR) is a nuclear reactor design that uses thorium as its primary fuel and employs an inert gas coolant (such as helium or carbon dioxide). The gas provides efficient heat transfer, operates at high temperatures (~700–1000°C), and inherently enhances safety due to the low reactivity of inert gases. Its goal is to produce low-cost, safe, and sustainable energy by achieving a self-sustaining fuel cycle, primarily through thorium’s ability to breed uranium-233 (U-233). The reactor can operate in both breeding and burning modes, ideally achieving self-sustenance over its lifetime.
Image & data copyright: Dr. Remond Pahladsingh
Typically features a graphite moderator that works in tandem with the gas coolant. The fuel is often in solid form (e.g., coated particles or rods) to facilitate handling and fuel management. It’s designed to have passive safety mechanisms, such as negative temperature feedback and natural convection cooling.
The reactor breeds U-233 efficiently and recycles it within the core, maintaining criticality without external fissile input. Excess breeding allows for fuel recycling and waste minimization.
Image & data copyright: John Troughton
Image & data copyright: John Troughton
Case study
Assessment of the scope of tasks to completely phase out fossil fuels in Hawaiʻi
This study set out to examine the various different options to phase out fossil fuels in the State of Hawai’i. The use of Thorium MSR units was documented in one of several case studies.
The Purple Transition
Page 461 of Main Report. Page 477 of pdf. Figure 244
What would happen if Hawaii applied liquid fuel fission, Thorium Molten Salt MSR systems for power generation?
Page 508 of Main Report. Page 524 of pdf.
Appendix Theta (q ): Liquid fuel fission with a thorium salt fuel. Appendix 2. Page 1202 of pdf.
Comparison of footprint of liquid fuel Th MSR to solid fuel U ALWR, Appendix 2. Page 1240 of pdf.
ERoEI of Th MSR, Appendix 2. Page 1258 of pdf.
