Thorium, a mildly radioactive metal roughly three times more abundant in the Earth’s crust than uranium, powered a working nuclear reactor at Oak Ridge National Laboratory more than half a century ago. The experiment ran successfully for years, then the program was shelved. Today, with no domestic thorium production in the United States and full import reliance for what little is consumed, the element sits on the sidelines of energy policy while nations scramble for low-carbon power sources.
Oak Ridge Proved It Could Work in the 1960s
The Molten Salt Reactor Experiment, or MSRE, achieved its first self-sustaining reaction on June 1, 1965, at Oak Ridge National Laboratory in Tennessee. The reactor used a liquid fluoride salt mixture as both fuel and coolant, a radical departure from the solid-fuel, water-cooled designs that dominated the nuclear industry. Three years later, on October 8, 1968, the MSRE became the first reactor ever to run on uranium-233, a fissile isotope bred from thorium. That milestone demonstrated a complete thorium fuel cycle in a working machine, not just on paper, and it emerged from a laboratory that had already built deep expertise in neutron behavior and materials through its broader neutron-science programs.
The technical foundations for this work stretched back even further. A task force report commissioned by the U.S. Atomic Energy Commission had already evaluated molten-salt and other fluid-fuel concepts for their technical feasibility. By the early 1960s, Oak Ridge engineers were publishing detailed progress reports on materials, salt chemistry, and component development for the molten-salt program. The groundwork existed decades ago, as documented in a 1960–1961 progress summary that tracked engineering milestones in real time. Yet despite this record, federal priorities shifted toward solid-fuel reactors optimized for plutonium production and light-water power generation, and the liquid-fueled breeder concept ultimately lost its funding and political sponsorship.
Why Thorium Remains Sidelined in the U.S.
Thorium composes roughly 0.0006% of the Earth’s crust, which, while a small fraction, still makes it significantly more available than uranium in geological terms. A Stanford analysis notes that the cost per kilogram of thorium would likely be similar to uranium, meaning the raw material itself poses no obvious economic barrier. Thorium shares key nuclear properties with uranium that allow it to sustain a chain reaction once converted to U‑233, but unlike uranium, it cannot be directly split to release energy. Instead, it must first absorb a neutron and convert into fissile U‑233, adding a step that existing commercial reactor designs were never built to accommodate and complicating licensing pathways for any new design that depends on it.
The supply picture tells its own story. According to the Mineral Commodity Summaries 2024, U.S. thorium production is nonexistent, and the country is entirely import reliant for the small quantities used in niche applications like welding electrodes and ceramics. Broader assessments by the U.S. Geological Survey have long treated thorium as a byproduct or curiosity rather than a strategic fuel, reflecting the absence of a commercial market. Without a reactor fleet that burns thorium, there is no market pull to develop domestic mining or processing. This creates a circular problem: no reactors means no demand, and no demand means no investment in supply chains. Breaking that cycle would require either a policy mandate that explicitly values thorium’s waste and fuel-security advantages, or a private-sector bet large enough to absorb years of technical, regulatory, and financing risk.
Molten Salt Reactors and the Breeding Advantage
The reactor type best suited to thorium is the same one Oak Ridge tested: the liquid-fueled molten salt reactor. In these systems, thorium dissolved in a molten fluoride salt can absorb neutrons and breed U‑233 continuously while the reactor operates, turning the core into both power plant and fuel factory. An Oak Ridge synthesis explains how liquid-fueled designs enable U‑233/thorium breeding through online or batch processing and fission-product removal, capabilities that solid-fuel reactors cannot easily replicate. The two major reference designs that emerged from Oak Ridge’s work are the Molten Salt Breeder Reactor (MSBR), optimized for high breeding ratios, and the Denatured Molten Salt Reactor (DMSR), which trades some breeding performance for simpler operation and improved proliferation resistance.
Thorium is well suited to at least three advanced configurations, according to a 2013 study published in Energy Policy, including molten-salt and high-temperature gas-cooled systems. The element has a long history of investigation across multiple reactor types, but the molten-salt approach highlights its distinctive strengths. For energy planners, the practical difference matters: thorium fuel cycles produce far less long-lived transuranic waste than conventional uranium cycles, which could sharply reduce the volume and toxicity of spent fuel that must be stored for millennia. That waste reduction is not just a theoretical bonus but a direct consequence of the physics, since thorium‑232 breeds U‑233 rather than plutonium, sidestepping many of the isotopes that make conventional nuclear waste so persistent and politically contentious.
Real Hazards the Optimists Overlook
Enthusiasm for thorium often glosses over hard lessons from the only facility that actually ran a thorium-derived fuel cycle at scale. When workers returned to the shut-down MSRE decades after its closure, they discovered uranium hexafluoride and fluorine in the off-gas lines in March 1994, hazardous compounds that required complex safety mitigations. The cleanup effort revealed that managing residual salts, radioactive gases, and corrosion products in a dormant molten-salt system is far from trivial. Those experiences underscore that while molten salts avoid some of the failure modes of water-cooled reactors, they introduce their own chemical and radiological challenges that must be engineered and regulated just as rigorously.
Regulators have also cataloged non-trivial proliferation and safeguards questions around thorium cycles. A contract study for the U.S. Nuclear Regulatory Commission examined how thorium-based fuel cycles might affect safeguards, waste streams, and security. It concluded that while thorium can reduce plutonium production, it does not eliminate proliferation concerns, because U‑233 itself is a weapons-usable material that demands tight control and monitoring. Any future thorium deployment would therefore have to integrate robust safeguards, remote handling, and long-term stewardship plans from the outset, rather than assuming that a different fuel automatically solves the political and security problems that have dogged nuclear power for decades.
What It Would Take to Bring Thorium Back
Rehabilitating thorium from historical curiosity to serious energy option would require more than nostalgia for Oak Ridge’s achievements. First, designers would need to translate the MSRE’s experimental insights into modern, licensable reactor concepts that can meet today’s safety and performance expectations. That means extensive materials testing, corrosion studies, and fuel-cycle development, informed by both legacy documentation and contemporary tools such as advanced neutron-characterization facilities and high-fidelity simulation. Second, regulators would have to develop clear guidance for liquid-fuel reactors and thorium fuel cycles, building on prior technical assessments and NRC-sponsored analyses to define how such systems can demonstrate safety, security, and environmental protection.
On the economic side, any serious thorium program would need a credible path from demonstration to fleet deployment. That implies aligning long-term power purchase agreements, government-backed risk-sharing, and staged investments in mining, separation, and fuel-processing infrastructure. It also means confronting the reality that competing low-carbon options (renewables, conventional nuclear, and gas with carbon capture) are all vying for the same capital and policy attention. Thorium’s advantages in fuel abundance and waste characteristics are real, but they will only matter if governments and private investors are willing to fund the slow, technically demanding work of turning a half-century-old experiment into a commercial technology. Until then, thorium will remain what it is today: a proven but politically orphaned fuel, waiting on the margins of the energy transition while the world debates how bold it is willing to be with nuclear power.
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*This article was researched with the help of AI, with human editors creating the final content.
