Why the world’s most abundant nuclear material was abandoned — and why it’s back
Thorium has been waiting in the wings of nuclear energy for decades. Three to four times more abundant in the Earth’s crust than uranium, this silvery radioactive metal — named after Thor, the Norse god of thunder — was seriously considered as a reactor fuel in the early nuclear age before uranium won out for largely non-technical reasons. Today, a combination of energy security concerns, advances in molten salt reactor technology, and China’s groundbreaking experimental results are putting thorium back in the spotlight.
How Thorium Works
Thorium-232, the only naturally occurring thorium isotope, is not itself a fissile fuel — it cannot directly sustain a chain reaction. However, when bombarded with neutrons, Th-232 absorbs one and undergoes a series of radioactive decays to eventually produce uranium-233, which is an excellent fissile fuel. This makes thorium a ‘fertile’ material: a breeding feedstock rather than a direct fuel. Reactors can be designed to continuously breed U-233 from thorium while simultaneously burning it as fuel, creating a partially self-sustaining cycle. According to the IAEA, thorium can generate more fissile material than it consumes when fueling a water-cooled or molten salt reactor — a form of breeding that uranium-235 does not easily achieve in conventional thermal reactors.
The Advantages Are Compelling
The IAEA’s four-year coordinated research project, summarized in a 2023 report, identifies several key advantages of thorium-based fuel cycles. First is abundance: the Earth’s upper crust contains approximately 10.5 parts per million (ppm) of thorium, compared to roughly 3 ppm of uranium. Countries like India — which has the world’s largest thorium reserves — stand to benefit enormously from a thorium-based nuclear economy. Second, thorium-fueled reactors produce significantly less long-lived nuclear waste. They generate fewer long-lived minor actinides than plutonium-based fuels, meaning the waste decays to safe background radiation levels in hundreds rather than thousands of years. Third, thorium has inherently favorable safety properties, including a negative temperature coefficient of reactivity — meaning the reactor naturally slows down if it gets too hot, a passive safety feature that makes runaway meltdown scenarios extremely difficult.
China Takes the Lead
The most significant recent milestone in thorium research came in October 2024, when China’s Shanghai Institute of Applied Physics announced that its 2-megawatt thermal Thorium Molten Salt Reactor (TMSR-LF1) had successfully transmuted thorium fuel into uranium-233 for nuclear fission — a world first. The reactor reached full-power operation at 650°C in June 2024, and the subsequent detection of protactinium-233 in October confirmed the establishment of the complete thorium-to-uranium conversion chain. This achievement verified the technical feasibility of the thorium fuel cycle for the first time in a real reactor environment. China plans to complete a 100-megawatt thermal prototype by 2035, with commercial-scale application to follow.
Copenhagen Atomics and the European Push
In Denmark, Copenhagen Atomics is developing mass-manufacturable molten salt reactors that fit inside a standard 40-foot shipping container. Their thorium-capable reactor uses separated plutonium from spent nuclear fuel as the initial fissile load, transitioning to a thorium breeder cycle over time. In July 2024, the company announced its reactor is ready for a critical experiment at the Paul Scherrer Institute in Switzerland in 2026. European interest is also reflected in the 2024 SMR market data: approximately 27% of new SMR concept announcements in 2024 included thorium fuel options, and at least 18 MSR concepts globally reference thorium molten-salt fuel.
India’s Long-Term Bet
India has pursued thorium energy for decades, driven by its massive domestic reserves and relatively poor uranium supplies. The country is developing the Advanced Heavy Water Reactor (AHWR), a pressurized heavy water reactor designed to breed U-233 from thorium until the thorium fuel cycle becomes self-sustaining. India’s three-stage nuclear program explicitly envisions thorium powering the country’s long-term energy future.
Remaining Challenges
Despite its promise, thorium faces real hurdles. The thorium fuel cycle has not been demonstrated at commercial scale — almost all large-scale nuclear technology investment over the past 70 years has gone into uranium systems. The handling of U-233 is complicated by the presence of U-232, a contaminant that produces intense gamma radiation requiring specialized remote-handling facilities. Regulatory frameworks for thorium reactors are essentially non-existent, requiring development from scratch. Roughly 30% of pilot thorium reactor projects in 2023 faced delays due to regulatory licensing and fuel qualification issues.
Thorium is not a guaranteed revolution, but China’s 2024 milestone has demonstrated that the thorium fuel cycle is physically real. As the world seeks both energy security and cleaner nuclear waste profiles, this long-overlooked metal may finally find its moment.