What are thorium and molten-salt reactors, and do they live up to the hype?

Thorium and molten-salt reactors are old ideas being tried again, and neither is yet in widespread commercial use. Thorium is a different nuclear fuel: it is a fertile element that is turned into uranium-233 inside a reactor, and the uranium-233 then fissions. A molten-salt reactor is a reactor in which the fuel, the coolant or both is a mixture of molten salt with fissile material. The two ideas are often paired, but they are separate.

The history is older than most people think. Glenn Seaborg discovered in 1940 that thorium can breed uranium-233. In the 1960s Oak Ridge National Laboratory in Tennessee ran the Molten-Salt Reactor Experiment, which operated critical for roughly 15,000 hours from 1965 to 1969, as a test of a thorium power plant. Thorium also has a real reactor record: the thorium core at Shippingport produced over 2.1 billion kilowatt hours of electricity between 1977 and 1982.

Supporters list potential advantages: thorium is much more abundant, its fuel cycle can produce less nuclear waste, and the uranium-233 it breeds is difficult to weaponize, though the United States tested several uranium-233 bombs between 1955 and 1968. For molten salt, the fuel is designed to drain into a containment vessel in an emergency and solidify. Challenges are real too: hot salts are corrosive, and some MSR designs could be converted to breeders that produce weapons-grade material.

The 1950s to 1970s interest in thorium declined largely after large uranium deposits were found. In 2024, China's TMSR-LF1 became the first operational molten-salt reactor to use thorium fuel.