Thorium-based nuclear power

Based on Wikipedia: Thorium-based nuclear power

The Road Not Taken

In 1973, the United States government made a choice that would shape global energy policy for half a century. They picked uranium. They buried thorium.

The decision wasn't obviously wrong at the time—uranium-fueled reactors seemed more efficient, the research was further along, and the liquid metal fast breeder reactor had a larger breeding rate than its thorium competitor. But as physicists Ralph Moir and Edward Teller would later observe, abandoning thorium research entirely, rather than keeping it as a backup option, was "an excusable mistake." Excusable, perhaps. Consequential, certainly.

Today, most nuclear scientists have never seriously studied thorium power. According to Chemical and Engineering News, "it's possible to have a Ph.D. in nuclear reactor technology and not know about thorium energy." Nuclear physicist Victor Stenger admitted in 2012 that learning about thorium's potential "came as a surprise"—this from a man who had spent his career studying atomic physics. The element that could have changed everything became an obscure footnote.

But here's the thing about obscure footnotes: sometimes they contain the most important stories.

What Thorium Actually Is

Thorium sits at atomic number ninety on the periodic table, a silvery-gray metal named after Thor, the Norse god of thunder. It's about three times more abundant than uranium in Earth's crust—roughly as common as lead and gallium. The United States alone has enough thorium deposits to power the country at current energy consumption levels for over a thousand years. Much of it sits buried as a byproduct of rare earth mining, waiting.

But thorium itself isn't directly fissile. You can't just pack thorium atoms together and expect them to start splitting apart in a chain reaction the way enriched uranium does. Thorium is what nuclear physicists call "fertile"—it can become fissile material, but it needs help getting there.

The process works like this: when thorium-232, the naturally occurring isotope, absorbs a neutron, it transforms through a series of radioactive decays into uranium-233. And uranium-233 is intensely fissile. It splits readily, releasing energy and more neutrons, which can then convert more thorium into more uranium-233. A properly designed reactor can breed more fuel than it consumes—a self-sustaining cycle that needs only thorium and an initial spark to keep running indefinitely.

Glenn Seaborg, the legendary nuclear chemist who would later win a Nobel Prize and have an element named after him, discovered this conversion process in 1940. He bombarded thorium with neutrons in a cyclotron and watched it transmute into a new form of uranium. During the Manhattan Project, Seaborg quickly grasped uranium-233's potential as a fissile material. But the weapons program moved forward with plutonium instead—Seaborg had discovered that element too, just months later—and thorium was set aside for later consideration.

"Later" would prove to be a very long time.

The Molten Salt Dream

Oak Ridge National Laboratory in Tennessee became the crucible of thorium research. In the 1960s, a team led by nuclear engineer Alvin Weinberg built something genuinely revolutionary: the Molten-Salt Reactor Experiment, or MSRE.

Traditional nuclear reactors use solid fuel rods cooled by water. The MSRE dissolved its uranium fuel directly into a bath of molten fluoride salts heated to around seven hundred degrees Celsius. This liquid fuel circulated through the reactor core, eliminating many of the mechanical complexities and failure modes of solid-fuel designs.

The concept had an elegant safety feature built into its physics. At the bottom of the reactor sat a small plug made of frozen salt, kept solid by a cooling fan. If power failed, the fan stopped, the plug melted, and the entire fuel mixture drained by gravity into underground tanks designed for safe storage. No operators required. No emergency protocols to follow. The reactor simply shut itself down.

The MSRE operated successfully from 1965 to 1969, running critical for roughly fifteen thousand hours. In 1968, Glenn Seaborg—by then chairman of the Atomic Energy Commission—publicly announced that a uranium-233-based reactor had been successfully developed and tested. The technology worked.

Then Milton Shaw killed it.

The Bureaucratic Assassination

Shaw ran the Reactor Development and Testing Division at the Atomic Energy Commission. He was a protégé of Admiral Hyman Rickover, the father of the nuclear Navy, and he had strong opinions about which reactor designs deserved government support. Molten salt wasn't one of them.

Shaw pressured Weinberg's team relentlessly. He favored the liquid metal fast breeder reactor, a competing design that used sodium coolant and the uranium-plutonium fuel cycle rather than thorium. By Shaw's metrics, the fast breeder looked better on paper—it had a higher breeding ratio, meaning it produced more excess fuel per cycle.

In December 1969, the MSRE shut down. By 1973, thorium research in America was essentially over.

Weinberg himself reflected on this period with evident frustration. He recalled a confrontation with Congressman Chet Holifield:

"Chet Holifield was clearly exasperated with me, and he finally blurted out, 'Alvin, if you are concerned about the safety of reactors, then I think it may be time for you to leave nuclear energy.' I was speechless. But it was apparent to me that my style, my attitude, and my perception of the future were no longer in tune with the powers within the AEC."

A man was told, essentially, that caring too much about safety made him unsuitable for the nuclear industry. He left Oak Ridge not long after.

Science writer Richard Martin initially believed Weinberg lost his job because he championed thorium reactors that couldn't easily produce weapons-grade material. The military wanted plutonium; thorium gave you uranium-233 contaminated with uranium-232, which made handling the material dangerous and weapons design difficult. It seemed like a tidy explanation: cold warriors sacrificing safer civilian power on the altar of the nuclear arsenal.

But Martin later concluded this theory was too simple. The evidence that Shaw and the Atomic Energy Commission deliberately killed molten salt technology because it was bad for weapons production is thin. The real story is more mundane and arguably more tragic: institutional momentum, competing technical constituencies, limited budgets, and the simple fact that uranium technology was further along. Nobody made a conspiracy. They just made a choice.

The liquid metal fast breeder became America's chosen path forward. The flagship project, the Clinch River Breeder Reactor, launched in 1970 with tremendous government backing. It encountered massive cost overruns, technical challenges, and political opposition. Congress finally killed it in 1983, having spent over a billion dollars with nothing to show for it.

The Light Water Breeder

Not everyone gave up on thorium. Admiral Rickover, ironically the mentor to Shaw, had pushed for a thorium breeder project since 1963. While Shaw was shutting down molten salt research, Rickover was organizing something different: a light-water breeder reactor that could use existing reactor infrastructure.

The project converted the Shippingport Atomic Power Station in Pennsylvania—America's first peacetime nuclear power plant—into a thorium demonstration reactor. It reached criticality on August 26, 1977, and operated successfully until 1982, generating over 2.1 billion kilowatt-hours of electricity.

More importantly, it proved the breeding concept worked. The reactor converted enough thorium-232 into uranium-233 to achieve a breeding ratio of 1.014—meaning it produced slightly more fissile material than it consumed. This was lower than the fast breeders promised, but it was real, demonstrated technology operating at commercial power plant scale.

Sixty megawatts of electrical power, five years of successful operation, genuine fuel breeding. Proof of concept, delivered.

Almost nobody noticed.

The Case for Thorium

Why does any of this matter? Because thorium offers solutions to nearly every problem that haunts nuclear power.

Consider nuclear waste. A thorium reactor using molten salt technology could reduce high-level waste by up to two orders of magnitude compared to conventional uranium reactors. Moir and Teller estimated this could eliminate the need for large-scale, long-term storage entirely. Chinese scientists have claimed hazardous waste from thorium systems would be a thousand times less than from uranium. The radioactivity of thorium waste drops to safe levels after one to a few hundred years, rather than the tens of thousands of years required for conventional spent nuclear fuel.

Consider weapons proliferation. Thorium reactors produce plutonium at less than two percent the rate of standard uranium reactors, according to Alvin Radkowsky, who designed the world's first full-scale atomic electric power plant. The uranium-233 they do produce is contaminated with uranium-232, which emits intense gamma radiation that makes the material difficult to handle and tends to cause pre-detonation in weapons—the bomb goes off before it's supposed to, resulting in a fizzle rather than a full nuclear yield. The Atomic Energy Commission did successfully separate uranium-232 from uranium-233 at Rocky Flats, using multiple chemical isolation steps, but the process was complex and expensive. Newer laser isotope separation techniques might make it easier, but thorium remains a significantly harder path to nuclear weapons than uranium or plutonium.

Consider fuel supply. Almost all natural thorium is the useful isotope, thorium-232. Compare this to uranium: 99.3 percent is uranium-238, which requires conversion before use, and only 0.7 percent is the directly fissile uranium-235. Getting uranium ready for a conventional reactor requires expensive enrichment facilities. Thorium needs no enrichment at all.

Consider efficiency. Nobel laureate Carlo Rubbia of CERN—the European Organization for Nuclear Research—estimated that one ton of thorium can produce as much energy as two hundred tons of uranium or 3.5 million tons of coal. The numbers are almost absurd.

Consider mining. Thorium ore, primarily the mineral monazite, typically contains higher concentrations of the desired element than uranium ores contain uranium. Thorium mining can be done in open pits requiring no ventilation, unlike underground uranium mines where radon gas accumulates to dangerous levels.

Consider safety. A properly designed molten salt reactor cannot melt down. There's nothing to melt—the fuel is already liquid. The freeze plug mechanism provides passive safety that requires no human intervention and no external power.

In 2011, scientists at the Georgia Institute of Technology assessed thorium-based power as "a 1000+ year solution or a quality low-carbon bridge to truly sustainable energy sources solving a huge portion of mankind's negative environmental impact." Kirk Sorensen, a former NASA scientist who became the most prominent thorium advocate in America, believes that if the United States had not discontinued research in 1974, the country "could have probably achieved energy independence by around 2000."

The Complications

This all sounds too good to be true, which should trigger appropriate skepticism. No technology is without drawbacks, and thorium is no exception.

The production of activation products and fission products—the radioactive materials created when neutrons interact with reactor components or when atoms split—is broadly similar between thorium and uranium fuel cycles. Thorium doesn't magically eliminate all radioactive byproducts; it changes their composition and timing.

The technology requires significant development work. A 2012 report from the Bulletin of the Atomic Scientists examined using thorium fuel with existing water-cooled reactors and concluded it would "require too great an investment and provide no clear" advantage over incremental improvements to uranium technology. The expertise, supply chains, regulatory frameworks, and operational experience that exist for uranium reactors would need to be rebuilt essentially from scratch for thorium systems.

Molten salt technology specifically presents materials challenges. The fluoride salts are corrosive at high temperatures, requiring specialized alloys for reactor vessels and piping. The MSRE operated for only four years; whether the materials would hold up over the forty-to-sixty-year lifespan expected of commercial power plants remains to be proven at scale.

Licensing a fundamentally new reactor type through the Nuclear Regulatory Commission would be an expensive, years-long process. The NRC's entire framework was built around light-water uranium reactors; thorium molten salt designs don't fit neatly into existing categories.

And there's the simple problem of institutional momentum. Uranium works. Reactors running on uranium have operated safely for decades, generating about ten percent of global electricity. The devil you know has considerable appeal when the alternative requires billions of dollars in research and development with uncertain outcomes.

The International Picture

Other countries have experimented with thorium over the decades. Germany built the AVR pebble-bed reactor, which used mixed uranium-thorium fuel and operated from 1967 to 1988. The Germans followed it with the THTR-300, a three-hundred-megawatt commercial thorium reactor commissioned in 1985. Both projects were plagued with design problems. The THTR-300 shut down in 1989 after only four years of operation.

India, with vast thorium reserves and limited uranium deposits, has long planned for a thorium-based nuclear future as the third stage of its nuclear power program. Progress has been slow.

China announced an ambitious program to develop thorium molten salt reactors in 2011, drawing on design documents from the Oak Ridge MSRE that have been available since declassification. By 2022, they had begun testing research reactors. The Shanghai Institute of Applied Physics has stated goals of commercial thorium power by the 2030s.

Between 1999 and 2022, the number of operational thorium reactors worldwide rose from zero to a handful of research facilities, with commercial plans moving forward in several nations. The technology isn't dead. It's waking up.

What Might Have Been

In 2004, Moir and Teller—the latter being Edward Teller, father of the hydrogen bomb—wrote a paper arguing that thorium nuclear research should be restarted after its three-decade shutdown. They proposed building a small prototype plant and estimated the cost at "well under $1 billion with operation costs likely on the order of $100 million per year." Within a decade, they believed, a large-scale nuclear power plan usable by many countries could be established.

The paper had little immediate impact. The nuclear industry continued on its uranium path, building large light-water reactors when it built anything at all.

In May 2022, Senator Tommy Tuberville introduced the Thorium Energy Security Act, a bill to preserve and store uranium-233 for future thorium molten salt reactor development. Kirk Sorensen had been urging such legislation since 2006. The bill was not adopted by Congress.

There's a melancholy quality to the thorium story. The technology works—demonstrated at Shippingport, proven at Oak Ridge. The advantages are substantial and well-documented. The path forward is understood, at least in broad strokes. What's missing is simply the will to walk it.

Nuclear power broadly remains controversial. Environmental groups who might otherwise champion zero-carbon electricity generation often oppose it reflexively. The memory of Chernobyl, Three Mile Island, and Fukushima lingers. The waste problem, unsolved for uranium reactors, provides easy ammunition for critics even though thorium could largely solve it.

Perhaps thorium's moment will come as climate change forces harder choices about energy. Perhaps China will demonstrate commercial success and other nations will follow. Perhaps some breakthrough in materials science or regulatory reform will finally clear the path.

Or perhaps thorium will remain what it has been for fifty years: the road not taken, the proof of concept that proved nothing to the people making decisions, the technology that works but somehow never quite works out.

Richard Martin, who wrote a book called SuperFuel about thorium's potential, summarized the case simply: "Thorium could provide a clean and effectively limitless source of power while allaying all public concern—weapons proliferation, radioactive pollution, toxic waste, and fuel that is both costly and complicated to process."

Could. The conditional tense carries a lot of weight in the thorium story. It always has.

Closing the Loop

There's one more potential use for thorium that deserves mention, though it sounds almost too convenient: the thorium fuel cycle could help dispose of existing weapons-grade plutonium.

The world's nuclear powers possess stockpiles of plutonium from decommissioned weapons—material that poses proliferation risks simply by existing. A thorium reactor can be designed to "burn" this plutonium, converting it into less dangerous isotopes while generating electricity. The waste from one nuclear era becomes the fuel for a cleaner one.

It's an elegant solution in an industry that desperately needs them. Whether anyone will actually implement it remains, as always, to be seen.

Glenn Seaborg bombarded thorium with neutrons in 1940 and created a new element. More than eighty years later, we're still arguing about what to do with his discovery. The physics hasn't changed. The politics moves slower than radioactive decay.

Thorium waits. It's been waiting a long time. It's patient like that—stable isotope, long half-life, abundant and unassuming in the Earth's crust. Named for a god of thunder, but making no noise at all.