The Passion of Alvin Weinberg

The Humanitarian Behind China's Great Thorium Push

To read a translation of this article in French, click here.

The 59-year-old physicist was in something of a panic. The earth was getting hotter, and nobody in Washington seemed to care. Nuclear power — the only realistic way to produce a lot of electricity with few carbon emissions — was the solution. But rising costs for nuclear power and the power of the coal lobby appeared to be trumping environmental concerns, and rationality itself.

He started writing articles. The first he published in Science. It was called “Global Effects of Man’s Production of Energy.” Next, he co-authored an article evaluating what would happen if the U.S. moved away from nuclear. “Continued energy demands during the first few decades of the next century will push atmospheric carbon dioxide concentrations to levels which warrant serious concern, even for the low energy growth case.”

The problem was time. “The inertial effect in energy supply systems makes it clear that decisions made now on the nuclear / nonnuclear issue,” the man wrote, “will have an impact reaching many years into the future.” In other words, future generations depend on the decisions we make today about energy.

The physicist went to the Capitol, seeking supporters. “I went from office to office in Washington, curves of the carbon dioxide buildup in hand,” the man recalled. “I reminded them that nuclear energy was on the verge of dying. Something must be done. I almost screamed.”

The year was 1974, and the man’s name was Dr. Alvin Weinberg. A veteran of the Manhattan Project and the director of Oak Ridge National Laboratory, Weinberg created the prototype for a new kind of nuclear energy source, one that can’t melt down nor make useful weapons.

Where the vast majority of today’s nuclear reactors are cooled by plain old water, Weinberg invented a radically new reactor cooled by molten salt. Loss of coolant was behind the meltdowns at Three Mile Island and Fukushima. By contrast, Weinberg’s reactor could not melt down because the fuel was already melted and dissolved in the molten salt coolant itself.

If you want to understand why nuclear energy from thorium and cooled by molten salt has aroused the passion of American scientists and engineers, as well as the Chinese government, which recently invested $350 million in a new molten-salt project, then you have to understand the life and times of Alvin Weinberg.


Alvin Weinberg was born in Chicago in 1915 and earned his PhD in physics in 1939 from the University of Chicago. His master’s thesis dealt with the infrared absorption spectrum of CO2, presaging his later efforts to warn of global warming. At the University Chicago Metallurgical Laboratory he rubbed shoulders with physicists Edward Teller, Leo Szilard, and Nobel-prize-winners Arthur Compton, Eugene Wigner, and Enrico Fermi. Soon after he was working to help build the Bomb. In a 1944 memo he advanced the idea of harnessing nuclear energy for power, “…it may be possible to run such a system under pressure and obtain high-pressure steam which could be used for power production.”

In 1945, after the war, Weinberg went to work for Oak Ridge National Laboratories. There he persuaded Navy Admiral Hyman Rickover that a water-cooled reactor would work better on submarines — an achievement he was ambivalent about since it led to the use of water as coolant for civilian nuclear power reactors. “Thus was born the pressurized-water reactor, not as a commercial power plant, and not because it was cheap or inherently safer than other reactors, but rather because it was compact and simple and lent itself to naval propulsion,” he wrote wistfully.

The Air Force tasked him with building a nuclear-powered airplane. Powering a jet engine requires 860°C heat — far higher than the 315°C temperature achieved by water-cooled reactors. Weinberg’s team hit on the idea of a molten mixture of zirconium and sodium fluorides into which they put the uranium fuel. The stable fluoride salts did not corrode the stainless steel container. And because the salt would stay liquid at atmospheric pressure even at 1400°C, there could be no radioactivity release from overheating. 

The experiment worked. In 1954 this Aircraft Reactor Experiment produced 2.5 MW of thermal power at red-hot 860°C for 100 hours. It demonstrated intrinsic reactivity stability, automatically adjusting power with no control rods, as the heat exchanger airflow varied. But in the end it made more sense for power generation than for powering aircrafts (which to this day are powered by kerosene).

In 1955, at the age of 40, Weinberg became the director of Oak Ridge. By 1966, Weinberg’s Oak Ridge team had a prototype of uranium dissolved in the molten fluoride salts of lithium and beryllium, which ran until 1969.

Weinberg was thrilled. Such a reactor could provide the world with limitless energy and allow it to protect the environment. It could create electricity for the poor and freshwater from seawater. And if thorium rather than uranium were used, we would never run out of fuel, as the element is abundant in the Earth’s surface.


Weinberg was more focused than his colleagues on safety and was dismayed that reactors based on a design made for submarines had achieved dominance over the market. “The boiler band wagon has so much pressure that everyone climbed on it, pell mell,” he remarked later.

In 1959 he established the journal Nuclear Safety, and he put one hundred scientists and engineers at Oak Ridge on safety research. As nuclear reactors became larger, Weinberg’s Lab expressed concerns that in a loss-of-cooling accident — like the kind behind the nuclear accidents at TMI and Fukushima — residual afterheat, not a continuing chain reaction, might breach all three barriers.

Throughout the early ‘60s, Weinberg and his colleagues conducted a series of tests that surfaced safety flaws in the light-water reactor design. Superior safety was very important to Weinberg: to him, a molten salt reactor that used thorium fuel would provide immense benefits over light-water designs. As a coolant, atmospheric-pressure molten salt withstands much higher temperatures and reduces mechanical stress upon the reactor vessel. As a fuel, thorium cannot be used to build useful weapons; in a reactor it can breed new uranium fuel that is consumed to produce energy.

Weinberg’s innovations extended beyond molten-salt reactors. Oak Ridge’s safety work influenced the creation of the high-temperature gas-cooled pebble-bed reactor operating at Tsinghua University in China. And the new nuclear plants being built in Georgia incorporate passive safety features. The Westinghouse AP1000 overhead water reservoir can cool a powerless reactor for three days after shutdown. The B&W mPower continues passive cooling for three days on battery power. And the smaller NuScale reactor, funded by the Department of Energy, continues with air cooling indefinitely after its water reservoir boils away.

But Weinberg’s obsession with safety rubbed some of his colleagues the wrong way. The chair of the Joint Committee on Atomic Energy in 1970 was outraged at Weinberg’s efforts with Senators Howard Baker and Edmund Muskie to establish a National Environmental Laboratory at Oak Ridge. Holifield “didn’t want nuclear labs tainted with the environmentalist brush,” Weinberg recalled. Holifield told him, “Alvin, if you are concerned about the safety of reactors, then I think it may be time for you to leave nuclear energy.” Weinberg was fired shortly thereafter. Six years later, the Three Mile Island meltdown occurred.


In the fall of 2013, four of the world’s leading climate scientists including former NASA scientist James Hansen sent an open letter to environmentalists, asking that they reverse their opposition to nuclear power in order to save the climate. The letter was treated as something of a novelty in the media. Environmental scientists — for nuclear power? What a strange thing.

And yet there was Dr. Weinberg, one of America’s most respected scientists, making the climate case for nuclear power nearly 40 years before the open letter and a decade and half before Hansen famously told reporters that his scientific colleagues should stop “waffling” and acknowledge that humans were changing the climate.

Climate and energy for Weinberg and many after him are two sides of the same coin. After a stint as Director of the US Office of Energy Research and Development in 1974 Weinberg had managed to found the Institute for Energy Analysis (IEA) at Oak Ridge Associated Universities, concerned with the future of energy. IEA invented today’s energy-return-on-investment (EROI) analysis concept.

In 1976 at IEA Weinberg predicted that “…atmospheric concentration of 375-390 ppm may well be a threshold range at which climate change from CO2 effects will be separable from natural climate fluctuations … The consequences of an increase of this magnitude in atmospheric CO2 make it prudent to proceed cautiously in the large-scale use of fossil fuels."

Weinberg’s view of energy stood in marked contrast to the views held by antinuclear greens who argued that poor people across the globe wouldn’t benefit from cheap, reliable electricity. “Giving society cheap, abundant energy,” Stanford professor Paul Ehrlich famously wrote in 1975, “would be the equivalent of giving an idiot child a machine gun.” Weinberg fiercely argued the opposite: low-energy societies were far less free and “probably suffer from more pollution of air and water and urban environments than do high-energy societies.” More, not less, energy was central to well-being.

A new generation of engineers concerned about climate change are rediscovering Weinberg and his design. Bill Gates’s company TerraPower is investigating molten salt reactors. MIT engineer Leslie Dewan co-founded Transatomic Power, which uses a MSR design. And former NASA employee Kirk Sorensen published the original Oak Ridge R&D documents on the Internet, and started Flibe Energy.

The technology has aroused global interest. Former antinuke environmentalist Baroness Bryony Worthington helped found the London-based Weinberg Foundation, to “re-catalyze the research, development and deployment of MSRs first designed, built and proven by Alvin Weinberg … to combat climate change.” And an article I wrote in American Scientist in 2012 sparked the Chinese Academy of Science’s undertaking its $350 million development project announced last year.

With the open letter from climate scientists, a growing number of environmentalists calling for nuclear power, and a growing number of philanthropists like Bill Gates and Paul Allen investing in next generation nuclear, the altruism that originally motivated nuclear scientists and engineers is finally coming to redefine nuclear power. “What made him unique,” said Alexander Zucker, a physics professor and colleague of Weinberg’s, “was his profound concern for the welfare of man. He never stopped thinking about it.”


Robert Hargraves is the author of THORIUM: energy cheaper than coal. With coauthor Ralph Moir, he has written articles for the American Physical Society Forum on Physics and Society: Liquid Fuel Nuclear Reactors (Jan 2011) and American Scientist: Liquid Fluoride Thorium Reactors (July 2010). His presentation “Aim High” about the technology and social benefits of the liquid fluoride thorium reactor has been presented to audiences at Dartmouth ILEAD, Thayer School of Engineering, Brown University, Columbia Earth Institute, and beyond. For a more detailed history of Weinberg's work at the ORNL, read an accompanying piece by Hargraves at Atomic Insights.