Alvin Weinberg first came to Oak Ridge in 1945, just after its laboratories had been built in the northern Tennessee hills to make weapons-grade uranium and plutonium. A veteran of the Manhattan Project, Weinberg became director of the rapidly growing national lab in 1955 and held the position until 1973. He was a pioneering nuclear physicist and a philosopher of nuclear power who used the phrase “Faustian bargain” to describe the tension between industrialized society’s thirst for abundant energy and the extreme vigilance needed to keep nuclear power safe. To make this energy source both clean and extremely cheap, he believed, the link between nuclear power and nuclear weapons would have to be severed. And the way to break that link was the thorium molten-salt reactor.
Under Weinberg’s leadership, a team of enthusiastic young chemists, physicists, and engineers operated a small, experimental molten-salt reactor from 1965 to 1969. That reactor at Oak Ridge ran on uranium; Weinberg’s eventual goal was to build one that would run exclusively on thorium, which, unlike uranium, cannot easily be made into a bomb. But the molten-salt experiment was abandoned in the early 1970s. One big reason was that Weinberg managed to alienate his superiors by warning of the dangers of conventional nuclear power at a time when dozens of such reactors were already under construction or in the planning stages.
By the end of the century, the U.S. had built 104 nuclear reactors, but construction of new ones had all but come to a halt, and the technology remained stuck in the 1970s. Because conventional reactors require huge, costly containment vessels that can blow up in extreme conditions, and because they use extensive external cooling systems to make sure the solid-fuel core doesn’t overheat and cause a runaway reaction leading to a meltdown, they are hugely expensive. Two new reactors being built now in Georgia could cost $21 billion, 50 percent over the original estimate of $14 billion. All that for 40-year-old technology.
Today, though, as climate change accelerates and government officials and scientists seek a nuclear technology without the expensive problems that have stalled the conventional version, molten salt is enjoying a renaissance. Companies such as Terrestrial Energy, Transatomic Power, Moltex, and Flibe Energy are vying to develop new molten-salt reactors. Research programs on various forms of the technology are under way at universities and institutes in Japan, France, Russia, and the United States, in addition to the one at the Shanghai Institute. Besides the work going into developing solid-fuel reactors that are cooled by molten salt (like the one I toured virtually in Shanghai), there are even more radical designs that also use radioactive materials dissolved in molten salt as the fuel (as Weinberg’s experiment did).
Like all nuclear plants, molten-salt reactors excite atoms in a radioactive material to create a controlled chain reaction. The reaction unleashes heat that boils water, creating steam that drives a turbine to generate electricity. Solid-fuel reactors cooled with molten salt can run at higher temperatures than conventional reactors, making them more efficient, and they operate at atmospheric pressures—meaning they do not require expensive vessels of the sort that ruptured at Chernobyl. Molten-salt reactors that use liquid fuel have an even more attractive advantage: when the temperature in the core reaches a certain threshold, the liquid expands, which slows the nuclear reactions and lets the core cool. To take advantage of this property, the reactor is built like a bathtub, with a drain plug in the bottom; if the temperature in the core gets too high, the plug melts and the fuel drains into a shielded tank, typically underground, where it is stored safely as it cools. These reactors should be able to tap more of the energy available in radioactive material than conventional ones do. That means they should dramatically reduce the amount of nuclear waste that must be handled and stored.
Because they don’t require huge containment structures and need less fuel to produce the same amount of electricity, these reactors are more compact than today’s nuclear plants. They could be mass-produced, in factories, and combined in arrays to form larger power plants.
All of that should make them cheaper to build. Unlike wind and solar, which have gotten far less expensive over time, nuclear plants have become much more so. According to the U.S. Energy Information Administration, the inflation-adjusted cost of building a nuclear plant rose from $1,500 per kilowatt of capacity in the early 1960s to more than $4,000 a kilowatt by the mid-1970s. In its latest calculation, in 2013, the EIA found that the figure had risen to more than $5,500—more expensive than a solar power plant or onshore wind farm, and far more than a natural-gas plant. That up-front cost is amplified by the large size of the reactors; at the average cited by the EIA, a one-gigawatt plant would cost $5.5 billion, a risky investment for any company.
Those up-front costs are balanced by the fact that nuclear plants are relatively cheap to operate: at new plants the levelized cost of electricity, which measures the cost of power generated over the lifetime of the plant, is $95 per megawatt-hour, according to the EIA—comparable to the cost of electricity from coal-fired plants, and less than solar power ($125 a megawatt-hour). Still, natural-gas plants are far cheaper to build, and the cost of the electricity they produce ($75 a megawatt-hour, according to the EIA) is also lower. Tightening regulations on carbon emissions makes nuclear more attractive, but lowering the cost of construction is critical to the future of zero-carbon nuclear power.
