Humanity is in a serious pinch for energy.The world population may balloon to 9 billion people by 2040, up from 7.36 billion in 2016, and researchers believe this will translate to a 48% jump in energy consumption.Fossil fuels could slake the world's thirst for energy, but burning more would exacerbate climate change and threaten millions. And it'd be temporary, since known reserves are expected to run out within a century or two.Meanwhile, renewable energy like wind and solar, though key parts of a solution, are no silver bullets ' especially if the world is to meet a 2050 deadline set by the Paris Agreement."You've got to be able to generate energy reliably. You've got to be able to generate energy on demand. And that's what wind and solar can't do, and will never be able to do," Kirk Sorensen, the CTO of nuclear energy startup Flibe Energy, told Business Insider in an episode of our podcast Codebreaker, produced with Marketplace.Nuclear reactors fit the bill: They're dense, reliable, emit no carbon, and ' contrary to popular belief ' are among the safest energy sources on Earth. They currently supply 20% of America's energy, but this share may decline by 50% through 2040 as companies take decades-old reactors offline, according to a July 2016 report released by Idaho National Laboratory.Fortunately, a powerful yet relatively unknown solution may have started with a Cold War-era airplane: "The Crusader" NB-36H jet bomber, which flew over two US states with a nuclear reactor in its belly.The effort was part of the Aircraft Nuclear Propulsion (ANP) program. Although it was ultimately canceled, ANP spawned the development of a radical new type of power plant, called the molten-salt reactor.Today, engineers like Sorensen are trying to revive the molten-salt reactor, which the US abandoned in the early 1970s, and fuel it with a practically infinite source of carbon-free energy: thorium.A push for nuclear-powered flightThe US government in 1946 launched ANP as an effort to develop a nuclear-powered jet bomber.It was an extreme means to a practical (and deadly) end: Fly at least 15,000 miles without refueling to give the plane a fearsome range of attack, according to Scientific American.Physicist Alvin Weinberg, who invented the most well-known type of nuclear reactor in 1945 ' the light-water reactor (LWR) ' rose to the occasion and began working up a solution as the director of Oak Ridge National Laboratory (ORNL) in Tennessee.But Weinberg didn't want to put a LWR into an airplane.LWRs, which now provide 100% of America's nuclear energy, rely on solid nuclear fuel, typically one that contains uranium-235. If this "fissile" isotope of uranium is struck by a flying neutron, it can split, release gobs of energy, and shoot out more neutrons. This process is called fission. If there's enough fuel in one place, there will be enough neutrons flying around to self-sustain a fission chain-reaction.The problem is that solid fuel is terribly inefficient. In fact, LWRs fission or "burn up" just a few percent of their fuel before it needs to be replaced. This is because waste products slowly accumulate in the fuel, absorb more and more neutrons, and poison the process of fission.So Weinberg instead chose to develop an idea he'd heard during the Manhattan Project, which had since become "kind of an obsession" for him: a reactor that fissioned its fuel in a fluid of molten salt.Molten-salt reactors are unlike any commercial nuclear power plants that exist today. Instead of using solid pellets of nuclear fuel, they dissolve nuclear fuel in a stable, blazing-hot fluid.The fluid can dramatically increase the efficiency of nuclear fission by making it easy to remove fission products. This helps it burn up almost all the nuclear fuel and boosts energy output. Such reactors are essentially meltdown-proof, too, since cooling down the salt solidifies and expands it, slowing fission to a crawl.Weinberg and others knew such efficiency might allow engineers to shrink a reactor to fit inside an airplane. So he and his team at ORNL built a small molten-salt reactor as part of an offshoot program, called the Aircraft Reactor Experiment (ARE).The birth of the molten-salt reactorBy 1954, Weinberg and his team had built a working prototype: a 2.5-megawatt power plant that used a small amount of uranium-235 dissolved in molten salt made of fluorine, sodium, and zirconium.It was the first working molten-salt reactor ever built.Inside the ARE's molten-salt fuel, uranium powered a fission chain-reaction. The atomic heat warmed up an adjacent loop of coolant (made of molten sodium) from 1,200 to 1,500 degrees Fahrenheit. Incoming air cooled the sodium, and pumps returned it to the fluid-fueled reactor core for reheating."The Air Force was delighted by the aircraft reactor experiment," Weinberg wrote in his 1994 autobiography, "The First Nuclear Era," since this was hot enough to drive jet engines.The Air Force immediately began retrofitting a B-36 jet bomber ("The Crusader") to carry a nuclear reactor. It also funded ORNL's follow-up molten-salt reactor, called the Aircraft Reactor Test (ART).But the Air Force canceled ART in 1957 to cut ballooning costs ' and instead flew a different reactor it had funded in tandem.The reactor, which was not a molten-salt reactor but a light-water reactor, was never connected to the plane's engines, since "The Crusader" was only intended to test radiation shielding. (The Air Force planned to later incorporate it into a purpose-built nuclear bomber called the WS-125.)"The Crusader" flew 47 demonstration flights from 1955 through 1957 over New Mexico and Texas. It weighed nearly 18 tons fully-loaded and logged 218 hours of flight, of which the reactor ran for nearly 90 hours. And the crew lived.But strapped with high costs of about $7 billion and faced with other priorities, including the creation of intercontinental ballistic missiles and the space race, President John F. Kennedy canceled all Aircraft Nuclear Propulsion projects in 1961.Still, by that time, Weinberg had squeezed in several years' worth of research and $1 billion on molten-salt reactors.By 1960, with the government funding the development of commercial nuclear power plants, Weinberg poured all of that knowledge into the Molten-Salt Reactor Experiment (MSRE). The MSRE went critical in 1965, produced power for thousands of hours through 1969, and was hailed a success.The next stage was to develop MSRE into something called a breeder reactor.The death of Weinberg's dreamBreeder reactors can create more fuel than they burn through fission, thanks to a process called neutron capture: a "fertile" atom will absorb a neutron from fission, then decay into (and "breed") the fuel. The fuel can then be fissioned to breed more fuel, and so on.As long as fertile material is around, this can go on indefinitely. But breeding only works with a few radioactive isotopes, since it requires so many neutrons to work.One is uranium-238, a fertile isotope which makes up more than 99% of natural uranium ore. It can be bred into plutonium-239, a fissile weapons material. (This is how the US made most of its nuclear arsenal.)Another is thorium, which can be bred into uranium-233 ' another fissile fuel, yet one that is very difficult to handle or make into bomb material."Right now we extract thorium inadvertently as a function of rare-earth mining," Sorensen said. "We go looking for neodymium, and other rare-earths ' ironically for magnets for things like wind turbines ' and we bring up quite a bit of thorium in the process, which right now is treated like a waste."But it's no waste.According to "SuperFuel," a 2013 book on thorium energy's demise and promise by journalist and author Richard Martin:"Thorium is around four times as abundant as uranium and about as common as lead. Pick up a handful of soil at your local park or ball-field; it contains about 12 parts per million of thorium. The United States has about 440,000 tons of thorium reserves, according to the Nuclear Energy Agency; Australia has the world's largest resources, at about 539,000 tons. Like uranium and plutonium, thorium makes a dense and highly efficient energy source: scoop up a few ounces of sand on certain beaches on the coast of India, it's said, and you'll have enough thorium to power Mumbai for a year."Inside a molten-salt breeder reactor, which burns up almost all of its fuel and generates hundreds of times less waste than LWRs, Weinberg estimated that thorium could meet the world's energy needs for billions of years.But the government canceled the MSRE in 1972, Weinberg retired soon after, and he never revived his research.The US ultimately favored the LWR design for its nuclear reactors because more of them had been built, the military liked the design for nuclear submarines, and they could also make nuclear weapons material.A push for next-generation nuclear powerIn the 2000s, Sorensen and others (including China and India) began rediscovering the idea of thorium-fueled molten-salt reactors.Sorensen is one of a few entrepreneurs who is trying to revive, modernize, and license his own version of the technology, called the liquid fluoride thorium reactor (LFTR)."The technology is viable, the science has been demonstrated," Hans Gougar, a nuclear physicist at Idaho National Laboratory (INL), told Business Insider.But it's been rough. Developing nuclear power technology requires billions of dollars and is very slow, since it has to be proven safe at multiple stages before commercial-scale plants can be built ' and the LFTR is unlike anything in service today."Maneuvering the licensing process is a huge challenge. The regulatory framework is not currently streamlined to support these novel innovative technologies," Rita Baranwal, a materials engineer at INL, told Business Insider.The Department of Energy estimates it may take until 2040 or 2050 to license a full-scale and commercial molten-salt power plant. Meanwhile, America's aging yet vital nuclear power plants aren't getting any younger.That's why, this summer, INL tapped Baranwal to direct its new Gateway for Accelerating Innovation in Nuclear (GAIN) program, which is a technology accelerator and support system created for small-time nuclear entrepreneurs.However it gets built ' or whoever builds it ' Sorensen is convinced that thorium-fueled molten-salt reactors are the key to solving Earth's energy blues for good."This is something that's going to benefit their future tremendously, it's going to lead to a new age of human success," he said, speaking to the world at large. "And if they want that, they need to be talking to their elected officials and demanding it, in fact, and saying 'we want to see these things happen.' Because only a society that decides to embrace this kind of technology is going to ultimately realize its benefits."For more on molten salt reactors and solving climate change, listen to the "world building" episode of the Codebreaker podcast from Business Insider and Marketplace. Subscribe to the whole series on iTunes or wherever you get your podcasts. SEE ALSO:8 terrifying ways the world could actually endDON'T MISS:25 images of Earth that will bring you to your kneesJoin the conversation about this storyNOW WATCH: This Cold War-era technology could safely power the world for millions of years
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