But here's why we're not using it.
The lifeblood of modern civilisation is affordable, free-flowing energy. It gives us the power to heat our homes. Grow and refrigerate food. Purify water. Manufacture products. Perform organ transplants. Drive a car. Go to work. Or procrastinate from work by reading a story about the future of energy.
Today's cheap, bountiful supplies make it hard to see humanity's looming energy crisis, but it's possibly coming within our lifetimes.
Our numbers will grow from 7.36 billion people today to 9 billion in 2040, an increase of 22 percent. Rapidly developing nations, however, will supercharge global energy consumption at more than twice that rate.
Fossil fuels could quench the planet's deep thirst for energy, but they'd be a temporary fix at best. Known reserves may dry up within a century or two.
And burning up that carbon-based fuel would accelerate climate change, which is already on track to disrupt and jeopardise countless lives.
Meanwhile, renewable energy sources like wind and solar, though key parts of a solution, are not silver bullets - especially if the world is to meet a 2050 deadline set by the Paris Agreement.
Energy from fusion is promising, but it's not yet proved to work, let alone on a commercial and competitive scale.
Nuclear reactors, on the other hand, fit the bill: they're dense, reliable, emit no carbon, and - contrary to bitter popular sentiment - are among the safest energy sources on Earth.
Today, they supply about 20 percent of America's energy, though by the 2040s, this share may drop to 10 percent as companies shut down decades-old reactors, according to a July 2016 report released by Idaho National Laboratory (INL).
The good news is that a proven solution is at hand - if we want it badly enough.
Called a molten-salt reactor, the technology was conceived during the Cold War and forgoes solid nuclear fuel for a liquid one, which it can "burn" with far greater efficiency than any power technology in existence.
It also generates a small fraction of the radioactive waste that today's commercial reactors - which all rely on solid fuel - do. And, in theory, molten-salt reactors can never melt down.
"It's reliable, it's clean, it basically does everything fossil fuel does today," Kirk Sorensen, the chief technology officer of nuclear-energy startup Flibe Energy, told Business Insider.
Sorensen was speaking during an episode of Business Insider's podcast Codebreaker, which is produced with National Public Radio's 'Marketplace'.
"And it does a whole bunch of things it doesn't do today, like make energy without emitting carbon," he added, though the same could be said of any nuclear reactor technology.
What's more, feeding a molten-salt reactor a radioactive waste from mining, called thorium (which is three to four times more abundant than uranium), can 'breed' as much nuclear fuel as it burns up.
Manhattan Project scientist Alvin Weinberg calculated in 1959 that if we could somehow harvest all the thorium in the Earth's crust and use it in this way, we could power civilisation for tens of billions of years.
"The technology is viable, the science has been demonstrated," Hans Gougar, a nuclear engineer at INL, told Business Insider.
Demonstrated, because government scientists built two complementary prototypes during the 1950s and '60s.
They weren't good for making nuclear weapons, though, among other reasons, so bureaucrats pulled funding for the revolutionary energy technology. The last working molten-salt reactor shut down in 1969.
Today, entrepreneurs such as Sorensen are working tirelessly to revive and modernise the technology. So are foreign governments like India and China.
China now spends more than $US350 million a year developing its variation of the Cold War-era design.
The story of how we got here is neither short nor simple, but it explains why Sorensen and others are betting big on humanity's coming 'Thorium Age' - and why governments are stumbling at its dawn.
The argument for nuclear energy
Its brutalist architecture may not be sexy, but nuclear energy unlocks a truly incredible source of carbon-free fuel. Ounce per ounce, uranium provides roughly 16,000 times more energy than coal and creates millions of times less pollution.
The argument to support growth in nuclear energy is so clear to James Hansen, a seasoned climatologist and outspoken environmentalist, that he passionately advocates for the use and development of the technology.
"To solve the climate problem, policy must be based on facts and not on prejudice. The climate system cares about greenhouse gas emissions - not about whether energy comes from renewable power or abundant nuclear power," Hansen and three other well-known scientists - Ken Caldeira, Kerry Emanuel, and Tom Wigley - wrote in an editorial for The Guardian in 2015.
"Nuclear energy can power whole civilisations, and produce waste streams that are trivial compared to the waste produced by fossil fuel combustion," they wrote.
"Nuclear will make the difference between the world missing crucial climate targets or achieving them."
Climate science aside, the economics of nuclear energy are enough of a draw to make the technology worthwhile.
Today, the industry is already profitable, albeit well subsidised.
Still, if you level the energy playing field against other power sources by taking into account government subsidies and tax breaks, capital costs, fuel costs, and other factors that affect the price-per-megawatt-hour of a power plant, nuclear energy remains a financial win in the long run.
Nuclear power's 2016 levelised costs make it about twice as cheap as natural gas 'peaking' plants (which fire up to meet sudden peaks in energy demand). Nuclear also beats the overall cost of many coal-fired power plants.
And that's before you account for the extraordinary hidden costs of fossil fuels against public health and the environment, including particulate pollution (which kills tens of thousands of people a year) and exacerbating climate change.
Nuclear also wins financially against solar rooftops, many fuel-cell energy schemes, and some geothermal and bioenergy plants.
That isn't to say that current nuclear power plants are flawless. However, they're irrefutably amazing power sources, currently meeting one-fifth of the US's energy needs with just 61 power plants.
They're also incredibly reliable, always-on sources of baseload electricity, heat, and medically useful radioisotopes. Yet great titans fall hard, and the reasons why are key to the continued delay of the Thorium Age.
Why nuclear energy use is collapsing
While new reactors are planned or are coming online soon in the US, many have stalled and the industry has stagnated, with eight of the US's 99 decades-old reactors planned for shutdown by 2025.
What gives?
Subsidies
Flibe Energy's Sorensen partly blames aggressive government subsidisation of wind and solar energy, which leads to the problem of negative pricing.
"We've created rules that disturb the energy market substantially," Sorensen said.
"The first rule is that whenever wind and solar come online, we have to take the power. That's called grid priority. The second rules is, they're paid no matter how much power they make."
Sorensen characterised this as the "murder" of nuclear energy, since those plants can't be shut on and off quickly. He also said this is hurting the environment by causing companies to invest more heavily in gas plants (which can be ramped up and down quickly).
"These two put together create negative prices, and if you're a nuclear power-plant operator, and you're trying to obviously make money selling power to the grid and the prices go negative for large portions of the day, that's economically unviable," he said. "That's what's causing reactors to get shut down."
But other issues are kneecapping nuclear too.
Time and cost
Energy sources such as hydroelectric and wind are still cheaper than nuclear, and a fracking boom has fuelled investment in natural-gas-fired power plants.
As a result, nuclear is having a harder time finding a seat at the energy-pricing table.
Reactors also take many years and billions of dollars to permit, build, and licence for operation: They're exceedingly large and complex works of engineering (though you only need a high school diploma to operate one once they're finished).
Old age
The average US reactor is about 35 years old. They can run for decades with constant maintenance. The Oyster Creek nuclear generating station outside of New York City, for example, has operated since 1969.
But many are being eyed for shutdown, and once they're shut off, reactors can take more than a decade to decommission, demolish, and bury.
A dysfunctional uranium fuel cycle in the US has not helped, where just 3 to 6.5 percent of solid uranium fuel is burned up - and the remaining 93 to 97 percent is treated as radioactive waste and not reprocessed and recycled.
Fear
Then there is society's pervasive anxiety toward nuclear power, often amped to irrational levels. While events such as Three Mile Island, Chernobyl, and the Fukushima Daiichi disaster stand out in people's minds, the reality does not match up by a long shot.
"Nuclear radiation ticks all the boxes for increasing the fear factor," David Spiegelhalter, a statistician at Cambridge University, told New Scientist after the Fukushima disaster in 2011:
"It is invisible, an unknowable quantity. People don't feel in control of it, and they don't understand it. They feel it is imposed upon them and that it is unnatural. It has the dread quality of causing cancer and birth defects."
But as Spiegelhalter, Sorensen, and others have said, the actual safety record of nuclear power is remarkable.
Fukushima's reactor meltdowns killed no one, according to a 2013 World Health Organisation report. Even in "the two most affected locations of Fukushima prefecture", people in the first year would receive only two to three CT chest scans' worth of radiation exposure.
"Let me throw out other names you might not be familiar with: San Bruno. Banqiao Dam," Sorensen said, referring to the two accidents that killed eight (in a 2010 California gas-line explosion) and as many as 230,000 people (in a series of 1975 Chinese dam collapses), respectively.
"These are catastrophic incidents with hydropower and natural gas that really did result in large losses of human life," he said. "And yet the public doesn't have a terror of hydroelectric power or natural gas."
What does the data say about nuclear energy's safety?
Measuring immediate deaths against gigawatts of electrical power is a typical way to assess the safety of energy sources, and a 2010 analysis by the Organisation for Economic Cooperation and Development (OECD) used this.
But adding in incidental deaths that occur later, such as 9,000 estimated cancer fatalities from Chernobyl (which the OECD left out), does change the numbers, as does including pollution deaths and incidental Banqiao Dam deaths.
In a more apples-to-apples comparison, New Scientist crunched the numbers. That maximum death-toll estimates from that analysis show:
- Natural gas is 1.3 times as dangerous as nuclear
- Coal is 27 times as dangerous as nuclear
- Hydroelectric is 46 times as dangerous as nuclear
In absolute terms, nuclear energy prevents about 80,000 air-pollution-related deaths a year, according to a 2013 study. Groups with antinuclear positions, such as Greenpeace, have struggled to spin these numbers.
"Nuclear power has consistently been proven to be the safest and most effective form of power that we have today, and by using thorium nuclear power, we can take that admirable safety record and go even further," Sorensen said.
But grasping the promise and potential perils of a thorium-powered future, or any other atomic-energy scheme, means you've got to know a thing or two about nuclear physics.
Nuclear Physics 101
In the United States, about 100,000 people work in the nuclear industry, and each year only a few thousand are awarded an undergraduate degree in physics.
These numbers suggest that more than 99 percent of us aren't intimately familiar with how nuclear energy works - so here's a bit of background about the atomic magic that provides roughly one-fifth of US power.
What reactors do
A commercial nuclear reactor's job, like any fossil-fuel-burning plant, is to generate heat.
Systems around the reactor harvest that flow of energy, use it to boil water into steam, drive turbines, and ultimately create electricity. Instead of burning fossil fuels, though, nuclear reactors 'burn' heavy elements, typically uranium.
But uranium isn't just uranium.
The element is found as, and can be transformed into, different isotopes, or various weights or 'flavours' of the same atomic element:
- uranium-238 (U-238), which makes up 99.27 percent of natural uranium ore
- uranium-235 (U-235), which is just 0.72 percent of natural ore, but a key ingredient in weapons and reactor fuel
- uranium-233 (U-233), which isn't found in nature yet is essential to thorium molten-salt reactors (more on this later)
The larger the number, the more chargeless neutrons are jammed into an atomic nucleus, and the heavier it is.
Take away or add a neutron, and you can radically alter an isotope's stability (and radioactivity), the types of radiation it emits, and what happens when it's blasted by more neutrons.
The most common isotopes of uranium aren't very radioactive.
For half of any U-238 to decay into lighter atoms - a measure called half-life - it takes 4.6 billion years.
That's a very, very long time to spread out a set amount of radiation. U-235 isn't much more radioactive with a half-life of 704 million years. Compare that to radon-222 (Rn-222), a gas with a half-life of nearly four days.
It's tens of billion times as radioactive as U-235, ounce for ounce, simply because Rn-222 decays so much faster. (Which is why it's a problem if it seeps out of the ground and into your basement.)
Yet we don't use Rn-222 as a nuclear fuel. One atomic property matters much more than all the others inside a reactor core.
Going critical
One of the most important things about a nuclear fuel is the chance its nucleus will react with a flying neutron, a property called neutron cross section.
Physicists measure cross section as an area, in 'barns', which you can imagine as a baseball glove. The larger the cross section the bigger the glove, and the more likely it is to catch a neutron - the baseball in this analogy.
The speed of a neutron greatly affects what happens next, and it can get weird.
A neutron can scatter, get captured (and turn a nucleus into a new isotope), or, of tremendous importance, get caught in the glove, suddenly fission it into pieces, and spit out two or three more baseballs in the process.
When those extra neutrons slam into neutrons and cause them to fission, it's a chain reaction.
Energy vs. bombs
Fission chain reactions are the key to nuclear reactors (and nuclear bombs), since each fission event turns a little bit of mass into pure energy.
However, only a handful of isotopes are fissile - meaning they spit out enough neutrons and have the right cross section to 'go critical' in a chain reaction.
U-238's thermal cross section is about 0.00003 barn. That is a very tiny glove. Meanwhile, U-235's cross section is 583 barns, making its figurative 'glove' millions of times as big, and a highly fissile fuel. U-233 is also fissile with a respectable cross section of 529 barns.
This is all gravely important.
A controlled chain reaction is a nuclear reactor. A runaway fission reaction is a nuclear disaster, or a weapon of mass destruction.
It took thousands of the world's brightest scientists in the Manhattan Project many years to crack open these and deeper mysteries of nuclear physics, then design technologies like bombs and reactors, so we'll skip most of that backstory.
The Making of the Atomic Bomb by Richard Rhodes is one of the best books to explore that history.
But in addition to figuring out how to 'breed' Pu-239 from U-238, scientists learned to transmute thorium into U-233.
Breeding atoms: as real as alchemy gets
If you press Sorensen for a simple analogy that illustrates how energy from thorium works, he may plunk you down in a wet forest.
"If you've ever gone camping as a Boy Scout or something like that, and been caught in a rainstorm and had to start a fire, you know that you're really looking hard for dry wood. Wood that will immediately burn. That's kind of how some of the uranium we have today is," Sorensen said.
"It's like the dry wood. It's the kindling."
Which makes thorium the wet wood: Get your nuclear fire hot enough and it will burn too.
"That's an imperfect analogy, but what happens in a thorium reactor is thorium absorbs neutrons and it forms a new fuel - uranium-233 - that can then sustain the reaction," he said. "It can produce enough neutrons to continue turning more thorium into U-233."
This transformative process is called breeding, and it's the key that unlocks the promise of thorium - and explains its eventual abandonment during the Cold War.
Manhattan Project scientists, who embraced a 'try everything' race to the bomb, didn't figure out thorium breeding until late in World War II.
They initially focused on enriching U-235 in natural ore from less than 1 percent to about 90 percent, which is considered weapons-grade material.
But enrichment was painfully inefficient, requiring city-size industrial complexes with mile-long buildings.
All $US1 billion worth of enriched uranium went inside the "Little Boy" bomb, which killed more than 100,000 people in Hiroshima.
Plutonium, an element not found in nature - and specifically the isotope Pu-239 - eventually changed everything, since it was a simpler (though still arduous) path to nuclear weapons.
The highly fissile isotope could be "bred" from common U-238 by pounding it with neutrons, then chemically removing the fresh Pu-239 with a bath of nitric acid - no mile-long buildings full of machinery required.
But in tandem, the Manhattan Project also explored making a third fissile material, U-233, from thorium.
Thorium's first fizzle
Glenn Seaborg, who discovered plutonium in 1940, "may have seen uranium-233 as a backup plan to the plutonium effort", Sorensen wrote in his 2014 University of Tennessee master's thesis about early research into thorium.
The scheme involved fuelling up a reactor, then using the neutrons to bombard thorium - and breeding it into U-233. But U-233 quickly became a dead end for the military.
For one, U-235 and Pu-239 were precious bomb-making materials, so burning them up in reactors was risky. Breeding U-233 from thorium also created significant amounts of a worrisome contaminant called U-232, which scientists had not yet figured out how to remove.
U-232 emits a lot of alpha radiation, which can trigger spontaneous fission - not good for a nuclear weapon you don't want to randomly explode.
Its decay products also emit a lot of gamma radiation, which can wreck electronics and harm or kill people who handle bombs.
In addition, gamma rays can blow a bomb's cover, since they are detectable by aeroplane or satellite, and pass through all but the heaviest radiation shielding.
Scientists like Seaborg weren't even certain a U-233-powered bomb would blow up very well. Apparently, they were right: A 1955 "Operation Teapot" weapons test using U-233 fizzled (the US government has yet to declassify all the details, though).
So in 1945, with Pu-239 production firmly in place, confidence in that weapons material, and a looming Japanese surrender, the defenders of breeding thorium into U-233 "went to zero," Sorensen told Business Insider.
"Was that the right decision? It's very hard to know," Sorensen said.
"Those people thought that they were making a decision to preserve the future for their children. ... So I hesitate to levy judgments on those decisions made in the past."
But in the years leading up to the war's end, Manhattan Project scientists were dreaming up ways to turn their wartime research into commercial power sources, and one group arrived at a brilliant concept: a super-fuel-efficient 'breeder' reactor that ran on thorium and U-233.
A powerful postwar revival for thorium
The concept of the breeder reactor was fairly straightforward.
It would dramatically increase the chances for fission, boost the flow of neutrons, and breed more fissile fuel from a 'fertile' material than the reactor burned up.
Breeding U-238 into Pu-239 created an excess of plutonium. Meanwhile, breeding thorium into U-233 broke even, burning up just as much fuel as it made. The choice of fuel makes all the difference.
The plutonium fuel cycle is a great way to make weapons. Meanwhile, the thorium fuel cycle can produce almost limitless energy.
A fluid-fuelled design was ultimately envisioned by Manhattan Project scientists to "eliminate the considerable difficulty of fabricating solid fuelled elements," Sorensen wrote in his thesis.
Liquid fuel also made it easy to remove both useful fission products - for example, for medical procedures, and those that poison nuclear chain reactions.
The gas xenon-135 (Xe-135) is a common uranium fission product, and its 3-million-barn cross section gobbles up neutrons and chokes reactors.
Physicist Alvin Weinberg later wrote the idea to use fluid fuels "kind of an obsession" of his, to the extent he eventually succeeded at building his first molten-salt reactors in Tennessee.
Nuclear jets and the first molten-salt reactor
When the Air Force launched an effort to build a nuclear-powered bomber in 1947 - part of the Aircraft Nuclear Propulsion (ANP) program - Weinberg, who in 1945 invented the now industry standard light-water reactor (LWR), rose to the occasion.
But Weinberg, then the director of Oak Ridge National Laboratory (ORNL), thought LWRs were too heavy and inefficient for a jet aeroplane.
In fact, even modern LWRs - which all US commercial nuclear power plants operate today - fission or 'burn up' just a few per cent of their fuel before it needs to be replaced. That's because neutron-absorbing waste builds up in the fuel, can't be removed, and chokes fission.
"When you go to gas station, do you feel good about burning 10 percent of it? What about 5 percent?" Sorensen said, referencing the low burn-up rate of solid-fuelled commercial reactors. "You want to burn it all. Why should we expect anything different?"
A molten-salt reactor emerged as the clear choice, since it could be built small: The fluid dramatically increases the efficiency of nuclear fission by making it easy to remove fission products, helping it burn up almost all the nuclear fuel and boosting energy output.
By 1954, Weinberg's team had built the proof-of-concept Aircraft Reactor Experiment (ARE), a 2.5-megawatt power plant that ran on 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.
Dissolved inside the reactor's molten salt, U-235 fuel powered a fission chain reaction.
The atomic heat warmed up an adjacent loop of coolant (filled with molten sodium) by 300 degrees Fahrenheit (166 degrees Celsius), from 1,200 to 1,500 degrees Fahrenheit (650 to 815 degrees Celsius).
Incoming air cooled the sodium, and pumps returned it to the fluid-fuelled 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-engine turbines.
Weinberg's new technology never made it inside the 'the Crusader' nuclear B-36 bomber (which actually did fly carrying a working reactor) before President John F. Kennedy cancelled the entire USAF project in 1961.
However, Weinberg had squeezed years' worth of research on molten-salt reactors out of the effort by then - and wasted no time spinning his work into the Molten-Salt Reactor Experiment (MSRE).
Weinberg's thorium dream is born
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