Photo: Thomas Hannich

The thick hardbound volume was sitting on a shelf in a colleague's office when Kirk Sorensen spotted it. A rookie NASA engineer at the Marshall Space Flight Center, Sorensen was researching nuclear-powered propulsion, and the book's title — Fluid Fuel Reactors — jumped out at him. He picked it up and thumbed through it. Hours later, he was still reading, enchanted by the ideas but struggling with the arcane writing. "I took it home that night, but I didn't understand all the nuclear terminology," Sorensen says. He pored over it in the coming months, ultimately deciding that he held in his hands the key to the world's energy future.

Published in 1958 under the auspices of the Atomic Energy Commission as part of its Atoms for Peace program, Fluid Fuel Reactors is a book only an engineer could love: a dense, 978-page account of research conducted at Oak Ridge National Lab, most of it under former director Alvin Weinberg. What caught Sorensen's eye was the description of Weinberg's experiments producing nuclear power with an element called thorium.

At the time, in 2000, Sorensen was just 25, engaged to be married and thrilled to be employed at his first serious job as a real aerospace engineer. A devout Mormon with a linebacker's build and a marine's crew cut, Sorensen made an unlikely iconoclast. But the book inspired him to pursue an intense study of nuclear energy over the next few years, during which he became convinced that thorium could solve the nuclear power industry's most intractable problems. After it has been used as fuel for power plants, the element leaves behind minuscule amounts of waste. And that waste needs to be stored for only a few hundred years, not a few hundred thousand like other nuclear byproducts. Because it's so plentiful in nature, it's virtually inexhaustible. It's also one of only a few substances that acts as a thermal breeder, in theory creating enough new fuel as it breaks down to sustain a high-temperature chain reaction indefinitely. And it would be virtually impossible for the byproducts of a thorium reactor to be used by terrorists or anyone else to make nuclear weapons.

Weinberg and his men proved the efficacy of thorium reactors in hundreds of tests at Oak Ridge from the '50s through the early '70s. But thorium hit a dead end. Locked in a struggle with a nuclear- armed Soviet Union, the US government in the '60s chose to build uranium-fueled reactors — in part because they produce plutonium that can be refined into weapons-grade material. The course of the nuclear industry was set for the next four decades, and thorium power became one of the great what-if technologies of the 20th century.

Today, however, Sorensen spearheads a cadre of outsiders dedicated to sparking a thorium revival. When he's not at his day job as an aerospace engineer at Marshall Space Flight Center in Huntsville, Alabama — or wrapping up the master's in nuclear engineering he is soon to earn from the University of Tennessee — he runs a popular blog called Energy From Thorium. A community of engineers, amateur nuclear power geeks, and researchers has gathered around the site's forum, ardently discussing the future of thorium. The site even links to PDFs of the Oak Ridge archives, which Sorensen helped get scanned. Energy From Thorium has become a sort of open source project aimed at resurrecting long-lost energy technology using modern techniques.

And the online upstarts aren't alone. Industry players are looking into thorium, and governments from Dubai to Beijing are funding research. India is betting heavily on the element.

The concept of nuclear power without waste or proliferation has obvious political appeal in the US, as well. The threat of climate change has created an urgent demand for carbon-free electricity, and the 52,000 tons of spent, toxic material that has piled up around the country makes traditional nuclear power less attractive. President Obama and his energy secretary, Steven Chu, have expressed general support for a nuclear renaissance. Utilities are investigating several next-gen alternatives, including scaled-down conventional plants and "pebble bed" reactors, in which the nuclear fuel is inserted into small graphite balls in a way that reduces the risk of meltdown.

Those technologies are still based on uranium, however, and will be beset by the same problems that have dogged the nuclear industry since the 1960s. It is only thorium, Sorensen and his band of revolutionaries argue, that can move the country toward a new era of safe, clean, affordable energy.

Named for the Norse god of thunder, thorium is a lustrous silvery-white metal. It's only slightly radioactive; you could carry a lump of it in your pocket without harm. On the periodic table of elements, it's found in the bottom row, along with other dense, radioactive substances — including uranium and plutonium — known as actinides.

Actinides are dense because their nuclei contain large numbers of neutrons and protons. But it's the strange behavior of those nuclei that has long made actinides the stuff of wonder. At intervals that can vary from every millisecond to every hundred thousand years, actinides spin off particles and decay into more stable elements. And if you pack together enough of certain actinide atoms, their nuclei will erupt in a powerful release of energy.

To understand the magic and terror of those two processes working in concert, think of a game of pool played in 3-D. The nucleus of the atom is a group of balls, or particles, racked at the center. Shoot the cue ball — a stray neutron — and the cluster breaks apart, or fissions. Now imagine the same game played with trillions of racked nuclei. Balls propelled by the first collision crash into nearby clusters, which fly apart, their stray neutrons colliding with yet more clusters. Voilè0: a nuclear chain reaction.

Actinides are the only materials that split apart this way, and if the collisions are uncontrolled, you unleash hell: a nuclear explosion. But if you can control the conditions in which these reactions happen — by both controlling the number of stray neutrons and regulating the temperature, as is done in the core of a nuclear reactor — you get useful energy. Racks of these nuclei crash together, creating a hot glowing pile of radioactive material. If you pump water past the material, the water turns to steam, which can spin a turbine to make electricity.

Uranium is currently the actinide of choice for the industry, used (sometimes with a little plutonium) in 100 percent of the world's commercial reactors. But it's a problematic fuel. In most reactors, sustaining a chain reaction requires extremely rare uranium-235, which must be purified, or enriched, from far more common U-238. The reactors also leave behind plutonium-239, itself radioactive (and useful to technologically sophisticated organizations bent on making bombs). And conventional uranium-fueled reactors require lots of engineering, including neutron-absorbing control rods to damp the reaction and gargantuan pressurized vessels to move water through the reactor core. If something goes kerflooey, the surrounding countryside gets blanketed with radioactivity (think Chernobyl). Even if things go well, toxic waste is left over.

When he took over as head of Oak Ridge in 1955, Alvin Weinberg realized that thorium by itself could start to solve these problems. It's abundant — the US has at least 175,000 tons of the stuff — and doesn't require costly processing. It is also extraordinarily efficient as a nuclear fuel. As it decays in a reactor core, its byproducts produce more neutrons per collision than conventional fuel. The more neutrons per collision, the more energy generated, the less total fuel consumed, and the less radioactive nastiness left behind.

Even better, Weinberg realized that you could use thorium in an entirely new kind of reactor, one that would have zero risk of meltdown. The design is based on the lab's finding that thorium dissolves in hot liquid fluoride salts. This fission soup is poured into tubes in the core of the reactor, where the nuclear chain reaction — the billiard balls colliding — happens. The system makes the reactor self-regulating: When the soup gets too hot it expands and flows out of the tubes — slowing fission and eliminating the possibility of another Chernobyl. Any actinide can work in this method, but thorium is particularly well suited because it is so efficient at the high temperatures at which fission occurs in the soup.

In 1965, Weinberg and his team built a working reactor, one that suspended the byproducts of thorium in a molten salt bath, and he spent the rest of his 18-year tenure trying to make thorium the heart of the nation's atomic power effort. He failed. Uranium reactors had already been established, and Hyman Rickover, de facto head of the US nuclear program, wanted the plutonium from uranium-powered nuclear plants to make bombs. Increasingly shunted aside, Weinberg was finally forced out in 1973.

That proved to be "the most pivotal year in energy history," according to the US Energy Information Administration. It was the year the Arab states cut off oil supplies to the West, setting in motion the petroleum-fueled conflicts that roil the world to this day. The same year, the US nuclear industry signed contracts to build a record 41 nuke plants, all of which used uranium. And 1973 was the year that thorium R&D faded away — and with it the realistic prospect for a golden nuclear age when electricity would be too cheap to meter and clean, safe nuclear plants would dot the green countryside.

The core of this hypothetical nuclear reactor is a cluster of tubes filled with a fluoride thorium solution. 1// compressor, 2// turbine, 3// 1,000 megawatt generator, 4// heat exchanger, 5// containment vessel, 6// reactor core.

Illustration: Martin Woodtli

When Sorensen and his pals began delving into this history, they discovered not only an alternative fuel but also the design for the alternative reactor. Using that template, the Energy From Thorium team helped produce a design for a new liquid fluoride thorium reactor, or LFTR (pronounced "lifter"), which, according to estimates by Sorensen and others, would be some 50 percent more efficient than today's light-water uranium reactors. If the US reactor fleet could be converted to LFTRs overnight, existing thorium reserves would power the US for a thousand years.

Overseas, the nuclear power establishment is getting the message. In France, which already generates more than 75 percent of its electricity from nuclear power, the Laboratoire de Physique Subatomique et de Cosmologie has been building models of variations of Weinberg's design for molten salt reactors to see if they can be made to work efficiently. The real action, though, is in India and China, both of which need to satisfy an immense and growing demand for electricity. The world's largest source of thorium, India, doesn't have any commercial thorium reactors yet. But it has announced plans to increase its nuclear power capacity: Nuclear energy now accounts for 9 percent of India's total energy; the government expects that by 2050 it will be 25 percent, with thorium generating a large part of that. China plans to build dozens of nuclear reactors in the coming decade, and it hosted a major thorium conference last October. The People's Republic recently ordered mineral refiners to reserve the thorium they produce so it can be used to generate nuclear power.

In the United States, the LFTR concept is gaining momentum, if more slowly. Sorensen and others promote it regularly at energy conferences. Renowned climatologist James Hansen specifically cited thorium as a potential fuel source in an "Open Letter to Obama" after the election. And legislators are acting, too. At least three thorium-related bills are making their way through the Capitol, including the Senate's Thorium Energy Independence and Security Act, cosponsored by Orrin Hatch of Utah and Harry Reid of Nevada, which would provide $250 million for research at the Department of Energy. "I don't know of anything more beneficial to the country, as far as environmentally sound power, than nuclear energy powered by thorium," Hatch says. (Both senators have long opposed nuclear waste dumps in their home states.)

Unfortunately, $250 million won't solve the problem. The best available estimates for building even one molten salt reactor run much higher than that. And there will need to be lots of startup capital if thorium is to become financially efficient enough to persuade nuclear power executives to scrap an installed base of conventional reactors. "What we have now works pretty well," says John Rowe, CEO of Exelon, a power company that owns the country's largest portfolio of nuclear reactors, "and it will for the foreseeable future."

Critics point out that thorium's biggest advantage — its high efficiency — actually presents challenges. Since the reaction is sustained for a very long time, the fuel needs special containers that are extremely durable and can stand up to corrosive salts. The combination of certain kinds of corrosion-resistant alloys and graphite could meet these requirements. But such a system has yet to be proven over decades.

And LFTRs face more than engineering problems; they've also got serious perception problems. To some nuclear engineers, a LFTR is a little ... unsettling. It's a chaotic system without any of the closely monitored control rods and cooling towers on which the nuclear industry stakes its claim to safety. A conventional reactor, on the other hand, is as tightly engineered as a jet fighter. And more important, Americans have come to regard anything that's in any way nuclear with profound skepticism.

So, if US utilities are unlikely to embrace a new generation of thorium reactors, a more viable strategy would be to put thorium into existing nuclear plants. In fact, work in that direction is starting to happen — thanks to a US company operating in Russia.

Located outside Moscow, the Kurchatov Institute is known as the Los Alamos of Russia. Much of the work on the Soviet nuclear arsenal took place here. In the late '80s, as the Soviet economy buckled, Kurchatov scientists found themselves wearing mittens to work in unheated laboratories. Then, in the mid-'90s, a savior appeared: a Virginia company called Thorium Power.

2. Uranium-Fueled Light-Water Reactor3. Fuel Uranium fuel rods 4. Fuel input per gigawatt output 250 tons raw uranium 5. Annual fuel cost for 1-GW reactor $50-60 million 6. Coolant Water 7. Proliferation potential Medium 8. Footprint 200,000-300,000 square feet, surrounded by a low-density population zone 2. Seed-and-Blanket Reactor3. Fuel Thorium oxide and uranium oxide rods 4. Fuel input per gigawatt output 4.6 tons raw thorium, 177 tons raw uranium 5. Annual fuel cost for 1-GW reactor $50-60 million 6. Coolant Water 7. Proliferation potential None 8. Footprint 200,000-300,000 square feet, surrounded by a low-density population zone 2. Liquid Fluoride Thorium Reactor3. Fuel Thorium and uranium fluoride solution 4. Fuel input per gigawatt output 1 ton raw thorium 5. Annual fuel cost for 1-GW reactor $10,000 (estimated) 6. Coolant Self-regulating 7. Proliferation potential None 8. Footprint 2,000-3,000 square feet, with no need for a buffer zone Founded by another Alvin — American nuclear physicist Alvin Radkowsky — Thorium Power, since renamed Lightbridge, is attempting to commercialize technology that will replace uranium with thorium in conventional reactors. From 1950 to 1972, Radkowsky headed the team that designed reactors to power Navy ships and submarines, and in 1977 Westinghouse opened a reactor he had drawn up — with a uranium thorium core. The reactor ran efficiently for five years until the experiment was ended. Radkowsky formed his company in 1992 with millions of dollars from the Initiative for Proliferation Prevention Program, essentially a federal make-work effort to keep those chilly former Soviet weapons scientists from joining another team.

The reactor design that Lightbridge created is known as seed-and-blanket. Its core consists of a seed of enriched uranium rods surrounded by a blanket of rods made of thorium oxide mixed with uranium oxide. This yields a safer, longer-lived reaction than uranium rods alone. It also produces less waste, and the little bit it does leave behind is unsuitable for use in weapons.

CEO Seth Grae thinks it's better business to convert existing reactors than it is to build new ones. "We're just trying to replace leaded fuel with unleaded," he says. "You don't have to replace engines or build new gas stations." Grae is speaking from Abu Dhabi, where he has multimillion-dollar contracts to advise the United Arab Emirates on its plans for nuclear power. In August 2009, Lightbridge signed a deal with the French firm Areva, the world's largest nuclear power producer, to investigate alternative nuclear fuel assemblies.

Until developing the consulting side of its business, Lightbridge struggled to build a convincing business model. Now, Grae says, the company has enough revenue to commercialize its seed-and-blanket system. It needs approval from the US Nuclear Regulatory Commission — which could be difficult given that the design was originally developed and tested in Russian reactors. Then there's the nontrivial matter of winning over American nuclear utilities. Seed-and-blanket doesn't just have to work — it has to deliver a significant economic edge.

For Sorensen, putting thorium into a conventional reactor is a half measure, like putting biofuel in a Hummer. But he acknowledges that the seed-and-blanket design has potential to get the country on its way to a greener, safer nuclear future. "The real enemy is coal," he says. "I want to fight it with LFTRs — which are like machine guns — instead of with light-water reactors, which are like bayonets. But when the enemy is spilling into the trench, you affix bayonets and go to work." The thorium battalion is small, but — as nuclear physics demonstrates — tiny forces can yield powerful effects.

Richard Martin (rmartin@newwest.net), editor of VON, wrote about the Large Hadron Collider in issue 12.04.