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Volume 87 Issue 46 | pp. 44-46
Issue Date: November 16, 2009

Reintroducing Thorium

A largely forgotten natural resource holds vast nuclear power potential
Department: Science & Technology
Keywords: Thorium, Nuclear Power, Nuclear Reactor, Molten-salt Reactor, Uranium
Thorium Energy Alliance Conference
Credit: YouTube
A nuclear waste burning liquid salt thorium reactor.
Credit: YouTube
Using thorium energy to address environmental problems.
Credit: YouTube
What fusion wanted to be.
Credit: YouTube
A new beginning for an old idea.
Credit: YouTube

Don't know much about thorium? Don't worry. You're not alone.

Most people—including scientists—have hardly heard of the heavy-metal element and know little about it even though it was first identified almost two centuries ago. Yet in just the past few weeks, a handful of people in a small number of countries have been beating the thorium drum loudly. They're convinced that the element, which sits near uranium in the periodic table, could serve as a nearly inexhaustible fuel for commercial nuclear reactors, providing low-cost, inherently safe, and nonpolluting power to an increasingly power-hungry planet. And they're trying to tell the world all about it.

Thorium's potential as an energy source isn't some classified government secret to be divulged only on a need-to-know basis. Far from it. A simple Google search will turn up a number of useful links. But although the information isn't hidden, details of thorium-fueled-reactor concepts aren't exactly common knowledge—not even to many seasoned nuclear power aficionados.

Michael H. Montgomery is one of the exceptions. For most of the 45 years he's worked in the nuclear industry, his focus—like nearly everyone else's—has been almost exclusively on uranium. That changed recently when he took a position as vice president of fuel technology with Mclean, Va.-based Lightbridge, one of the few U.S. companies working to develop thorium-derived nuclear power.

As Montgomery sees it, in the early days of the U.S. nuclear industry, thorium was regarded as falling somewhere between "a scientific curiosity and an avenue of interest." Decades ago, U.S. government labs studied the feasibility of converting 232Th into 233U by neutron bombardment and using that isotope of uranium in post-World War II nuclear projects, he says.

But as today's thorium-power proponents point out, that idea never took hold primarily because thorium-fueled reactors don't provide the opportunity to make and collect materials that can be used to build nuclear bombs. It made no sense to Cold War-era policymakers to devote resources to developing thorium-based technology, given that plutonium, especially 239Pu, an ideal and much-needed bomb material at that time, could be readily produced in nuclear reactors fueled by uranium (which consists of roughly 99.3% 238U). So thorium never made it into mainstream nuclear technology and has never been commercialized even though it is more abundant, potentially less expensive to process, and boasts other key advantages relative to uranium.

Since those days, with the exception of a few pockets of advanced research in a small number of countries, including India, Russia, and Norway, element 90 has largely fallen off the radar screen. But a few dedicated enthusiasts are working to put it back on.

A number of them recently formed the Thorium Energy Alliance. The alliance is a small U.S.-based advocacy group made up of members with wide-ranging scientific and professional backgrounds who share an interest in energy security and environmental issues. Almost none of the members has a direct financial stake in promoting thorium. Yet they work energetically to help spread the word about thorium-fueled nuclear power to government leaders and the public.

Last month, the group convened its first conference, which drew about 50 people to Washington, D.C. At the same time that the Washington conference was in session, the International Thorium Energy Organization, a brand-new European group with the same goals as the U.S.-based alliance, announced its existence by launching the IThEO.org website. And just one month earlier, the Chinese Nuclear Society ran a thorium workshop in Baotou, Inner Mongolia.

A few minutes is all it takes for thorium supporters like Kirk F. Sorensen, who participated in the Washington meeting, to outline a couple of arguments that catch the attention of energy-conscientious listeners. By day, Sorensen is an aerospace engineer at the National Aeronautics & Space Administration's Marshall Space Flight Center, in Huntsville, Ala. But he's also a nuclear engineering researcher affiliated with the University of Tennessee, Knoxville, and an ardent blogger at thoriumenergy.blogspot.com.

Thorium itself is not actually a nuclear fuel. But it can be converted (transmuted) into one by exposing it to low-energy neutrons. Neutron capture converts 232Th into 233U, a material that liberates enormous amounts of energy when it undergoes nuclear fission.

[+]Enlarge
PACKED WITH POWER
Relatively abundant yet obscure, thorium—found in minerals such as this crystal of thorite (a thorium-uranium silicate)—could fuel nuclear power reactors for generations.
Credit: Theodore Gray
8746sci2_thoritecxd
 
PACKED WITH POWER
Relatively abundant yet obscure, thorium—found in minerals such as this crystal of thorite (a thorium-uranium silicate)—could fuel nuclear power reactors for generations.
Credit: Theodore Gray

Sorensen explains that the same "shortcoming" that led government officials during the Cold War to decide against pursuing thorium-reactor technology—namely the inability to generate weapon materials—is clearly recognized as an advantage today. At no point in the thorium cycle, from mining thorium minerals to preparing and "burning" reactor fuel to managing the waste, can fuel or waste products be converted into nuclear bomb materials. Unlike uranium, thorium is nuclear-proliferation proof.

Nuclear proliferation is precisely the reason Iran and North Korea dominate today's headlines and generate considerable international angst. Iran claims it is enriching uranium—meaning increasing the concentration of 235U (a fissile isotope) relative to 238U—to low levels, sufficient only for peaceful purposes such as generating electricity in a nuclear power reactor. Yet the world wonders whether Iran really aims to make highly enriched weapons-grade uranium.

North Korea makes no secret of its intentions. The country proudly announced earlier this month that it has reprocessed 8,000 spent uranium fuel rods, thereby extracting the 239Pu formed in reactors from 238U. North Korea is now believed to have enough plutonium to make a half-dozen nuclear weapons.

In addition to the nonproliferation argument, thorium advocates quickly rattle off a slew of other reasons to push forward with thorium-based power. For example, the element is roughly four times more abundant than uranium and accessible via mining techniques that are simpler and less costly than the ones used to extract uranium. According to James Hendrick, a recently retired U.S. Geological Survey scientist who spoke at the Washington meeting, estimates of U.S. reserves of the metal are on the order of 300,000 metric tons —about 20% of the world's supply—much of which is found in Idaho's Lemhi Pass.

Not only is thorium more plentiful than uranium, it also does not need to undergo a costly and complex enrichment process to render it usable in a nuclear reactor. Uranium needs to be enriched because the desirable fissile isotope 235U comprises just 0.7% of the total material. Thorium exists in nature almost entirely as 232Th.

Proponents also point out that although waste products from thorium usage are radioactive, radiotoxicity persists for just tens of years rather than thousands of years as uranium waste does. They also stress that, unlike coal- and natural-gas-fired power plants, thorium-fueled power plants would not emit greenhouse gases such as CO2 and could generate power almost continuously, unlike solar- and wind-driven systems.

Extracting thorium's latent energy requires some type of nuclear reactor. Descriptions of the reactor best suited to that job depend on who's doing the describing. One design idea that's generating a big buzz in this little community calls for using thorium in a liquid state in an updated version of an experimental molten-salt reactor that ran for several years during the 1960s at Oak Ridge National Laboratory. That reactor idea grew out of an uncompleted project to design a nuclear-powered military airplane.

In designs for the updated device, known as the liquid fluoride thorium reactor (LFTR, pronounced "lifter"), a molten blanket of 232ThF4 dissolved in a lithium-beryllium fluoride solvent surrounds a fluid core containing 233UF4 in the same solvent. As 233U nuclei fission, they generate heat that is transferred to a gas that drives a turbine to generate electricity. At the same time, the uranium nuclei emit neutrons that convert 232Th in the blanket to 233U. As uranium fuel accumulates in the blanket, it is gasified (converted to UF6), separated, and fed into the core gradually and continuously as fresh thorium is injected into the blanket.

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Salt Power
The fission of 233U nuclei in a molten-salt nuclear reactor core liberates heat, which is used to generate electricity, and neutrons, which convert 232Th in the blanket to additional 233U fuel.
8746sci2_sch_lg
 
Salt Power
The fission of 233U nuclei in a molten-salt nuclear reactor core liberates heat, which is used to generate electricity, and neutrons, which convert 232Th in the blanket to additional 233U fuel.

A key source of interest in LFTR is the design's inherent safety features. David LeBlanc, a staff physicist at Carleton University, in Ottawa, and a nuclear reactor specialist, points out several safety-related differences between LFTRs and today's commercial reactors. To begin with, LFTRs would operate at low pressure. Furthermore, an increase in the temperature of LFTR fuel (a molten salt) would reduce the medium's density and thereby lower its nuclear reactivity. In addition, if the reactor leaked or was drained of its fuel, the molten salt would solidify. In the event of reactor malfunction, those features would terminate nuclear reactions and prevent the spread of radioactive material without the need for plant-operator intervention.

Despite considerable enthusiasm for the LFTR concept among thorium advocates, several attendees at the Washington conference acknowledged that an enormous investment of time, effort, and money would be required before any new type of nuclear reactor could be licensed for commercial operation.

That's one of the main reasons Lightbridge scientists are developing thorium-based fuels for today's commercial reactors. As Montgomery explains it, Lightbridge fuels feature a "seed and blanket" design, which appears identical to commercial fuel assemblies but has a unique composition. Seed rods at the center of the assembly are made of a metallic uranium-zirconium matrix. Blanket rods positioned along the periphery contain thorium-uranium oxide pellets. As with the molten-salt design, uranium fission generates heat and converts 232Th to 233U, thereby creating more fuel. Montgomery notes that Lightbridge is scheduled to insert three test fuel-rod assemblies into commercial light-water reactors in Russia in the 2012–13 time frame.

HANDFUL OF ENERGY
A ball of thorium this size could provide one person with a lifetime supply of energy. (Thorium is mildly radioactive; this prop is stainless steel.)
Credit: Robert Hargraves
8746sci2_thoriumcxd
 
HANDFUL OF ENERGY
A ball of thorium this size could provide one person with a lifetime supply of energy. (Thorium is mildly radioactive; this prop is stainless steel.)
Credit: Robert Hargraves

Meanwhile, India is developing its own thorium-fueled nuclear industry to exploit that country's large reserves of thorium minerals. India's unique multistage approach is based on light- and heavy-water reactors and various combinations of uranium, plutonium, and thorium fuels. In September, India's prime minister, Manmohan Singh, articulated India's commitment to boosting its nuclear energy output in the coming decades by a factor of more than 100, largely by tapping thorium's power.

Thorium proponents in the U.S. are trying to breed the same kind of commitment among their leaders. At the meeting last month, conference organizer and alliance founder John H. Kutsch ran down a list of essential factoids for successfully spreading the word on thorium to members of Congress and handed out a brochure for that purpose. He pressed for volunteers to write a Wikipedia entry on LFTRs, update nuclear engineering textbooks, and put up funds to support education on thorium. And he and his fellow believers are not afraid to turn to out-of-the-box means to communicate the good word about thorium. Rather than publishing papers in conventional journals, this group runs blogs, posts podcasts, and gives Google TechTalks, which are available on YouTube.

One attendee observed that "it's possible to have a Ph.D. in nuclear reactor technology and not know about thorium energy." The thorium advocates are working to change that and to undo what Sorensen describes as the element's status "as Earth's forgotten treasure."

 
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