TWO

THE THUNDER ELEMENT

Thorium is a lustrous silvery-white metal, denser than lead, that occurs in great abundance in Earth’s crust. It’s slightly radioactive, but, like naturally occurring uranium, you could carry a lump of it in your pocket without harm. When heated, thorium incandesces a brilliant white. Its atomic number—the number of protons in an atom’s nucleus—is 90. On the periodic table it’s found on the bottom row, along with the other heavy radioactive elements, or actinides—protactinium, uranium, neptunium, plutonium, and so on. With an atomic weight of 232, it is the second-heaviest element found in measurable amounts in nature, behind uranium (atomic weight essentially equals the number of protons plus the number of neutrons). It has a half-life—the time it takes for half of any sample to decay to a nonradioactive state—of about 14 billion years, or about the age of the universe.¹

Thorium is around four times as abundant as uranium, or about as prevalent as lead. The international Nuclear Energy Agency estimates the United States has 440,000 tons of thorium reserves.*

It was discovered in 1828 by Swedish chemist Jöns Jacob Berzelius, who named it for the Norse god of thunder. Berzelius, who was born in 1779, originally trained as a physician, and his early medical studies included some daring ideas, among them the influence of a galvanic electrical current on various diseases. More practically, he helped develop the technique of electrolysis, or using direct current to stimulate a chemical reaction, and invented the method of notating chemical formulae that is still in use today. In his career he also discovered selenium and cesium, and he was the first to isolate a long list of elements that includes calcium, barium, silicon, and titanium. Along with Robert Boyle and Antoine Lavoisier (the French chemist who devised the first periodic table and whose law of the conservation of matter would be overturned by Einstein’s theory of relativity), Berzelius is considered one of the fathers of modern chemistry. On thorium, though, he had already made an embarrassing blunder: he’d mistakenly “discovered” it once before.

People who knew of Berzelius’s fame, and his skill at identifying strange new materials, had a habit of bringing him samples to test in his lab at Stockholm. In 1815 he obtained an unfamiliar material, a black earth, and subjected it to the usual chemical analysis. At the time he believed he had discovered a new element. Wishing to honor the Scandinavian deities, he named it after Thor. As Berzelius’s knowledge of chemical interactions deepened, however, he began to have doubts about thorium, and in 1824, almost a decade after he’d announced his discovery, he realized that the black earth was actually a form of a previously discovered element called yttrium.

Four years later he got another chance. A Swedish pastor named H. M. T. Esmark was hiking on the island of Lövö, off the west coast of Norway, when he spotted a black mineral with a darkly gleaming surface. The islands of Norway are formed of granite and gneiss, with stony uplands dotted by fantastic outcroppings. Reverend Esmark reportedly liked to walk these desolate heaths, glorying in the works of the Lord and occasionally collecting interesting rock samples for his father, a prominent mineralogist named Jens Esmark.

In this case Professor Esmark could not identify the material. So he sent it on to Berzelius, who at age 49, despite the thorium misstep, was considered the leading chemist in Sweden. This time Berzelius was certain he was right, and in 1829, after subjecting the material to a series of chemical analyses and isolating the 60 percent of the sample that was an unknown element in pure form, he announced the rediscovery of thorium in a paper in a Swedish geological journal. It was 40 years after the discovery of uranium by a German apothecary and 66 years before the accidental discovery of radiation by the German physicist Wilhelm Conrad Röntgen.

Like all heavy elements, the material Berzelius identified is literally not of this earth. All the thorium on Earth was created in supernovas—the tremendous stellar explosions that mark the end of large stars’ stable life.

Nothing is lost in the immolation of a star. In the core the elements hydrogen and helium are combined in nuclear fusion, in which smaller elements collide and fuse to make larger ones—including the basic stuff of life, carbon, nitrogen, and oxygen. Stellar fusion, however, is not powerful enough to create elements heavier than iron (atomic number 26). In the cataclysm of star death, the heart of the star heats a furnace in which new heavy elements are formed, a process called nucleosynthesis. These new elements are flung into the vacuum of deep space like embers from a conflagration. Debris condenses in the gravity of other, younger stars to form rocky or gaseous planets. Our solar system took form from the vast elemental clouds drifting in space and came into being roughly 4.5 billion years ago. At Earth’s center thorium and uranium give off the energy from long-dead stars.

That energy, in fact, still fuels the heat generated underground in the mantle that underlies the continent-forming crust. Earth generates an enormous amount of radioactive heat—something on the order of 2.1 x 10¹³ watts, or about 20 million megawatts. Heat emanating from the depths creates Earth’s magnetic field (which shields the planet from the corrosive solar wind), enables the plate tectonics that formed today’s continents, and sustains life itself. Only three radioactive elements are found in enough abundance to generate significant amounts of heat in the terrestrial skin: uranium, potassium, and thorium. Three isotopes of uranium occur naturally: U-238, U-234, and U-235. The vast majority of the uranium on Earth—about 99.3 percent—is U-238. For use in conventional reactors uranium must be enriched to 3 to 5 percent U-235, a blend known as low-enriched uranium (LEU). Highly enriched uranium, anything better than 20 percent, can be used in nuclear weapons, although for a reliable modern warhead guaranteed to produce an explosion equivalent to 465 kilotons or so of TNT and to destroy a good-sized city, you need uranium that is about 85 percent pure U-235. The hard part about building an atomic bomb is not building the bomb; it’s the expensive and complicated process of enriching enough uranium to cause an uncontrollable fission reaction.

Like potassium, thorium has only one naturally occurring isotope, Th-232, with 90 protons and 142 neutrons. (The difference between U-235, with 143 neutrons, and thorium, with one fewer neutron, is a critical one.) The decay of these three elements is responsible for the great majority of geothermal heat; uranium, with a half-life that is 10 billion years shorter than thorium’s, produces more heat by volume than thorium, but thorium is much more abundant in Earth’s crust. It is safe to say that if not for the thorium created in the explosion of dying stars five or six billion years ago, life on Earth would not exist, and you would not be reading this book.

Like all radioactive elements, thorium has a characteristic decay chain— the series of elements into which the material spontaneously transforms as it sheds particles in the form of radiation. The decay chain of thorium includes two isotopes each of radium and polonium; another thorium isotope (Th-228); radon; and, eventually, lead, the stable resting element into which both uranium and thorium decay. Almost all the thorium currently in Earth has been present since the planet was formed.

A fissile element is one whose atoms will split apart when bombarded by low-energy neutrons. The primary fissile elements are uranium-235, plutonium-239, and uranium-233. Naturally occurring uranium-238 is not fissile; that’s why you have to enrich it to increase the percentage of U-235. Thorium is not fissile, but it is fertile: under the right conditions—say, bombarded by neutrons in the core of a nuclear reactor—it can be converted into U-233, a highly fissile isotope of uranium. Thorium-232 actually captures a neutron to become Th-233, which then decays quickly, by way of protactinium-233, into U-233. The difference between fissile U-235 and fertile thorium plays a large role in this story.

Although it was critical to the late nineteenth- and early twentieth-century physicists who blazed the pathway into the mysteries of the atom, thorium is much less well known than its cousins uranium and plutonium. The dramatic part played by uranium in the history of the twentieth and the twenty-first century completely overshadows thorium’s potential and its unique powers. If elements were celebrities, thorium wouldn’t even make the B list.

In many ways thorium is a shadow element to its more infamous neighbor on the periodic table, uranium: chemically similar and exhibiting closely related but utterly tangential behavior, the two are deeply linked on some elemental level. Thorium is the first element into which U-238 decays; in a nuclear reactor thorium transmutes into an isotope of uranium, U-233, that has qualities more suited for power generation than the U-235 version. It is as if thorium had a complementary or contrasting feature for every response and quality of uranium. Yin and yang. Masculine and feminine. Light—the light of Trinity, of Bikini Atoll, and of Hiroshima—and shadow.

Thorium could be the younger sister—less volatile, slower to self-consume—of her flamboyant and domineering older brother, uranium. Their differences have defined the history of nuclear power—and on the threshold of World War II this pair of radioactive siblings enabled some of the key theoretical insights that led directly to the development of nuclear weapons.

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“IT IS REMARKABLE THAT THE TWO most active elements, uranium and thorium, are the ones which possess the greatest atomic weight.”

In that short sentence, included in the epochal 1898 paper, “Rays Emitted by Compounds of Uranium and of Thorium,” Marie Curie embedded many of the concepts that would underlie that next half century of astonishingly rapid discoveries in nuclear physics, quantum mechanics, and the science of nuclear fission.

From Henri Becquerel’s accidental discovery of radioactivity in 1896 to August 6, 1945, when Hiroshima was destroyed by an atomic bomb, occurred without question the most remarkable series of discoveries—revelations is a better word—in the history of science. This quest of modern physics was catalyzed in 1905 by Albert Einstein’s insights into the inextricable identity of energy and matter; pushed forward by a remarkably colorful and brilliant group of European physicists who were led, driven, and inspired by the Danish master Niels Bohr; and culminated, in the high desert of New Mexico, in the realization of the worst fears surrounding the power of the shattered atom. Because the quest culminated in the utter destruction of two ancient cities—not military targets—by the most fearsome weapons conceived up to that point, it is a story of absolute betrayal, of the subversion of humanity’s highest ideals and its most brilliant minds by a technology that led irrevocably to a fundamental change not only in warfare but the human condition and potentially to the eradication of the human race itself.

This is the fundamental dilemma of twentieth-century nuclear physics: this band of brilliant, humane, cosmopolitan scientists led us all to the brink of annihilation. In Faust in Copenhagen, his entertaining account of the famous Copenhagen gathering of 1932, the physicist Gino Segre makes it clear that by the mid-1930s, as fascism spread malignantly across Europe, Niels Bohr, Werner Heisenberg, Paul Dirac, Leo Szilard, Enrico Fermi, and the rest possessed a dawning awareness of the awful trade-offs—the Faustian bargains—they were making on the road to full understanding of the power that lurked inside radioactive elements. I’d read that story in several well-told and exhaustive accounts, most powerfully in Richard Rhodes’s The Making of the Atomic Bomb. I considered myself, as a science-and-technology journalist, fairly well versed in the subject. Now, though, while researching thorium’s tale, I realized that there was a shadow version, and, seen through the lens of thorium, it cast the whole enterprise in a new and surprising light. A year after I first heard of thorium, I came to see it as a key element in the remarkable history of discoveries that transformed our world in the first half of the twentieth century.

My purpose here is to examine how the story of thorium is woven throughout the astonishing scientific and technological advances of that magical and terrible half century—to shed some light on a shadowy element.

In the case of Marie and Pierre Curie, the distinctions between uranium and thorium opened the way to their greatest discoveries. Since Becquerel’s discovery of radioactivity, Marie Curie—whose biography is a prefeminist tale of unthwarted ambition and conventional barriers overcome—had become fascinated by this effect. The mystery of Becquerel’s “rays,” she would recall later, “seemed to us very attractive and all the more so because the question was entirely new and nothing yet had been written upon it.” ² What’s more, the radioactive properties of thorium were still undiscovered; Berzelius, working before radiation was even detectable, had no idea that the element he discovered was one of the most radioactive in nature.

In 1894, having completed her math degree at the Sorbonne, Marie Sklodowska had set up a laboratory in a disused storage space, little more than a closet, in the school in Paris where Pierre Curie (whom she would marry a year later) taught. As a woman and a Pole, Marie had endured terrible hardship during her early years as a student in Paris, but she quickly set her challenging circumstances to productive use. Along with Pierre, she “arrived at ‘a new method of chemical analysis’ based on very precise measurements of what we now call radiation.”³ She started out with uranium, which Becquerel had used, but she quickly moved on to other elements, including thorium.

To test the properties of these materials she used leftover wooden grocery crates to build a primitive but extremely sensitive electrometer, which had been devised by Pierre and his brother. Inside were two metal plates held one above the other. The lower plate, charged with a high-voltage battery, held the material being tested; the upper one could be tested for electrical current. Sklodowska, a gifted scavenger, first used white uranium powder obtained from the French chemist Henri Moissan, who had supplied Becquerel with the same material. After measuring the current produced in the upper plate by uranium, she moved on to other elements, including gold and copper and a bizarre substance called pitchblende. Mined in Joachimsthal in eastern Germany, a major silver-mining district since the sixteenth century, pitchblende was a black tarry mineral from which Martin Heinrich Klaproth had first extracted uranium in the late eighteenth century. Then she tested the mineral aeschynite, which she knew contained thorium.

The literature and Marie’s detailed lab diary are not clear about where Curie obtained her thorium. Some of her mineral samples came from chemist friends of Pierre’s; some came from the Museum of Natural History in Paris. Wagons regularly arrived outside the school to deliver loads of exotic minerals. At any rate, the results of the examination were startling.

“I have studied the conductance of air under the influence of the uranium rays discovered by M. Becquerel,” she wrote in the 1898 paper, “and I examined whether substances other than compounds of uranium were able to make the air a conductor of electricity.”⁴ The other substances included various oxides of uranium and thorium as well as potassium, sodium, ammonium, chalcite, and pitchblende.

At this point, at the end of the nineteenth century, uranium was seen as a curiosity with few practical applications beyond its use as a coloring agent for ceramics. Thorium, on the other hand, was already the key element of a thriving industry, mostly centered on the manufacture of mantles for gas lanterns. Readily available, thorium was prized above other metals for its capacity to phosphoresce brilliantly at high temperatures. First invented in the 1700s, gas lighting became a major source of illumination in the early nineteenth century. By the 1830s much of Paris was lit by gas streetlights, earning the French capital its sobriquet “City of Light.” None of the strollers on the Boulevard Haussman in Belle Époque Paris realized that the streetlights contained one of the greatest sources of energy on Earth, one that helped power the fires at the core of the planet.

In fact, Marie Curie was not the first to discover that thorium is radioactive: that honor technically goes to Gerhard Carl Schmidt, a German chemist who reported the finding in the Deutsche Physikalische Gesellschaft in March 1898, a few weeks before Curie presented her paper to the French Academy of Sciences. Schmidt, however, delved no further into thorium’s mysterious properties; he promptly disappears from the story.

Marie Curie’s 1898 paper gave the magnitude of the current, in amperes, emitted by the substances she tested using Pierre’s electrometer and a piezoelectric quartz (piezoelectric refers to the ability of certain quartz crystals to transform kinetic energy into electrical current; tap one end with a hammer and a minute electrical current emerges from the other). To her surprise, the most active material was not purified uranium; it was raw pitchblende, followed by thorium oxide. Thorium, in fact, was nearly twice as potent as uranium (pitchblende was nearly four times as strong).

The astonished scientist realized that the ability to produce an electrical current in air was not unique to uranium; it was a fundamental property of the universe, found to varying degrees in many elements. Not only that, but the qualities of this strange activity could be used to discover new and even stranger elements.

With this understanding the revolution began. Marie Curie, though, had noticed another unique aspect of thorium, one that she did not pursue: the current produced by thorium oxide was not steady. Early on Curie realized that the activity in the electrometer would gradually increase when she was testing thorium. “She was intrigued enough to open the chamber up, renew the air, and make further measurements. Once again, she noticed a slight increase in activity within the chamber.”⁵

“Had these experiments been more clear-cut,” wrote Irene Curie, Marie’s daughter, many years later, “the entire orientation of future work might have been changed.”⁶

For the first of many times, thorium’s shadowy qualities were pushed aside in favor of uranium, which afforded more clearcut results. In a few more years the question of thorium’s unstable activity would arise again.

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WHILE MARIE CURIE WAS TESTING SUBSTANCES in her wooden electrometer, a young New Zealander working at the Cavendish Laboratory at Cambridge under J. J. Thompson, the discoverer of the electron, was rapidly turning over the tables of fusty British physics. The New Zealander was Ernest Rutherford (later the first Baron Rutherford of Nelson), a peerless deviser of experiments who worked with a series of gifted assistants. He essentially fathered twentieth-century experimental physics, discovering the nucleus, chronicling the transformation of elements, and revealing the power of the free neutron, all during 25 years of groundbreaking work. Born in 1871, he made many of his most famous discoveries after winning the Nobel Prize in Chemistry in 1908. In 1898, the year of Marie Curie’s discovery of radiation, Rutherford joined the faculty of McGill University in Canada—not exactly a hotbed of advanced physics research at the time—where he would carry out a famous series of experiments in the following decade that would transform the understanding of matter and lay the groundwork for the discovery of nuclear fission in the interwar years.

The Curies had declared that the mysterious source of high-powered radiation inside Joachimsthal ore was an entirely new element, which they christened radium. Although its dreadful effects would not become apparent for years, the therapeutic wonders of radium were understood almost immediately; used to shrink tumors and to effect various other, less tangible cures, it quickly became the focus of a burgeoning medical industry that brought not only fame to the Curies but also wealth, almost all of which they plowed back into the lab. After thorium, radium became the second radioactive element to generate a lucrative commercial industry.

Much about these invisible rays was still not understood, and Rutherford, along with an enterprising young British graduate student named Frederick Soddy, performed a series of ingenious and influential experiments to tease out their nature. Rutherford had already noticed the same effect that Marie Curie had: thorium’s activity seemed to fluctuate over time. In 1899 one of Rutherford’s McGill assistants confirmed that the radiation from thorium changed “in response to events as irrelevant as the opening of a door.” Apparently thorium was sensitive to “slight currents of air.” Intrigued by this “capricious variation” of thorium, Rutherford performed a series of experiments to study it. In the summer of 1899 Rutherford solved the puzzle “and in so doing shook the foundations of chemistry as it was then known.”⁷

He found that thorium emits a gaseous substance that is different from the thorium itself. Rutherford had discovered the alchemist’s dream: the transmutation of elements, only not in response to some magical solution but simply as part of the strange processes at the heart of the atom. He called the new substance an “emanation” and reported that it has the power “of passing through the thin layers of metals, and, with great ease, through considerable thicknesses of paper.” The radioactivity of the new substance decreased in a geometric progression: half of its original strength after one minute, half of that half in another minute, and so on. What’s more, the emanation (which Rutherford suspected was a gas) “possesses the power of producing radioactivity in all substances on which it falls,” and the new, induced radiation “is of a more penetrating character than that given out by thorium or uranium.” Rutherford called this new activity “excited radioactivity.”⁸

At almost the same moment, the Curies, their imaginations fired by Rutherford’s relentless experimentation, were also uncovering evidence of excited radioactivity caused by radium and polonium. Rutherford, enlisting the help of Soddy, continued his work on the thorium emanation. Rutherford noted that “the duration of the conductivity” in ionized gas in his testing apparatus would have been far less if the experiment had been performed on a compound of uranium. Thus the first distinctions between these two closely similar elements began to emerge.⁹

But Rutherford was after bigger fish. Surmising that thorium’s emanation consisted of some kind of radioactive gas, he set Soddy—fresh from Oxford, a talented chemist, and an experimentalist almost equal to Rutherford himself—to work discovering its identity. Through a series of painstaking chemical analyses—physics was far more laborious in the days before research reactors and particle accelerators—Soddy came to “the tremendous and inevitable conclusion that the element thorium was slowly and spontaneously transmuting itself into argon gas!”¹⁰

The emanation was in fact an inert gas incapable of reacting or combining with any other substance. In the fall of 1901 Rutherford and Soddy came to the conclusion that had been inescapable all along: the thorium was disintegrating. One element was being transformed into another. “Standing there transfixed as though stunned by the colossal import of the thing,” Soddy later recalled, he blurted out, “‘Rutherford, this is transmutation!’”¹¹

The two scientists fired off a paper to the Journal of the Chemical Society in London; in the paper they stated that radioactivity is “at once an atomic phenomenon and the accompaniment of a chemical change in which new kinds of matter are produced.” It was “one of the major discoveries of twentieth century physics,” Rhodes declares, and in it rested the destruction of Hiroshima and Nagasaki, the dawning of the Atomic Age, and the promise of thorium power.¹²

Working with thorium and other radioactive substances, Rutherford and Soddy in quick succession calculated the half-life of uranium and thorium and their decay products, defined the concept of an isotope (the term was Soddy’s), and distinguished between different forms of radiation: beta radiation comprises highly charged electrons, while alpha particles are actually helium atoms ejected during radioactive decay. After providing the emanations that enabled Rutherford to shatter the assumed indivisibility of the elements, thorium, as is its wont, sank from view, only to be taken up again a few decades later. In the interval I must pause to explain in more detail the nature of the atoms that Rutherford and his contemporaries were probing.

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IMAGINE A HARD PLASTIC SPHERE like a child’s ball. Now imagine a cluster of plastic balls, some colored red, some blue, all held together by an invisible but unfathomably strong glue. Other spheres (colored black, let’s say) circle this cluster in distant orbits and are held on their courses by some mysterious force, also powerful but many orders of magnitude weaker than the glue that holds the center cluster together.

This is the conventional representation of the atom, familiar to us from ten thousand logos. Like most such representations—including Niels Bohr’s early conception of the atom as a “liquid drop” with a smooth surface, it is inaccurate. So strange is the world of fundamental matter that it is literally impossible to picture it faithfully in our familiar physical terms. For one thing, the black spheres—electrons, discovered by Rutherford’s mentor J. J. Thomson in 1897—do not really orbit. (In fact, under the principles of quantum mechanics,* it is impossible to say exactly what they do.) For the purposes of an explanation of nuclear fission, though, it’s the best available image.

The red spheres are protons. The blue are neutrons; the black, as I have said, are electrons. Electrons are negatively charged, protons positive. Neutrons, as another Rutherford protégé, James Chadwick, demonstrated in 1932, have no charge at all. This is the key that would unlock the awesome energy at the heart of matter.

At the end of the nineteenth century, when scientists first began to understand that certain forms of matter emitted rays, or tiny fragments named particles, they knew that the positively charged protons (red) would necessarily attract the negatively charged electrons (black). Neither particle could move freely through intermatter space. There had to be a third particle, with “zero nuclear charge” (as Rutherford put it in a 1920 lecture to the Royal Society in London), that “should enter readily the structure of atoms, and may unite with the nucleus.” He would later describe the mysterious particle as “an invisible man passing through Piccadilly Circus—his path can be traced only by the people he has pushed aside.”¹³ This was the neutron, the blue ball, which can pass easily through the electron barrier to shatter or, in some cases, impregnate the nuclei of other atoms.

Some atoms, as I have said, decay spontaneously. In the process they give off three types of radiation: alpha (which consists of essentially helium ions, or the nuclei of helium atoms), beta (energetic electrons), or gamma (photons, with zero mass and no electric charge). Some are more prone to disintegrate than others when struck by neutrons. In the plastic ball model it is as if the nuclei of some elements—the heavy, radioactive ones on the bottom rows of the periodic table—are too densely packed, too loosely held together, to be stable. Balls fly off spontaneously as the cluster of spheres shifts to another form; some of the blue balls (neutrons) strike other clusters, disturbing them, and more balls fly outward in three-dimensional space. A certain number collide with more clusters and so on. If more than one blue ball (neutron) is ejected from every nucleus-struck ball, the reaction will continue almost ad infinitum.

When a stray neutron penetrates another atom’s nucleus, two things can happen: the atom splits (fission), or the atom absorbs the neutron to become a new isotope. I will explain further in chapter 3 how fission is controlled, and the speeding neutrons moderated, in a nuclear reactor; here I focus on the phenomenon itself. Experiments in harnessing fusion to drive generators and make electricity have been going on since the 1960s, to little avail. This was the dismal record that drove Kirk Sorensen to place his faith in the future of fission. So it is fission that I am explaining.

When the red and blue balls (protons and neutrons) split apart, they break the invisible glue (known as the strong force) and release incredible amounts of energy. Even before the full implications of fission were understood, physicists from Einstein onward realized how awesome that energy is. A single gram of matter contains 85 million British thermal units of heat, enough to generate 25 million kilowatt-hours of electricity. The trick to nuclear power is making that energy available without blowing everything to smithereens. Under the right conditions, the atoms of certain forms of matter—the fissile elements uranium and plutonium—can be induced to disintegrate in a predictable and orderly fashion. Because atoms are so tiny, the amount of energy released per collision is small—200 million electron-volts (MeV) or so—but there are many, many collisions, and their number increases exponentially if the reaction is uncontrolled. Unlike uranium, all of the thorium in a given volume can be “burned”—i.e., converted into fissile U-233 to power a reactor, meaning that thorium requires no expensive enrichment to be used as a nuclear fuel. Nobel laureate Carlo Rubbia estimates that one metric ton of thorium can produce as much energy as 200 metric tons of uranium.

It might appear, at first, that the faster and more energetic the neutrons are, the more effective they’ll be in shattering other atoms, but that’s not the case. Slower neutrons have a higher probability of interacting with other atoms. Physics instructors often use the baseball metaphor: the slower the pitch, the better chance the batter has of connecting with it. (In fact, conventional uranium reactors require a moderator—often graphite or plain water—to slow down the neutrons so as to increase the probability of further fission reactions enough to sustain the chain reaction in the core. See chapter 3 for more on how reactors work.) Here it’s worth keeping in mind the dimensions of the stadium around the batter.

One of the most astonishing discoveries of early twentieth-century physics was the vastness of inner space. As with the nonexistent orbits of the electrons, the metaphor of the solar system is often used to convey the gulfs of distance between the nucleus (the sun in this model) and the electrons (the planets). Another way to think of it is a large city plaza, say, St. Peter’s Square. If the atom were the size of the square, the nucleus would be the size of a grain of sand.

Put another way, a cubic meter of lead contains less than 1 percent actual matter, in the form of subatomic particles. The rest is vacuum. In the plastic ball model, the black balls are very, very far from the tiny cluster of red and blue balls at the center.

Nevertheless, the chances of one neutron smashing into another atom, in a common material like thorium-232 or uranium-238, are quite high. But the chances of fission happening, as opposed to absorption, are relatively low. That’s why naturally occurring uranium decays, but the ore does not fission: a much denser material—a critical mass—or a higher percentage of U-235 is required to produce the explosive chain reaction of a nuclear warhead. And thorium won’t chain-react at all, unless it transmutes into U-233. An image favored by nuclear engineers is a room filled with hundreds of mouse traps baited with ping-pong balls. When one trap goes off, the ping-pong ball flies off and hits another, setting it off, which in turn releases a ball that springs another trap. If the traps are packed closely enough, a single one sprung sets off a chain reaction until almost all go off. If they’re too far apart, as in natural uranium, the reactions quickly peter out.

By the last few years of the 1930s, as fascism swept across central Europe and war became inevitable, most of this had been understood and documented. Physicists had understood that new, transuranic (beyond uranium) elements could be created by bombarding heavy elements with neutrons. They knew that certain elements, including thorium and uranium, spontaneously decayed. At the end of 1938, thanks to the work of the radiochemist Otto Hahn, they realized that a neutron collision can overcome the tremendous binding force of the nucleus and split an atom apart—a process that the Austrian physicist Otto Frisch dubbed fission. Only one mystery remained, the solution of which would lead directly to Hiroshima, the nuclear arms race, and the dawn of nuclear power: Could fission be induced and prolonged to create a self-sustaining nuclear reaction?

The answer, as with other breakthroughs in this period of astonishingly rapid advances, would come through close examination of the related-yet-opposing qualities of uranium and its sister element, thorium.

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ON A VISIT TO THE UNITED STATES in early 1939, Niels Bohr had a flash of insight that would change the understanding of fission and the power of radioactive elements. Over breakfast one morning at the Nassau Club at Princeton, Bohr was discussing his liquid drop model of the atom with a skeptical Polish physicist named George Placzek.

Placzek pointed out a fundamental inconsistency in the prevailing understanding of the fission characteristics of uranium and thorium: while both tended to fission when bombarded by high-energy, fast neutrons (of more than one MeV), only uranium atoms split under bombardment by slow neutrons. Thorium, more resistant than uranium, was impervious to slow neutrons. “If the liquid-drop model had any validity at all, the difference made no sense,” Placzek argued.¹⁴

One of Bohr’s outstanding talents was the ability to shift course when presented with evidence that contradicted his notions. By this time he was less an active experimentalist than a teacher and leader, a mentor to younger scientists, and a guide and supporter of promising new research. But he was still capable of remarkable leaps of intuition. That day he had another such leap. He and Placzek hurried to Bohr’s office.

“Now listen,” Bohr exclaimed. “I have it all.”

He drew a series of graphs with the horizontal axis showing neutron energy, left to right, and the vertical axis depicting the cross section of nuclear reactions—the probability of a certain reaction’s occurring, increasing as the graph climbed. The affinity of nuclei to neutrons of a certain energy is called resonance. Both thorium and uranium are resonant with neutrons at more or less the same levels of energy, capturing them readily. Bohr was plotting the resonance curve of the two elements, or how they responded to neutrons of varying energies. He drew one graph for thorium, one for uranium-238, and one for uranium-235, the lighter and more rare isotope. The first two graphs were identical; the one for U-235 was quite different.

Up until this point, physicists had assumed that the differences in the fission profiles of thorium and uranium were more or less inconsequential. While each had its specific advantages for certain types of experiments, the differences had been considered an artifact of no great import. Placzek’s questions reexamined that assumption, and Bohr’s graphs disproved it. By comparing the resonance curves of thorium and uranium, he established that inside uranium there lurked a demon, an isotope that would burst apart upon encountering any neutron, of any energy. Tickle it and it would explode. Bohr understood that this tiny fraction of natural uranium was what makes a chain reaction—and thus a bomb—theoretically possible. Bohr rushed out a paper in early 1939.¹⁵ The invasion of Poland was less than seven months away. World War II had not yet begun, but Bohr had just opened the last gates to the scientific breakthrough that would decide it. Separating out the U-235, however, was fiendishly difficult, so much so that “it would take the entire efforts of a country to make a bomb,” Bohr stated. He was, of course, right.¹⁶

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THE FINAL STOP ON THIS THORIUM-BASED TOUR of nuclear physics history is Berkeley, California, where in the late 1930s Glenn Seaborg found and described the transuranic elements. The transuranics are elements with atomic numbers higher than 92. None occurs at more than trace levels in nature. By this time nuclear testing equipment had progressed far beyond Pierre Curie’s crude electrometer. Working on the Lawrence Cyclotron at the University of California at Berkeley, Seaborg and his graduate student assistant were able to bombard any element of their choosing with neutrons and analyze the results with exquisite precision.¹⁷

The career of Glenn Seaborg, one of the most remarkable scientist-administrators ever produced by the United States, exhibits the Janus-faced quality of many of the pioneers of nuclear physics: his work looked in one direction to developments that would vastly improve people’s lives and gazed in the other at the ultimate destruction of human society by nuclear weapons. The discoverer or codiscoverer of ten elements, he was a pioneer in nuclear medicine, the use of radioactive isotopes to diagnose and treat disease. He was the discoverer of plutonium, and he found uranium-233, the fissile isotope into which thorium-232 transforms and that has the potential to solve our current energy crisis and fuel a new era of inexpensive carbon-free energy.

After getting his Ph.D. at Berkeley in 1937, Seaborg set out in the footsteps of Frederic Soddy to uncover and investigate different isotopes of radioactive elements. At that time scientists understood that transuranic elements were not only possible but also, according to the tenets of theoretical physics, had to exist. There had to be an element 93 and an element 94, but no one had managed to produce them in the lab or to understand their properties. With his colleague John Livingood, in 1938 Seaborg had already created a new isotope of iodine, iodine–131, which is still used today to treat thyroid disease. Now he was hunting a very different beast.

In 1940 Seaborg was part of the team led by Edwin McMillan that discovered element 93, which they named neptunium. Then McMillan was called away to work on the development of radar technology for the war effort and granted his protégé permission to take the next step: the search for element 94.

Seaborg had perfected the oxidation-and-reduction technique, conceived by McMillan, that had tracked down neptunium. Now he and his colleagues used the same method to show that a new element could be formed from the induced decay of neptunium. In February 1941, after bombarding a sample of uranium, they isolated plutonium-239, the long-sought ninety-fourth element. A month later Seaborg showed that plutonium, like U-235, is highly fissile. Because separating U-235 from U-238 was such a difficult and costly process, the realization that plutonium, produced through the relatively simple bombardment of uranium with neutrons, was fissile led directly to the Manhattan Project and to the plutonium bomb that destroyed Nagasaki.

Seaborg understood the gravity of his discovery. He spent much of the rest of his life working for arms control. More immediately he turned to thorium.

Seaborg was curious whether thorium could be transformed into a fissile isotope. He knew of another isotope of uranium, U-233, which is found in trace amounts in nature but is not part of the direct decay chain of natural U-238. Suspecting that it might arise from thorium, he instructed his grad students to turn the cyclotron on thorium-232.

First they produced thorium-233, which had a half-life of only 23 minutes, decaying to protactinium, element 91. Little was known about protactinium at that time, other than that it had properties similar to zirconium (atomic number 40) and a half-life of about 27 days. Logically, because the protactinium decayed with the emission of beta radiation (an electron), the next step on the decay chain had to be one unit higher on the periodic table: uranium-233. The sequence looks like this:

Th-232 Th-233 Pa-233 U-233

And, sure enough, after bombarding a large amount of thorium, letting the protactinium decay, and concentrating the result, Seaborg and his team showed that they had created U-233, effectively a new isotope of uranium. When they tested it under bombardment, they discovered that not only was it fissile, but it produced more than two neutrons per fission reaction.

Though the full implications of his discovery took a while to dawn among other physicists, Seaborg grasped them readily. The creation of U-233 from thorium, he predicted, would be “a $50 quadrillion discovery.” Once again, though, thorium’s potential went untapped: it took Seaborg nearly 20 years to get around to patenting a commercial process for producing U-233. Under patent number 2951023, a body of thorium carbonate is compressed into a dense pellet and placed into the outer blanket of a thermal nuclear reactor. Once the ratio of U-233 to Th-232 reaches about 1:100, the pellet is removed from the core. Voilá: a limitless energy source, bred in a reactor.

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WITH THAT, LIKE A SWIMMER dropping beneath the waves of a troubled sea, thorium effectively disappeared from mainstream scientific research for six decades or so. Uranium, progenitor of both the Fat Man and Little Boy bombs, took over the show. The discoveries of Curie, Rutherford, Bohr, and Seaborg led to the creation of the largest scientific R&D project ever known, the Manhattan Project. Under the direction of Robert Oppenheimer, scientists raced to build an atomic bomb before Nazi Germany focused exclusively on uranium as the fuel source. Raw uranium was secretly imported from the rich uranium mines at Shinkolobwe in the Congo. At Oak Ridge in Tennessee and Hanford in eastern Washington, great industrial plants were built and run 24 hours a day to achieve vanishingly small amounts of highly enriched uranium (HEU). U-235 and plutonium instantly became the most valuable materials on Earth. By the end of the war, the two remaining superpowers were building cities where they developed bombs to destroy their enemies’ cities, and uranium took its place as the only element considered a potential fuel for both bombs and power plants.

There was a brief sideshow, however, that revealed the combatants’ burgeoning obsession with radioactive materials and the deep paranoia they engendered. Late in the war, anticipating the fall of the Reich, the United States created an intelligence team, code-named Alsos, to uncover the details of the German atomic bomb program. (Blocked by a lack of resources and indifference on the part of the Fuhrer, that program had fizzled out by 1944. The Allies didn’t know that.) Greek for grove, Alsos was named for Lieutenant General Leslie Groves, the gruff military head of the Manhattan Project. After the war the Alsos team would cross Germany in search of evidence of nuclear weapons production. Before that, as the Reich held out against the Allied invaders, Alsos agents collected intelligence and rumor in German-occupied Europe.

One of the German companies involved in procuring and processing uranium for weapons research was the Berlin chemicals conglomerate Auergesselschaft, which, among other things, had helped develop thorium mantles for gas lanterns as well as one of the earliest commercially produced lightbulbs, made from the element osmium and wolfram, or tungsten. Auergesselschaft’s roots reach deep in the history of nuclear physics; the scientific director, Nikolaus Riehl, had studied under Lise Meitner and Otto Hahn, the discoverers of nuclear fission.

In the fall of 1944, as Allied troops closed in on Berlin, Alsos agents in France learned that the German company had confiscated a French firm, Terres-Rares, soon after the Nazis occupied Paris. Terres-Rares had one of the world’s largest stockpiles of thorium. After D-Day the Germans had arranged to ship hundreds of tons of Terres-Rares thorium east into the Reich. When Alsos arrived at Terres-Rares’s Paris office after liberation, it was empty. According to Groves, a German chemist named Jansen was in charge of bringing the thorium to Berlin, but he was captured at the French-German border. In his briefcase was a pile of documents that included a file on the city of Hechingen, the center of atomic research in Germany. At Hechingen German researchers under Werner Heisenberg had built an isotope separation unit and an experimental pile—a rudimentary nuclear reactor—in a cave. A ton and a half of uranium was found buried in a nearby field. Adding two plus two to make six, the Alsos field agents concluded that the Nazis had gotten far closer to building an actual bomb than was previously understood. The agents also concluded that the Nazis had found a way to use thorium to make weapons-grade material.¹⁸

In the postwar years a less alarming explanation emerged: Auergesellschaft officials, realizing that the end of the Reich was near and that selling HEU to the German military was probably not a viable future business model, had decided to diversify, moving into cosmetics and other consumer products. Thorium had already been used to make toothpaste; in fact, Auer had a patent for it and an ad slogan: “Have sparkling, brilliant teeth—radioactive brilliance!”

The case of the purloined thorium had all the elements of a thriller. The real story was more pedestrian, but its coda had important implications for the Cold War. After Berlin fell, the Red Army sent its own detection team after German nuclear secrets. Equipment, technology, and even unlucky personnel were sent east, behind what would come to be known as the Iron Curtain. Along with intelligence from spies inside the Manhattan Project, Auergesellschaft helped the USSR fast-track its own atomic bomb program. The Soviet Union exploded its first nuclear weapon in 1949, a decade earlier than U.S. experts had predicted. The company itself was later taken over by a U.S. corporation, Mine Safety Appliances Corporation.

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DESPITE ITS PROVEN EFFICACY as a nuclear reactor fuel, thorium was relegated to various industrial uses for the next six decades. Gas lanterns continued to be produced with illuminators known as Welsbach mantles, built from thorium oxide (ThO₂) and cerium oxide. Used as a catalyst in a variety of chemical processes, including, ironically, the cracking of petroleum products (the breaking down of complex organic molecules into simpler forms, including light hydrocarbons), thorium is also a key ingredient in the production of high refractive index glass, used in high-end camera lenses. Its extremely high melting point, about 3,300 degrees centigrade, makes it an ideal material for high-temperature crucibles.

All of which is akin to using a Ferrari to drive to the corner market for milk. Despite the wide understanding of thorium’s energy potential, despite years of research on thorium and molten salt reactors at one of the preeminent nuclear labs in the United States, despite the evident problems raised by uranium reactors over the decades, a revival of interest in thorium as a source of energy would have to wait until the twenty-first century and the so-called renaissance of nuclear power.

 

* The Nuclear Energy Agency is a division of the Paris-based Organization for Economic Cooperation and Development.

* Werner Heisenberg’s Uncertainty Principle states that it is impossible to accurately measure both the location and the momentum of a particle.