The Winter Fortress

by Neal Bascomb

  A fraction of this water, which had by now coursed from the Vidda to Lake Møs through tunnels, then pipelines, then electrolysis cells, was sent through a cascade of specialized electrolysis cells that terminated in a basement cellar at Vemork. The water was then reduced and further reduced until it amounted to a steady drip similar in output to a leaky faucet. This water was now something unique and precious. It was heavy water.

  The American chemist Harold Urey won the Nobel Prize for his 1931 discovery of heavy water. While most hydrogen atoms consist of a single electron orbiting a single proton in the atom’s nucleus, Urey showed that there was a variant, or isotope, of hydrogen that carried a neutron in its nucleus as well. He called this isotope deuterium, or heavy hydrogen, because its atomic weight (the sum of an atom’s protons and neutrons) was 2 instead of 1. The isotope was extremely rare in nature (.015 percent of all hydrogen), and there was just one molecule of heavy water (D2O) for every 41 million molecules of ordinary water (H2O).

  Building on Urey’s work, several scientists found that the best method for producing heavy water was electrolysis. The substance didn’t break down as easily as ordinary water when an electric current ran through it, so any water remaining in a cell after the hydrogen gas was removed was more highly concentrated with heavy water. But generating the substance in any quantity demanded tremendous resources. A scientist noted that in order to produce a single kilogram (2.2 pounds) of heavy water, “50 tons of ordinary water had to be treated for one year, consuming 320,000 kilowatt hours [of electricity], and, then, the output had a purity no better than about ten percent.” That was a lot of electricity for a low level of purity in a very small quantity of deuterium.

  In 1933 Leif Tronstad, a celebrated young Norwegian professor, and his former college classmate Jomar Brun, who ran the hydrogen plant at Vemork, proposed to Norsk Hydro the idea of a heavy water industrial facility. They weren’t exactly sure what the substance might be used for in the end, but as Tronstad frequently said to his students, “Technology first, then industry and applications!” They did know that Vemork, with its inexhaustible supply of cheap power and water, provided the perfect setup for such a facility.

  They matched the plant’s natural advantages with an ingenious new design for the equipment. An early working plant, designed by Tronstad and Brun, had six stages. Think of a group of cans stacked in a pyramid. Now picture that pyramid upside down, with the single can at the bottom. In the Tronstad/Brun design, water flowed into the top row of cans—really 1,824 electrolysis cells, which treated the water (mixed with potash lye as a conductor) with a current. Some of the water was decomposed into bubbles of hydrogen and oxygen gas that were vacated from the cells, and the remainder, now containing a higher percentage of heavy water, cascaded down to the next row of cans in the pyramid (570 cells). Then it repeated the process through the third (228 cells), fourth (20 cells), and fifth (3 cells) rows of electrolysis cells. However, by the end of the fifth stage, with a huge amount of time and power exhausted, the cells still contained only 10 percent deuterium-rich water.

  Then the water cascaded into the bottom can of the pyramid. This sixth and final phase was called the high-concentration stage. Set in the cavernous, brightly lit basement of the hydrogen plant, it actually consisted of seven unique steel electrolysis cells lined up in a row. These specialized cells followed a similar cascade model to concentrate the heavy water in each cell. But they could also recycle the gaseous form of deuterium back into the production process, while it was essentially wasted in the other stages. As a result, the heavy water concentration rose quickly from one cell to the next. By the seventh, final cell in this high-concentration stage, the slow, steady drip had been purified to 99.5 percent heavy water.

  When the Vemork plant started production in earnest using this method, scientists around the world heralded it as a breakthrough, even though heavy water’s application remained uncertain. Because it froze at four degrees Celsius instead of zero, some joked it was only good for creating better skating rinks. Tronstad, who served as a consultant to Norsk Hydro and left the running of the plant to Brun, believed in the potential of heavy water. He spoke passionately of its use in the burgeoning field of atomic physics, and of its promise for chemical and biomedical research. Researchers found the life processes of mice slowed down when they were given minute amounts of heavy water. Seeds germinated more gradually in a diluted solution—and not at all in a pure one. Some believed that heavy water could lead to a cure for cancer.

  Vemork shipped its first containers of heavy water in January 1935 in batches of ten to one hundred grams, but business did not boom. Laboratories in France, Norway, Britain, Germany, the United States, Scandinavia, and Japan ordered no more than a few hundred grams at a time. In 1936 Vemork produced only forty kilograms for sale. Two years later, the amount had increased to eighty kilograms, a trifling amount valued at roughly $40,000. The company placed advertisements in industry magazines to little avail: there simply wasn’t sufficient demand.

  In June 1939 a Norsk Hydro audit of this small sideline business showed it to be a loser. Nobody wanted heavy water, at least not enough to make it worth the investment, and the company abandoned the venture.

  But only months after Brun shut off the lights in the basement and dust started to gather on the seven specialized cells in the high-concentration room, everything changed—and quickly—just as it had in the field of atomic physics.

  For decades, scientists had been plumbing the mysteries of “atoms and void,” which was how the ancient Greeks described the makeup of the universe. In dark rooms, experimenters bombarded elements with subatomic particles. Theoreticians made brilliant deductions on the blackboard. Pierre and Marie Curie, Max Planck, Albert Einstein, Enrico Fermi, Niels Bohr, and other scientists discovered an atomic world full of energy and possibilities.

  The English physicist Ernest Rutherford observed that heavy, unstable elements such as uranium would break down naturally into lighter ones such as argon. When he calculated the huge amount of energy emitted during this process, he realized what was at stake. “Could a proper detonator be found,” he suggested to a member of his lab, “a wave of atomic disintegration might be started through matter, which would indeed make this old world vanish in smoke . . . Some fool in a laboratory might blow up the universe unawares.”

  Then, in 1932, another English scientist, James Chadwick, discovered that proper detonator: the neutron. The neutron had mass, but unlike protons and electrons, which held positive and negative charges respectively, it carried no charge to hinder its movement. That made it the perfect particle to send into the nucleus of the atom. Sometimes the neutron was absorbed; sometimes it knocked a proton out, transforming the chemical element. Physicists had discovered a way to manipulate the basic fabric of the world, and with this ability, they could further investigate its many separate strands—and even create some of their own.

  Using radon or beryllium as neutron sources, physicists began flinging neutrons at all kinds of elements to produce changes in their nature. Led by the Italian Fermi, they found this process particularly effective when the neutrons had to pass through a “moderator” of some kind, which slowed their progress. Paraffin wax and plain water proved to be the best early moderators. Both contained lots of hydrogen, and when these hydrogen atoms collided with the neutrons (which had the same mass), they stole some of their speed, much like when two billiard balls collide. Bombarding uranium with neutrons in this manner brought the most mysterious results, including the unexpected presence of much lighter elements.

  In December 1938 two German chemists, the pioneering Otto Hahn and his young assistant Fritz Strassmann, proved that a neutron colliding with a uranium atom could do more than chip away at its nucleus or become absorbed within it. The neutron could split the atom in two—a process called fission. By early January 1939, word of the discovery had spread, bringing great excitement to the field of atomic research: Why, how, and to what effe
ct had the uranium atom split?

  Springboarding off an observation by the Danish theorist Niels Bohr, physicists realized that the uranium atom’s nucleus had acted like an overfilled water balloon. Its “skin” was stretched thin by the large number of protons and neutrons inside, and when a neutron was shot into it, it formed a dumbbell: two spheres connected by a thin waist. When the tension on the skin finally became too much, it snapped, and the two spheres—two lighter atoms—were flung apart with tremendous force, an amount equal to the energy that had once held the nucleus together (its binding energy). Researchers were quick to come to a figure, too: 200 million electron volts—enough to bounce a single grain of sand. A tiny amount, perhaps, but given that a single gram of uranium contained roughly 2.5 sextillion atoms (2.5 x 1021), the numbers alone obscured the potential energy release. One physicist calculated that a cubic meter of uranium ore could provide enough energy to raise a cubic kilometer of water twenty-seven kilometers into the air.

  The atom’s potential power became even clearer when scientists discovered that splitting the uranium nucleus released two to three fast-moving neutrons that could act as detonators. The neutrons from one atom could split two others. The neutrons from these two split four more. The four could cause the detonation of eight. The eight—sixteen. With an ever-increasing number of fast-moving neutrons flinging themselves about, splitting atoms at an exponential rate, scientists could create what was called a chain reaction—and generate enormous quantities of energy.

  Which prompted the obvious question: To what purpose? Some conceived of harnessing the energy release to fuel factories and homes. Others were drawn to—or feared—its use as an explosive. Within a week of Hahn’s discovery, American physicist J. Robert Oppenheimer sketched a crude bomb on his blackboard.

  Fermi, who had immigrated to the United States, trembled at the thought of what might come. Staring out the window of his office at Columbia University, he watched students bustling down the New York sidewalks, the streets crowded with traffic. He turned to his office mate, drew his hands together as if holding a soccer ball, and harked back to the words of Rutherford. “A little bomb like that,” he said solemnly before looking back outside, “and it would all disappear.” Given the aggression shown by Nazi Germany by the end of summer 1939, such a bomb, if it could be built, might be needed in a world on the precipice of war. Plans to obtain it were rapidly put together on both sides.

  By annexing Austria and occupying Czechoslovakia, Adolf Hitler had managed to pursue his goals without a fight until September 1, 1939, when at 4:45 a.m. his 103rd Artillery Regiment sent its first “iron greetings” into Poland. Panzer tanks swept across the border and bombers shot eastward overhead. The German Blitzkrieg had begun and, Hitler promised, bombs would be met with bombs.

  Britain and France responded with a declaration of war. On September 3, Winston Churchill, First Lord of the Admiralty, rose in the House of Commons and said, “This is not a question of fighting for Danzig or fighting for Poland. We are fighting to save the whole world from the pestilence of Nazi tyranny and in defense of all that is most sacred to man.”

  Less than two weeks later, on September 16, German scientist Kurt Diebner sat in his office at the headquarters of Berlin’s Army Ordnance Research Department, Hardenbergstraße 10, waiting for the eight German physicists he had ordered to report for duty a few days before. “It’s about bombs,” he told the recruit who drafted the list of attendees.

  Thirty-four years of age, Diebner was a loyal Nazi Party member with a presence as modest and retreating as his hairline. His suit fit too tightly over his short, thin frame, and he wore round schoolboy spectacles that constantly threatened to slip off his nose. In meetings, his words came out halting and unsure. But despite his appearance and manner of speech, he was an ambitious and eager man.

  Born into a working-class family outside the industrial city of Naumburg, Diebner got himself into university by dint of hard work and cleverness. First at Innsbruck, then Halle, he studied physics. While some of the other students dined out and had the means to care about the cut of their suits, he lived a threadbare existence. Drawn to the experimental side of physics, he worked diligently in the laboratory, his aim being to find a position as a university lecturer—and to achieve the salary and prestige that came with it. While a student at the University of Halle, Diebner joined an esteemed fencing club, an important rung on the social ladder, and earned several scars on his face from duels.

  Diebner gained his PhD in atomic physics in 1931. In 1934, the year Hitler became the führer of Germany, he joined the Army Ordnance Research Department, where he was tasked with developing hollow-shaped explosives. For years he pushed his boss to allow him to create an atomic research division instead. Such work, he was told, was “malarkey,” with no practical use. But rapid advances in the field in 1939 made it clear that atomic physics was anything but malarkey, and Diebner was finally given the mandate to form a team.

  When those among the best and brightest in German science arrived at Hardenbergstraße that mid-September day, they carried suitcases, not sure of where they were going to be sent. When they saw that it was Diebner who greeted them, they shook his hand enthusiastically, knowing that at least they were not to be delivered to the front. They assembled in a conference room and were told that German spies had discovered that the United States, France, and Great Britain were pursuing projects in nuclear fission. This much was already well known to the attendees. They had all read, and some had contributed to, the rush of international journal articles on the subject. Now that war had been declared, the curtain on this open theater of science had fallen. Diebner informed them that they had been called together to decide whether or not it was possible, in practice, to harness the atom’s energy for the production of weapons or electricity.

  One of the men in the room was already dedicated to the former goal. In April, Paul Harteck, a physical chemist at the University of Hamburg, had sent a letter to the Reich Ministry of War explaining recent developments in nuclear physics. In his estimation, he wrote, they held the “possibility for the creation of explosives whose effect would excel by a million times those presently in use . . . The country which first makes use of [this explosive] would, in relation to the others, possess a well-nigh irretrievable advantage.” Harteck believed the assembled group should pursue any such advantage.

  Otto Hahn, on the other hand, was distraught that his discovery was now being developed into a weapon to kill. He tried to extinguish any enthusiasm for the endeavor by pointing to the many technical challenges involved in engineering an explosive or designing a machine to produce energy.

  He noted from recent studies that it was the atoms of the rare uranium isotope U-235 (atomic weight 235: 92 protons, 143 neutrons) that fissioned most readily. Meanwhile, its more common cousin, U-238 (92 protons, 146 neutrons), tended to absorb neutrons that struck its nucleus, stealing their potential to foster a chain reaction. And unless fast-moving neutrons released from a split atom were properly slowed, the probability of U-235 fissioning was small. Natural uranium was made up of only seven parts U-235 for every thousand parts of U-238, and no method to separate the two isotopes existed. Furthermore, they would need to find an efficient moderator for U-235. Given all this, and likely other unseen challenges, Hahn believed that attempting to harness the atom for use in the current war was a fool’s errand.

  The debate continued for hours, until the scientists finally reached a consensus: “If there is only a trace of a chance this can be done, then we have to do it.”

  Ten days later, on September 26, Diebner called another meeting of his “Uranium Club.” This time Werner Heisenberg attended. Heisenberg was considered the leading light of German theoretical physics, particularly after Hitler’s rise had forced Albert Einstein and other Jewish physicists to flee the country. Initially, Diebner had resisted his inclusion in the group, because he wanted experimenters, not theoreticians, and because Heisenberg had c
alled Diebner’s academic research “amateurish.” But those Diebner did recruit urged him to reconsider: Heisenberg had won his Nobel Prize at the tender age of thirty-one, and he was too brilliant to leave out.

  Heisenberg proved to be a useful addition to the club. By the end of that meeting, the group had its orders. Some, like Harteck, would investigate how to extract sufficient quantities of U-235 from natural uranium. Others, like Heisenberg, would hash out chain reaction theory, both for constructing explosives and generating power. Still others would experiment with the best moderators.

  Heisenberg made quick work with the theory. By late October he’d started on a pair of breakthrough papers. If they separated the U-235 isotope and compressed sufficient quantities into a ball, the fast-moving neutrons would set off an immediate chain reaction, resulting in an explosion “greater than the strongest available explosives by several powers of ten.” Isotope separation, Heisenberg declared, was “the only way to produce explosives,” and the challenges of such separation were legion. But constructing a “machine” that used uranium and a moderator to generate a steady level of power was an attainable goal. After the machine went critical, the number of chain reactions would stabilize and it would sustain itself. The amount of U-235 was still key: they would need an enormous quantity of natural uranium in its processed purified form—uranium oxide—to provide suitable amounts of the rare fissile isotope.

  On the subject of moderators, Heisenberg dismissed plain water as an option. Its hydrogen atoms slowed the neutrons enough to promote the fissioning of U-235, but they also captured them at too high a rate. This left two known candidates: graphite, which was a crystalline form of carbon, and heavy water. In graphite, the carbon atoms acted as the moderator; in heavy water, it was deuterium. Both should prove effective in slowing neutrons down sufficiently and reducing to a minimum the number of neutrons parasitically absorbed.