by Neal Bascomb
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 effect 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.”
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nbsp; 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 called 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.
Once they had enough uranium and an effective moderator, Heisenberg concluded, it was simply a question of calculating the machine’s most efficient size (quantity of uranium and moderator), arrangement (mixed together or layered), and shape (cylindrical or spherical). His initial figures indicated that a sphere filled with at least a ton each of uranium and the chosen moderator separated in layers would be optimal. It was going to be big, but it would work.
Heisenberg gave Diebner the direction he needed to move forward—and the Nobel laureate’s reputation contributed to persuading others to follow this path. Experiments would continue to separate U-235, but most of the effort was now focused on building the uranium machine. If they were successful, they would prove at last the importance—and utility—of atomic physics. Constructing a bomb would follow.
In recognition of the Uranium Club’s work, Diebner was named head of the Kaiser Wilhelm Institute of Physics in Berlin, a body of preeminent reputation and the country’s most advanced laboratory. Heisenberg was appointed to the board as scientific adviser, to placate those who were upset at having Diebner, a physicist of no renown, directing the august institute.
By year’s end Diebner had dozens of scientists under his watch across Germany refining the uranium-machine theory and building the first small experimental designs. Progress had been made outfitting laboratories and ordering uranium oxide and other key materials.
Although the issue needed further study, the scientists’ calculations indicated that heavy water was the best presently known moderator. The Uranium Club would require a steady, robust supply of the precious liquid. Unfortunately, the world’s sole producer, Norsk Hydro’s Vemork plant, was far away in an inaccessible valley in Norway, a country whose neutral status in the war made it an unreliable partner. The plant had also only recently restarted heavy water production in November 1939 and could supply little more than ten kilograms a month. Diebner considered building a full-scale heavy water plant in Germany, though it would cost tens of millions of marks and consume a hundred thousand tons of coal for each single ton of heavy water. Before he made any such move, however, he and Heisenberg agreed that they needed to make sure heavy water was a viable moderator. For those experiments, twenty-five kilograms should do. Diebner had a representative of the German conglomerate IG Farben, which owned 25 percent of Norsk Hydro, put in the order, to conceal the involvement of Army Ordnance.
By January 1940, with more physicists in his group asking for their own supply, prospective orders had grown to one hundred kilograms a month, every month. Norsk Hydro wanted to know the purpose of such a large order, but with experiments using heavy water now labeled SH-200, a high-level military secret, the IG Farben representative offered only silence.
Not long after, the Norwegians did find out, from Jacques Allier, what that purpose was: the potential development of an atomic bomb.
When Allier visited Vemork on March 5, 1940, he presented himself merely as an official with Banque de Paris. Axel Aubert led the meeting with the plant’s lead engineer, Jomar Brun. From previously unsold supplies and the restart of production, the plant had a total of 185 kilograms at hand. All of it, Aubert told Brun, needed to be transported secretly to Oslo by truck. Brun asked to know why, just as he had asked why when Aubert had quietly told him earlier in the year to explore a fivefold increase in production to 50 kilograms a month. As before, Aubert declined to answer any questions and instructed that not a word of this special order was to be mentioned to anyone.
After settling these arrangements and finding a Rjukan welder to make twenty-six stainless-steel flasks that would fit neatly into suitcases, Allier returned to Oslo with Aubert to conclude their negotiations and prepare for spiriting the flasks out of Norway. The Norsk Hydro director general offered to give France the heavy water on loan, with no price attached, and told Allier that Norsk Hydro would provide France with first claim on what was produced in the future. Impressed by this generosity—and the alacrity with which Aubert had moved—Allier opened up about the intended use of the heavy water by Frédéric Joliot-Curie and his team.
On March 9, two trucks departed Vemork down the steep, ice-slick road. Brun rode in the first truck. At a nondescript house in Oslo, they unloaded the twenty-six flasks and entrusted them to Allier’s care. The house was owned by the French government and was a stone’s throw from an Abwehr safe house, but sometimes it was best to hide in plain sight.
To smuggle out the supply, Allier had grand visions of a submarine sneaking into Oslofjord and spiriting it away, but instead he settled for the old “bait and switch,” supported by the three French spies he had recruited in Stockholm. Through several ticket agents and under various assumed names, they booked flights on two planes leaving Oslo’s Fornebu airport at roughly the same time on the morning of March 12. One was headed to Amsterdam, the other to Perth, in Scotland. In case anything went wrong, they also bought seats on the same flights for two consecutive days after that.
At dawn on March 12, a frigid and cloudless morning, Allier and a fellow spy, Fernand Mosse, took a taxi three miles south of the city center to Fornebu. Dressed as businessmen, they made a big show of their upcoming trip to Amsterdam in front of the gate agents and baggage handlers who took their several large, heavy suitcases. Soon enough, they were crossing the tarmac toward the Junkers Ju-52 aircraft designated for their flight. Adjacent to their plane was an identical one, destined for Perth.
Once they were sure their baggage was loaded onto the Amsterdam-bound plane and its propellers began to spin, they readied to board. At that moment, a taxi drove onto the tarmac. Its passenger, Jehan Knall-Demars, another of Allier’s team, had pleaded with the
gate agent to let him through in the taxi so he could make his plane to Amsterdam. The spy had the taxi park between the two Junkers, out of view of the Fornebu terminal. From the trunk, he unloaded several suitcases, together containing thirteen of the heavy-water flasks. These were hauled into the baggage hold of the Scottish-bound plane, which Allier and Mosse boarded, instead of the one to Amsterdam. Knall-Demars left in the taxi, hiding in the back as he passed through the gate.
Minutes later, the plane to Amsterdam barreled down the runway and lifted into the sky. As it headed south over the Skagerrak, the strait of sea between Norway and Denmark, a pair of Luftwaffe fighters drew alongside. They ordered its pilots to divert their course to Hamburg. When the plane landed in Germany, Abwehr agents busted open the cargo hold. Rummaging through the suitcases, they found a few that were particularly heavy. Inside them? Granite rubble.
Meanwhile, Allier and Mosse landed safely in Scotland with their stash. The following day, Knall-Demars arrived with the other thirteen flasks.
By March 18, all twenty-six flasks were stored in the old stone-arched cellars of the Collège de France in Paris. The first battle of heavy water was won. The next, however, was all too shortly to begin.
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About the Author
Neal Bascomb is the national award-winning and New York Times best-selling author of The Winter Fortress, Hunting Eichmann, The Perfect Mile, Higher, The Nazi Hunters, and Red Mutiny, among others. A former international journalist, he is a widely recognized speaker on the subject of war and has appeared in a number of documentaries. He lives in Seattle.