by Neal Bascomb
For decades, scientists around the globe had been plumbing the mysteries of “atoms and void,” which was how the ancient Greeks described the substance that made up everything in the universe. Experimentalists bombarded elements with subatomic particles in dark laboratories. Theoreticians made brilliant deductions on their blackboards. Pierre and Marie Curie, Max Planck, Albert Einstein, Enrico Fermi, Niels Bohr, and other intellectual giants discovered a world full of energy that could be manipulated by humans.
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 released 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 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 shoot into the nucleus of the atom. Sometimes the neutron was absorbed; sometimes it knocked a proton out of the nucleus, which converted that element into a different one. For instance, when a nitrogen atom (with 7 protons in its nucleus, i.e., its “atomic number” on the periodic chart of elements) lost a proton, it became an isotope of carbon (atomic number 6). 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 strands of their own.
Soon they began flinging neutrons at all kinds of elements to transform their natures. 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, the moderators stole some of the neutrons’ speed, much like when two billiard balls collide. That allowed more opportunity for the atomic reaction to take place.
In December 1938, two German chemists, the elderly Otto Hahn and his young assistant, Fritz Strassman, proved that a neutron colliding with a uranium (U) 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 nuclear fission. When this happened, two lighter atoms were flung apart with a tremendous force equal to the energy that had held the nucleus together. The splitting of one atom could release 200 million electron volts. Given that a single gram of uranium contained some 2.5 sextillion atoms (that is, 2,500,000,000,000,000,000,000 atoms), numbers alone could not describe 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.
Otto Hahn.
The atom’s potential power became even clearer when scientists discovered that “fissioning” 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.
In nuclear fission, a neutron strikes an atom of uranium, which splits in two and causes a chain reaction.
Some wanted to use the energy to fuel factories and homes. Others were drawn to — or feared — its use as an explosive. Within a week of Hahn’s discovery, the American physicist J. Robert Oppenheimer sketched a crude bomb on his blackboard. Enrico Fermi, who had recently won a Nobel Prize for his work on neutron irradiation, stared out the window of his office at Columbia University. He watched students bustling down the New York sidewalks, the streets crowded with traffic. “A little bomb like that,” he said, drawing his hands together as if they held a soccer ball, “and it would all disappear.”
On September 16, 1939, Kurt Diebner sat in his office at the headquarters of Berlin’s Army Ordnance Research Department, waiting for eight of his fellow German physicists. At thirty-four years of age, Diebner was a loyal Nazi Party member with a presence as modest and retreating as his hairline. His suits fit too tightly over his short, thin frame, and he wore round schoolboy spectacles that constantly threatened to fall off his nose. In meetings, his words came out halting and unsure. But despite his appearance, he was an ambitious and eager man.
Diebner, who came from a working-class family, gained his PhD in atomic physics in 1931. In 1934, the year Hitler became the Führer of Germany, Diebner joined the Army Ordnance, where he was tasked to develop 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 Hahn’s splitting of the atom made it clear that atomic physics was anything but malarkey, and Diebner was finally given the mandate to form a team.
Diebner opened the first meeting of the Uranverein (the Uranium Club) with a statement of its purpose. German spies had discovered that the United States, France, and Great Britain were all pursuing projects in nuclear fission. This team had been called together to decide whether it was possible to harness the atom’s energy for the production of weapons or the generation of power — all for the good of the Third Reich.
Kurt Diebner.
One of the men present was already dedicated to the atom’s weapons potential. In April 1939, 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 view, 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 Germany should pursue any such advantage.
Otto Hahn, on the other hand, was distraught that his discovery was being developed into a weapon to kill. He tried to extinguish any enthusiasm by pointing to the many technical challenges involved in engineering an explosive or designing a machine to produce energy. He noted that fission occurred most readily in the rare uranium isotope U-235, while its more common cousin U-238 tended to absorb neutrons into its nucleus, stealing their potential to foster a chain reaction. 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. They also needed to find an efficient moderator for U-235. Given all this, and likely more unseen challenges, Hahn believed they were on a fool’s errand to build a weapon. The debate continued for hours, until the scientists finally concluded, “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 the 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 the others had urged him to reconsider: Heisenberg had won a Nobel Prize at the tender age of thirty-one, and he was too brilliant to leave out.
Werner Heisenberg.
Heisenberg proved to be a useful addition to the Club. By the end of the meeting, the group had its orders. Some, like Harteck, would investigate how to extract sufficient quantities of U-235 from natural uranium. Oth
ers, including Heisenberg, would hash out chain-reaction theory, both for constructing explosives and for generating power. Still others would experiment with the best moderators.
Heisenberg made quick work with the theory. By late October, he started on a pair of breakthrough papers. One showed that if they separated out the U-235 isotope and compressed enough of it into a ball, the 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. This machine might look like a giant sphere filled with at least a ton each of uranium and moderator, separated in layers. The amount of U-235 was still key: They would need an enormous quantity of natural uranium to provide enough of the rare isotope.
In his second paper, on the subject of moderators, Heisenberg dismissed plain water and paraffin wax as options. Their hydrogen atoms slowed the neutrons enough to promote the fissioning of U-235, but they also absorbed many of them away from the chain reaction, diminishing its effectiveness. This left two known candidates: graphite, which was a crystalline form of carbon, and heavy water. He recommended further research into these substances.
In recognition of the Uranium Club’s work, Diebner was named head of the Kaiser Wilhelm Institute of Physics in Berlin, 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 the end of 1939, Diebner had dozens of scientists under his watch across Germany, refining atomic theory and conducting experiments. They readied advanced laboratories and investigated — and ordered — key materials.
Although the Uranium Club agreed they needed to study the issue further, they calculated that heavy water was the best presently known moderator. Diebner and his scientists required a steady, robust supply of the precious liquid. Unfortunately, the world’s sole producer of heavy water, Norsk Hydro’s Vemork plant, was far away in a remote valley in Norway, a country whose neutral status in the war made it an unreliable partner. Furthermore, the plant had only recently restarted heavy-water production in November 1939 and could supply little more than 10 kilograms a month. By January 1940, Diebner was sending them orders for 100 kilograms a month, every month. The management of Norsk Hydro — a company whose majority stake was owned by French shareholders — asked for the purpose behind such a large order. Diebner offered no answer, as the German Army had made the use of heavy water, now labeled SH-200, a high-level military secret. Norsk Hydro refused to fulfill the order.
It was no matter. In April 1940, the Nazis overtook Norway and seized Vemork. Shortly after, a German general ordered production at the plant — and deliveries to Berlin — to increase at a rapid rate. Jomar Brun, who ran the heavy-water plant he helped build, was given no choice but to meet this demand. Although he was sworn to secrecy about his work, Brun consulted Tronstad to see if he knew what the Germans could want with such a large supply of heavy water. Tronstad knew there was interest in the substance in the new field of fission research, but he dismissed the idea that it could be applied to any great military use.
By the spring of 1941, Diebner had increased his orders to 1,500 kilograms of heavy water a year. Then only a few months later, orders rose to 5,000 kilograms. All of Vemork was to be used to produce heavy water. No effort would be spared. If their work achieved its expected results, Diebner and his fellow physicists aimed to deliver an atomic bomb to Hitler.
Throughout 1940 and much of 1941, Tronstad continued his teaching and research in Trondheim — and his work for the Norwegian resistance. Behind blackout curtains, he met with community leaders active in pushing back against German oppression in the city. Tronstad was also close to several bands of university students. Some published illegal newspapers. Others operated at a higher level in connection with the British Secret Intelligence Service (SIS), sending coded wireless radio transmissions to London to report on German troop movements and naval activity. Tronstad — code-named “The Mailman” — gave them any technical help they needed and provided intelligence of his own from his many industrial connections. He also supplied crucial information about Vemork’s supply of heavy water.
On the morning of September 9, 1941, a student visited Tronstad for advice on hiding the wireless radio that he had been using to send messages to the SIS. The Gestapo arrested the student that same afternoon. A week later, they seized the courier Tronstad had tasked to go to London by ship. On the wharf’s edge, the courier swallowed the cigarette paper with the information he was carrying, but it was a close call. Tronstad knew the Gestapo would likely come for him next.
On September 22, Tronstad went to Bassa and told her they must flee. That evening, the family boarded a 7:15 train to Oslo with their hastily packed suitcases. In the sleeping compartment, Tronstad wrote the first entry in the small black diary he would keep through the war: “Family, house and worldly goods have to be set aside for Norway’s sake.”
The next morning, they arrived in Oslo and took another train to a neighborhood a short distance outside the city. From the station, they climbed up the hill to Bassa’s childhood home. He made sure Bassa knew to tell anybody who asked that he was headed west toward Telemark, and they hugged. Then he kneeled beside his children. “Take care of your little brother,” he told nine-year-old Sidsel. Then he turned to four-year-old Leif. “You must be good for your mother while I’m gone.” He promised to bring back a little gift for each of them. “Be kind to each other,” Tronstad said before hurrying away, so overcome with emotion that he forgot to hug them.
In Oslo, he collected fake identity papers, and the next morning he borrowed a bicycle and rode twenty miles north, where the Milorg resistance network picked him up and ferried him out of the country. Weeks later in Stockholm, he boarded a transport plane that took him across the North Sea. On October 21, he arrived at King’s Cross Station in London. SIS arranged a room for him at St. Ermin’s Hotel in the heart of Westminster, a stone’s throw from the spy agency’s headquarters.
Tronstad knew London well from his student days, but now he found it a war zone. Soldiers crowded the streets, and a floating armada of gray barrage balloons hung in the sky to interfere with German bombers. The streets were strewn with the rubble of bombed-out buildings. Many thousands of people had been killed in the Blitz, and countless more had been wounded and left homeless.
A barrage balloon — designed to keep Nazi planes from dive-bombing their targets — hangs behind a devastated block in London in September 1940.
Within days of his arrival, he sat down with Eric Welsh, the SIS spy responsible for Norway. Tronstad revealed what he knew about Nazi interest in Vemork, particularly the huge spike in heavy-water production. He discovered the British were also pursuing an atomic bomb — and they were deeply concerned about the work of their Nazi rivals.
In September 1939, when Hitler invaded Poland, he boasted to the world that he would soon “employ a weapon against which there would be no defense.” This prompted Sir Henry Tizard, head of the Air Ministry’s research department, who was already fearful of Nazi advances in atomic science, to look even more urgently into the production of a British bomb.
Two physicists, Otto Frisch and Rudolf Peierls, both Jewish refugees from Germany, put the British firmly on their path. On March 19, 1940, their report, “On the Construction of a Super Bomb,” landed on Tizard’s desk. They detailed that one pound of pure uranium-235 — divided into two (or more) parts, which were then smashed together at a high velocity — would initiate an explosion that would “destroy life in a wide area … probably the center of a big city … at a temperature comparable to the interior of the sun.” Peierls and Frisch suggested that German sc
ientists might soon “be in possession of this weapon.” Britain could only counter this threat, they concluded, by obtaining a bomb as well.
The following month, the British government began exploratory research with some of its best scientists. In July 1941, the group delivered a road map for an atomic bomb program. On receiving it, Prime Minister Winston Churchill wrote to his War Cabinet: “Although personally I am quite content with the existing explosives, I feel we must not stand in the path of improvement.” The Cabinet agreed, promising “no time, labor, material or money should be spared in pushing forward the development of this weapon.” Thus, the “Directorate of Tube Alloys” —the code name for the British atomic bomb program — was formed.
Winston Churchill.
Throughout this period, fears over the German bomb continued. From far and wide came whispers, rumors, threats, and fact — which, mixed together, made for the typically confusing brew that governments called “intelligence.” Two drunken German pilots were overhead on a tram speaking about “new bombs” that were “very dangerous” and had the power of an earthquake. One émigré German physicist warned that there was pressure from high within the Nazi government to build a bomb, and the Allies “must hurry.” A military adviser in Stockholm reported, “A tale has again reached me that the Germans are well under way with the manufacture of an uranium bomb of enormous power, which will blast everything, and through the power of one bomb a whole town can be leveled.” SIS heard of a mysterious September 1941 meeting where Werner Heisenberg admitted to Niels Bohr, who was living in Nazi-occupied Denmark, that a bomb could be made, “and we’re working on it.”
The best intelligence the British received came through German activity at Vemork. As early as April 1940, a French spy with close ties to Norsk Hydro had alerted his British allies to Nazi efforts in uranium research using heavy water from the plant. Leif Tronstad provided another gold mine of information. What the Norwegian professor revealed about the increased levels of production left officials at the Directorate of Tube Alloys and high in the British government deeply on edge. Whatever position Tronstad decided to take in his fight against the Nazis, whether as a scientist or as an official in the exiled Norwegian government based in London, the British wanted him to continue to gather intelligence about Vemork and the German atomic program. Tronstad agreed.