The Making of the Atomic Bomb

Early in 1940, Otto Frisch, now working at the University of Birmingham, figures out that a uranium bomb would require several tons of the element and might not work. If the scarce U235 can be separated out and used alone, however, the chain reaction would definitely work and a bomb could be made from a mere 11 pounds of material. Divided into two parts and slammed together, it would release the energy of several thousand tons of TNT, including lethal radiation. The only defense against such a weapon “would be a counter-threat with a similar weapon” (325).

The English, French, Russians, and Japanese begin research into a possible super-bomb. In June 1940, the Advisory Committee on Uranium is folded into the newly formed National Defense Research Committee (NDRC), headed by engineer-inventor Vannevar Bush. In May 1940, Germany suddenly attacks and conquers Belgium and the Netherlands; by mid-June, German forces have swept through northern France and taken Paris. German planes bomb London; British bombers retaliate by striking Berlin. Hitler orders systematic bombing raids against the British capital that last from September 1940 to May 1941; this “Blitz,” as the English call it, kills over 40,000 Britons.

Several scientists, working separately, conclude that there may, indeed, be transuranics—elements larger and heavier than uranium—found among the byproducts of the neutron bombardment of uranium. These elements might fission more easily than U238, be easier to collect than the rare and elusive U235, and prove useful as explosive material. In March 1941, Segrè, chemist Glenn Seaborg, and others at Berkeley separate element 93, neptunium, and its daughter, element 94, from the decay products of uranium and find that element 94 fissions easily from slow neutrons. In keeping with its parent elements, uranium and neptunium—named for distant planets—the new element will be named plutonium. 

NDRC member James Bryant Conant, an internationally famous chemistry professor and president of Harvard University, travels to England early in 1941 to open contacts on war technology. Churchill’s chief science administrator, Frederick Lindemann, wants to discuss nuclear weapons, but Conant—who defers in such matters to the NDRC‘s Bush—reveals he’s unaware of the latest developments in nuclear physics: “[T]his was the first I had heard about even the remote possibility of a bomb” (359).

The British realize they’re ahead of the US in nuclear weapons research, and, shortly, so do the Americans. Bush, after much prodding by Lawrence, finally agrees that America should carry most of the load on this project; he arranges for a committee selected from members of the National Academy of Sciences (NAS) to review the progress so far and make recommendations. The committee is chaired by Arthur Compton, a University of Chicago physics professor and Nobel laureate.

The committee quickly concludes that nuclear research points to weapons in three forms: radiation products dispersed from airplanes onto the enemy, propulsion systems for submarines, and “violently explosive bombs” (365). These weapons will take two to four years to develop. Bush, however, remains skeptical. Then Hitler invades Russia, and the stakes go up.

Lawrence and Segrè show that plutonium’s fission rate is nearly twice that of U235. The English, however, share with the US its “MAUD” report that concludes that “Tube Alloys”—code name for a U235 bomb—is feasible and that an industrial plant should be built to extract the material. The NAS committee’s second report recommends making this project a priority. Briggs puts the MAUD report in his safe and ignores it. Meanwhile, German scientists have chosen to generate plutonium instead of U235.

University of Birmingham physicist Mark Oliphant visits America to urge the Americans to speed up their work; he discovers that Britain’s MAUD report has been held up by Briggs. Oliphant enlists Lawrence in a campaign to convince the reluctant American bureaucracy to get moving. Once onboard, Conant and Bush present the British case to Roosevelt, who approves it but keeps future decisions on the use of nuclear weapons strictly to a small group within his administration. The scientists will have no say in the matter.

At Columbia, Fermi suggests offhandedly to Teller that it might be possible to use an atom bomb to ignite a fusion bomb that converts hydrogen to helium and releases a thousand times as much energy as the atom bomb: “Teller found it a surpassing challenge and took it to heart” (374).

Heisenberg, whose progress with U235 experiments makes him believe a bomb is possible, meets Bohr in Copenhagen to discuss it. Bohr—who dismisses the idea that an atom bomb is feasible—seems surprised to hear that the Germans believe otherwise. Bohr refuses to discuss technical matters with his old protégé and is upset that Heisenberg so dutifully serves the Nazis. Heisenberg feels loyalty to Germany, not Hitler, but he must couch his thoughts carefully, given the German spies everywhere in Denmark. The meeting is a disaster that puts distance between the two longtime friends.

Compton's third NAS report, issued in November 1941, summarizes the latest experiments with U235 and its extraction but doesn’t mention plutonium research; Compton argues for plutonium a few weeks later and gets Bush’s approval. The project now will move forward on several fronts.

In a surprise maneuver, Japan attacks the US naval base at Pearl Harbor, Hawaii, on the morning of December 7, 1941. More than 3,500 servicemen and civilians are killed or wounded. Eighteen ships, including eight battleships, are sunk or damaged, along with nearly 300 aircraft. The next day, President Roosevelt gets from Congress a declaration of war against Japan, Germany, and Italy. 

At Columbia, Fermi—armed with new funding—builds a huge latticework of graphite bricks interspersed with large cans of uranium oxide, 38 tons in all, to study how to control uranium’s fission and decay products. Fermi casually refers to this structure as a “pile”; the name sticks. First tests show a “k” value—the number of neutrons coming out of an atom for every neutron fired in—of 0.87, meaning the reaction will peter out. The new Office of Scientific Research and Development (OSRD), under Conant, takes over from the NDRC: “[T]he bomb program had advanced from research into development” (398). Compton’s notes show that plutonium can make bombs half the size of U235 weapons but plutonium will take longer to generate. In April, Fermi redesigns the pile and gets a k reading of 0.918. Compton consolidates the various labs into one place at his own University of Chicago. Pile research will commence under the old football stadium’s stands.

Meanwhile, the Nazis, their war effort bogged down in wintry Russia, economize by shifting nuclear research to the ministry of education under incompetent bureaucrats. Armaments minister Albert Speer talks to Heisenberg and authorizes more money, but by mid-1942 Speer and Hitler believe an atomic bomb will take longer to build than the war will last, and they put the project on a back burner. Not knowing this, the US plows ahead: “Time, not money, was becoming the limiting factor in atomic bomb development” (406). Conant decides that all five methods of generating nuclear material—centrifuges, gaseous diffusion, cyclotron, graphite filtering and heavy-water filtering—should be used. This guarantees that the fastest method of the five would be found.

Glenn Seaborg takes over plutonium production at Chicago. His goal is to extract a dime-sized amount out of two tons of uranium. His team uses chemicals, a microscope, and miniaturized instruments to extract and weigh micrograms of plutonium. In August 1942 they succeed and celebrate over a tiny speck of pinkish material.

Teller develops a theory on how to use an atom bomb to ignite a hydrogen bomb 500 times more powerful; at a Berkeley conference, the other scientists are intrigued; they call such a weapon a “Super.” Teller also mentions the possibility that nuclear bombs might accidentally ignite the Earth’s oceans or atmosphere. Further calculations by Bethe, however, show that such a possibility would require temperatures 100 times hotter than those inside a “Super.” Bush reports the findings to Secretary of War Stimson: “The hydrogen bomb was thus under development in the United States onward from July 1942” (422).

The Army assumes authority over the nuclear program, but its brisk, top-down ways irk the scientists, and they protest. Army deputy chief of construction, Leslie Groves, who has recently finished building the Pentagon, is appointed to manage the nuclear project. Large, gruff, no-nonsense, and capable, General Groves irritates many but gets things done, and the work speeds up.

As graphite and uranium supplies pour in, Fermi runs pile tests that return a k value of 1.04, enough to sustain a chain reaction. Assuring himself that an out-of-control pile is unlikely, Compton approves a major test. They build it in the squash court next to the stadium, 250 tons of long graphite blocks arranged in 56 layers to slow neutron absorption, some drilled with holes to contain small cylinders of uranium whose collected weight is six tons. Long wooden strips covered in sheets of cadmium are slid by hand into and out of the pile; cadmium absorbs neutrons and can control the chain reaction. Special Geiger-counter-like instruments keep tabs on the radiation.

On December 2, 1942, Fermi directing, the control rods are pulled out slowly, one at a time. At each step, radiation levels rise as predicted. The last rod is pulled out six inches at a time, the radiation ratcheting up slightly with each pull. When a preset radiation level is reached, an automatic rod slides into place, preventing the pile from going critical. After lunch, they repeat the experiment, pulling out the control rods, the final rod to the level reached that morning. They pull this rod out further, and the radiation clicks increase to a roar as the pile goes critical. The rate of neutron intensity doubles every minute. After 4.5 minutes at half a watt, Fermi orders the rods replaced and the pile shut down: “Men had controlled the release of energy from the atomic nucleus” (440). It’s a new world. Szilard, who first imagined this event years earlier, recalls, “I shook hands with Fermi and I said I thought this day would go down as a black day in the history of mankind” (442).