The Making of the Atomic Bomb

He was willing to be impressed by a mighty progress of industry. “The work on atomic energy in the USA and in England proved to have advanced much further than my father had expected,” Aage Bohr understates. Robert Oppenheimer pitches his summary closer to the shock of surprise a refugee released from the suspended animation that had been occupied Denmark would have felt: “To Bohr the enterprises in the United States seemed completely fantastic.”

They were.

15 Different Animals

The 59,000 acres of Appalachian semiwildemess along the Clinch River in eastern Tennessee that Brigadier General Leslie R. Groves acquired for the Manhattan Engineer District as one of his first official acts, in September 1942, extended from the Cumberland foothills in a series of parallel, southwestern-running ridge valleys. Groves liked the geology, which offered isolation for his several enterprises, but the new reservation was nearly as primitive as Los Alamos would be. The Clinch, a meandering tributary of the Tennessee, defined the reservation's southeastern and southwestern boundaries. Eastward twenty miles was Knoxville, a city of nearly 112,000, farther east the wall of Great Smoky Mountains National Park. Five un-paved county roads traversed the ninety-two square miles of depleted valleys and scrub-oak ridges, an area seventeen miles long and seven miles wide that supported only about a thousand families in rural poverty. In the ridge-barricaded valleys of this impoverished hill country, far from prying eyes, the United States Army intended to construct the futuristic factories that would separate U235 from U238 in quantity sufficient to make an atomic bomb.

To do so it had first to improve communications and build a town. Into the gummy red eastern-Tennessee clay in the winter of 1942 and the spring of 1943 its contractors cut fifty-five miles of rail roadbed and three hundred miles of paved roads and streets. They improved the important county roads to four-lane highways. Stone & Webster, the hard-pressed Boston engineering corporation, laid out a town plan so unimaginative that the MED rejected it and passed the assignment to the ambitious young architectural firm of Skidmore, Owings and Merrill, which produced a well-sited arrangement of housing using innovative new materials that saved enough money to allow for such amenities in the best residences as fireplaces and porches. The new town, planned initially for thirteen thousand workers, took its name from its location lining a long section of the north-westernmost valley: Oak Ridge. The entire reservation, fenced with barbed wire and controlled through seven guarded gates, was named, after a nearby Tennessee community, the Clinton Engineer Works. Its workers would come to call it Dogpatch in homage to the hillbilly comic strip “Li'l Abner.” The new gates closed off public access on April 1.

Groves planned to build electromagnetic isotope separation plants and a gaseous-diffusion plant at Clinton; plutonium production, he realized during his first months on the project, would proceed at such a scale and generate so vast a quantity of potentially dangerous radioactivity that it would require a separate reservation of its own. Of the three processes, Ernest Lawrence's electromagnetic method was farthest along.

Electromagnetic isotope separation enlarged and elaborated Francis Aston's 1918 Cavendish invention, the mass spectrograph. As a 1945 report prepared by Lawrence's staff explains, the method “depends on the fact that an electrically charged atom traveling through a magnetic field moves in a circle whose radius is determined by its mass” — which was also a basic principle of Lawrence's cyclotron. The lighter the atom, the tighter the circle it made. Form ions of a vaporous uranium compound and start them moving at one side of a vacuum tank permeated by a strong magnetic field and the moving ions as they curved around would separate into two beams. Lighter U235 atoms would follow a narrower arc than heavier U238 atoms; across a four-foot semicircle the separation might be about three-tenths of an inch. Set a collecting pocket at the point where the U235 ion beam separately arrived and you could catch the ions. “When the ions strike the bottom of the collecting pocket… they give up their charge and are deposited as flakes of metal.” Schematically, with slotted electrodes to accelerate the ions, the arrangement would look like the illustration on page 488.

Late in 1941 Lawrence had installed such a 180-degree mass spectrometer in place of the dees in the Berkeley 37-inch cyclotron. By running it continuously for a month his crews produced a partially separated 100- microgram sample of U235. That was several hundred million times less than the 100 kilograms Robert Oppenheimer had originally estimated would be necessary to make a bomb. The demonstration proved the basic principle of electromagnetic separation even as it dramatized the method's monumental prodigality: Lawrence was proposing to separate uranium atom by individual atom.

^{Magnetic field perpendicular to plane of drawing}

Enlarging the equipment, increasing the accelerating voltage, multiplying the number of sources and collectors set side by side between the poles of the same magnet were obvious ways to improve output and efficiency. Lawrence had committed his time to winning the war; now he committed his beautiful new 184-inch cyclotron. Instead of cyclotron dees he had D-shaped mass-spectrometer tanks installed between the pole faces of its 4,500-ton magnet. Making the new instrument work, through the spring and summer of 1942, solved the most difficult design problems. It acquired a name along the way: calutron, another tron from the t/niversity of Ca/ifornia.

To separate 100 grams — about 4 ounces — of U235 per day, Lawrence estimated in the autumn of 1942, would require some 2,000 4-foot calutron tanks set among thousands of tons of magnets. If a bomb needed 30 kilograms — 66 pounds — of U235 for reasonable efficiency, as the Berkeley summer study group had just worked out, 2,000 such calutrons could enrich material enough for one bomb core every 300 days. That assumed the system worked reliably, which so far its laboratory predecessors had hardly done. Yet in 1942 electromagnetic separation still looked so much more promising to James Bryant Conant than either the plutonium approach or gaseous barrier diffusion that he had offered up for debate the possibility of pursuing it exclusively. Lawrence was self-confident but not foolhardy; he insisted that the two dark horses should continue to run the race alongside the favorite.

Groves was less impressed. So was the first Lewis committee that had visited Chicago and Berkeley when Fermi was building CP-1 in the winter of 1942. The Lewis committee judged gaseous diffusion the best approach because it was most like existing technology — diffusion was a phenomenon familiar to petroleum engineers and a gaseous-diffusion plant would be essentially an enormous interconnected assemblage of pipes and pumps. Electromagnetic separation by contrast was a batch process untested at such monumental scale; Berkeley planned a system of 4-foot tanks set vertically between the pole faces of large square electromagnets, two tanks to a gap and a total of 96 tanks per unit. To reduce the amount of iron needed for the magnet cores the arrangement would be not rectangular but oval, like a racetrack:

And racetrack it was called, though its official designation was Alpha. Berkeley could promise only 5 grams of enriched uranium per day per racetrack, but Groves thought 2,000 tanks well beyond Stone & Webster's capability and cut the number back to 500, reasoning, as Lawrence recalled later, “that the art and science of the process would go forward and that by the time the plant was built substantially higher production rates would be assured.” Five grams per day per racetrack with only five racetracks would mean 1,200 days per 30-kilogram bomb even if the Alpha calutrons produced nearly pure U235, which they did not — their best production was around 15 percent. Groves counted on improvements and forged ahead.

He had to begin building before he knew precisely what to build. He worked from the general to the particular, from outline to detail. Fully six months before he decided how many calutrons to authorize, his predecessors, Colonel James Marshall and Lieutenant Colonel Kenneth Nichols, had moved to solve one serious problem of supply. The United States was critically short of copper, the best common metal for winding the coils of electromagnets. For recoverable use the Treasury offered to make silver bullion available in copper's stead. The Manhattan District put the offer to the test, Nichols negotiating the loan with Treasury Undersecretary Daniel Bell. “At one point in the negotiations,” writes Groves, “Nichols… said that they would need between five and ten thousand tons of silver. This led to the icy reply: ‘Colonel, in the Treasury we do not speak of tons of silver; our unit is the Troy ounce.’” Eventually 395 million troy ounces of silver — 13,540 short tons — went off from the West Point Depository to be cast into cylindrical billets, rolled into 40-foot strips and wound onto iron cores at Allis-Chalmers in