The Making of the Atomic Bomb

surfaces; a swing saw cut the bricks to length. That processing produced 14 tons of bricks a day; each brick weighed 19 pounds.

To drill the blind, round-bottomed 3¹/₄-inch holes for the uranium pseudospheres, two to a brick, Zinn adapted a heavy lathe. He mounted a 3 %-inch spade bit in the headstock of the lathe, where the material to be turned would normally be mounted, and forced the graphite up against the tool with the lathe carriage. Dull bits caused problems. Zinn tried tough carballoy bits first, but they were tedious to resharpen. He began making bits from old steel files, sharpening them by hand whenever they dulled. One sharpening was good for 60 holes, about an hour's work. Before they were through they would shape and finish 45,000 graphite bricks and drill 19,000 holes.

General Groves made his first appearance at the Met Lab on October 5 and delivered his first pronouncement. The technical council was debating cooling systems again. “The War Department considers the project important,” Seaborg paraphrases Groves' formula, which they would all learn by heart. “There is no objection to a wrong decision with quick results. If there is a choice between two methods, one of which is good and the other looks promising, then build both.” Get the cooling-system decision into Compton's hands by Saturday night, Groves demanded. It was Monday. They had been debating for months.

Groves moved on to Berkeley more impressed with their work than his Met Lab auditors realized. “I left Chicago feeling that the plutonium process seemed to offer us the greatest chances for success in producing bomb material,” he recalls. “Every other process… depended upon the physical separation of materials having almost infinitesimal differences in their physical properties.” Transmutation by chain reaction was entirely new, but the rest of the plutonium process, chemical separation, “while extremely difficult and completely unprecedented, did not seem to be impossible.”

At the beginning of the month, to Compton's great relief, the brigadier had convinced E. I. du Pont de Nemours, the Delaware chemical and explosives manufacturers, to take over building and running the plutonium production piles under subcontract to Stone & Webster. He meant to involve the industrial chemists more extensively than that — meant for them to take over the plutonium project in its entirety. Du Pont resisted the increasing encroachment. “Its reasons were sound,” writes Groves: “the evident physical operating hazards, the company's inexperience in the field of nuclear physics, the many doubts about the feasibility of the process, the paucity of proven theory, and the complete lack of essential technical design data.” Du Pont also suspected, once it had sent an eight-man review team to Chicago at the beginning of November, that the plutonium project was the least promising of the several then under development and might even fail, tarnishing the company's reputation. Nor was it happy at the prospect of identifying itself with a secret weapon of mass destruction; it still remembered the general condemnation it had received for selling munitions to Britain and France before the United States entered the First World War. Groves told the Du Pont executive committee that the Germans were probably hard at work and the only defense against a Nazi atomic bomb would be an American bomb. And added what he took to be a clinching argument: “If we were successful in time, we would shorten the war and thus save tens of thousands of American casualties.” The second week in November Du Pont admitted the possibility of regular production by 1945 and accepted the assignment (limiting itself to a profit of one dollar to avoid arms-merchant stigma), but made its skepticism and reluctance clear.

By then Stone & Webster's construction workers had gone on strike. The pile building scheduled for completion by October 20 would be indefinitely delayed. Fermi lived with the problem only long enough to recalculate the risks of pile control. In early November he cornered Compton in his office and proposed an alternative site: the doubles squash court where his team had built its series of exponential piles. A k greater than 1.0 presented an entirely different order of risk from a A; of less than 1.0, however; Compton had, in Seaborg's words, a “dreadful decision” to make. “We did not see how a true nuclear explosion, such as that of an atomic bomb, could possibly occur,” Compton writes with more calm than he probably felt at the time. “But the amount of potentially radioactive material present in the pile would be enormous and anything that would cause excessive ionizing radiation in such a location would be intolerable.” He asked for Fermi's analysis of the probability of control.

No doubt Fermi discussed the various hand and automatic control rods he planned for the pile. But even slow-neutron fission generations had been calculated to multiply in thousandths of a second, which might flash the pile to dangerous levels of heat and radiation before any merely mechanical control system could move into position. The “most significant fact assuring us that the chain reaction could be controlled,” says Compton, was one of the Richard Roberts team's earliest discoveries at the Carnegie Institution's Department of Terrestrial Magnetism following Bohr's announcement of the discovery of fission in 1939 — in Compton's words, that “a certain small fraction of the neutrons associated with the fission process are not emitted at once but come off a few seconds after fission occurs.” With a pile operating at k only marginally above 1.0, such delayed neutrons would slow the response sufficiently to allow time for adjustment.

For once Compton made a quick decision: with control seemingly assured, he allowed Fermi to build CP-1 in the west stands. He chose not to inform the president of the University of Chicago, Robert Maynard Hut-chins, reasoning that he should not ask a lawyer to judge a matter of nuclear physics. “The only answer he could have given would have been — no. And this answer would have been wrong. So I assumed the responsibility myself.” The word meltdown had not yet entered the reactor engineer's vocabulary — Fermi was only then inventing that specialty — but that is what Compton was risking, a small Chernobyl in the midst of a crowded city. Except that Fermi, as he knew, was a formidably competent engineer.

In mid-November Fermi reorganized his team into two twelve-hour shifts, a day crew under Walter Zinn (who continued to supervise materials production as well), a night crew under Herbert Anderson. Construction began on Monday morning, November 16, 1942. From the balcony of the doubles squash court in the west stands of Stagg Field Fermi directed the hanging of the cubical dark-gray Goodyear balloon as his men hauled it into place with block and tackle. It dominated the room: bottom panel smoothed on the floor, top and three sides secured to the ceiling and the walls, the fourth side facing the balcony furled up out of the way like an awning. Someone drew a circle on the floor panel to locate the first layer of graphite and without ceremony the crew began positioning the dark, slippery bricks. The first layer was “dead” graphite that carried no load of uranium: solid crystalline carbon to diffuse and slow the neutrons that fission would generate. Up the pile as it stacked, the crews would alternate one layer of dead graphite with two layers of bricks each drilled and loaded with two five-pound uranium pseudospheres. That created a cubic cell of neutron-diffusing graphite around every lump of uranium.

To build the wooden framing, Herbert Anderson recalls, “Gus Knuth, the millwright, would be called in. We would show him… what we wanted, he would take a few measurements, and soon the timbers would be in place. There were no detailed plans or blueprints for the frame or the pile.” Since they had batches of graphite, oxide and metal of varying purity, they improvised the placement of materials as they went along. Fermi, says Anderson, “spent a good deal of time calculating the most effective location for the various grades of [material] on hand.”

They were soon averaging not quite two layers a shift, handing the bricks along from their delivery skids, sliding them to the workers on the pile, singing together to pass the time. The bricks in the dead graphite layers alternated direction, three running east and west and the next three north and south. That gave support to the oxide layers, which all ran together from front to back except at the outer edges, where dead graphite formed an outer shell. The physicist bricklayers had to be careful to line up the slots for the ten control-rod channels that passed at widely distributed points completely through the pile. “A simple design for a control rod was developed,” says Anderson, “which could be made on the spot: cadmium sheet nailed to a flat wood strip… The [thirteen-foot] strips had to be inserted and removed by hand. Except when the reactivity of the pile was being measured, they were kept inside the pile and locked using a simple hasp and padlock, the only keys to which were kept by Zinn and myself.” Cadmium, which has a gargantuan absorption cross section for slow neutrons, held the pile quiescent.

As it grew they assembled wooden scaffolding to stand on and ran loads of bricks up to the working face on a portable materials elevator. Before the arrival of the elevator, during the period when they were building large exponential piles, they had simply leaned over from the precarious 2 by 12-inch scaffolding and reached the bricks up from the men on the floor below. Groves walked in on them one day and dressed them down for risking their necks. The elevator appeared unbidden soon after.

When they achieved the fifteenth layer Zinn and Anderson began measuring neutron intensity at the end of each shift at a fixed point near the center of the pile with the control rods removed. They used a boron tri-fluoride counter Leona Woods had devised that worked much like a Geiger counter, clicking off the neutron count. Standard