Diana Preston - Before the Fallout

Feynman was an effervescent character, whom C. P. Snow later described as a cross between Groucho Marx and Einstein. He played the bongos and was once commissioned to paint a nude female toreador. He took delight in outwitting the increasingly sophisticated locking devices fitted to the Los Alamos filing cabinets. As he recalled in his memoirs, everybody thought their reports were safe, but, as he repeatedly demonstrated by presenting astonished colleagues with their own papers, the complex arrangements of steel rods, padlocks, and, later, combination wheels “didn’t mean a damn thing.”

Feynman’s wise-cracking boisterousness and passion for pranks masked a personal tragedy. His wife, Arlene, was dying of tuberculosis in a hospital in Albuquerque. Knowing that the end could come at any time, Feynman asked Fuchs whether he could borrow his car so he could get to Arlene’s bedside quickly. Fuchs—always obliging to his friends—readily agreed. When the summons finally came, Feynman tore off in Fuchs’s old blue Buick and, despite three flat tires, reached the hospital in time to be with Arlene when she died.

With the work at Los Alamos focused on two completely different designs of atom bomb—the uranium device, Little Boy, and the plutonium device, Fat Man—the scientists made their best estimates of how much uranium and plutonium respectively each would require to produce the necessary critical mass. They calculated that Little Boy would need between 87 and 133 pounds of U-235 to cause an explosion equivalent to the detonation of between 10,000 and 20,000 tons of TNT. Estimates of the necessary amount of plutonium for Fat Man were even more uncertain. As everyone knew, having a bomb of either or both types available in time to influence the course of the war depended, above all, on whether sufficient fuel could be produced in time. Groves had always believed this to be the hardest part of the project.

Manufacturing uranium and plutonium was by then a massive effort. Oak Ridge and Hanford were the heart, but they were supported by factories and laboratories in thirty-nine states. Groves later estimated that by the war’s end more than six hundred thousand people had contributed, directly or indirectly, to the Manhattan Project. The Y-12 electromagnetic uranium separation plant at Oak Ridge, where operators sat on high stools six feet apart, produced its first two hundred grams of U-235 in February 1944—barely a year after its construction began. However, production remained worryingly slow until the discovery that feeding the plant with uranium that had already been slightly enriched with U-235 significantly increased the yield. By late 1944, Y-12 was producing more-substantial amounts of U-235. Meanwhile, K-25—the plant using gaseous diffusion to separate U-235 and built in sections by the Chrysler Corporation in Detroit and then assembled at Oak Ridge—was nearing completion. It would not become fully operational in time to make a major contribution. However, the U-235 it began producing in April 1945, by pumping uranium gas against a porous membrane so that the lighter U-235 passed through more rapidly than the heavier U-238, could be used as feed for Y-12. [33] After the war, as the gaseous diffusion technique was perfected, it would replace electromagnetic separation—rejected as too costly and cumbersome for mass production—and become the sole technique used by the United States. In 1991 Western scientists were surprised by evidence that the Iraqi dictator Saddam Hussein was attempting to build a bomb using “old-fashioned” electromagnetism.

The first plutonium-producing plant at Hanford, where workers had labored in nine-hour shifts six days a week, was brought up to full power in late September 1944. Scientists observed the controlled chain reaction with satisfaction. However, after a while the power mysteriously began to drop, and the reactor effectively shut itself down. Soon after, the power level began to rise again, only to be followed once more by a seemingly inexplicable shutdown. Scientists discovered the reason to be a rare isotope—Xenon-135, created during the fission process—which sucked up neutrons, thereby causing the chain reaction to peter out. They solved the problem by increasing the amount of fuel loaded into the reactor. Fortunately, the Du Pont engineers had, with Groves’s backing, designed the reactor with a larger number of slots for fuel than the scientists had thought necessary. The first plutonium was extracted from the reactor around Christmas 1944 and dispatched to Los Alamos in early 1945, by which time the second and third plutonium plants at Hanford were also coming online. The plutonium was transported by military convoy. (Air transport was considered too risky in case of a crash, while train connections from Washington State were too few.)

The prospect that sufficient plutonium and U-235 would soon be available to build the bombs induced “great pressure to be ready with all the necessary developments for making and detonating them,” according to Rudolf Peierls. Oppenheimer drove his teams hard, determined, as he later wrote, “to interpose no day’s delay between the arrival of the material and the readiness of the bomb.” Scientists worked eighteen-hour days. One of the physicist’s wives, Ruth Marshak, recalled how “the Tech Area was a great pit which swallowed our scientist husbands out of sight, almost out of our lives. They worked at night, and often came home at three or four in the morning. Sometimes they set up army cots in the laboratories and did not come home at all.”

Oppenheimer was particularly anxious that everything for Fat Man, the plutonium-fueled implosion bomb, should be in place—that the essential physics research had been completed, that the explosive lenses designed by James Tuck had been made, and that an electric detonator system, developed by Luis Alvarez, was ready. Although the original plan had envisaged using uniform pressure waves to squeeze a thin, hollow shell of plutonium into a sphere, the necessary calculations had proved so complex that the idea had been abandoned for a simpler alternative. Robert Christy, by then working in Hans Bethe’s division, had proposed using a solid sphere of plutonium comprising two fused hemispheres, together roughly the size of an apple. Christy calculated that the force of the implosion would at least double the plutonium’s density, shortening the neutrons’ route between nuclei and thereby swiftly accomplishing the required chain reaction. The device also included an initiator and a natural uranium tamper, or shell. It was vital for achieving the chain reaction that the plutonium sphere remained spherical and did not distort, and the shell’s purpose was to compensate for any asymmetrical effects during implosion.

As significant quantities of fissionable material began reaching Los Alamos, radiological protection measures were increased. Since the cavalier days of the 1920s, scientists and the public had become increasingly aware of the adverse affects of radiation on those exposed to it. The use of radium in tonics and potions and even face creams sold over the counter had become strictly controlled during the 1930s. The painful death of a wealthy Pittsburgh industrialist, Ebert M. Byers, from the effects of a radium tonic, also advertised as an aphrodisiac, had been a particular catalyst for reform. In line with the instructions, he had daily consumed four doses, each containing two microcuries of radioactive material. The potential for radiation to cause genetic defects in unborn generations had also been recognized since 1927, following work on fruit flies by an American scientist named Herbert Muller. As a consequence, groups of experts had agreed on internationally accepted limits of radiation exposure for both public and workers, albeit considerably more lax than those in force today.