TWO ATOMIC BOMB DESIGNS DIVERGE

Various designs for this proposed weapon were debated, experiments were performed, and nuclear physics was given a thorough workout. The bomb had to be small and light enough to be delivered to the target by a large airplane, and it would be deployed by gravity, by dropping it from a high altitude. Simplicity was a goal, and eventually a design that would work using either uranium or plutonium emerged.

It was code-named “Thin Man.” It was 17 feet (5 m) long, with a bulge on the end. It looked like a telephone pole with fins on the end. The bulge at the end housed a subcritical cylinder of fissile material, with a three — inch (8-cm) hole in the center. A second subcritical piece of fissile mate­rial, cylindrical and just big enough to fit in the hole, was kept at the other end of the bomb, in a gun barrel running from end to end. To set off the bomb, an explosive charge behind the subcritical projectile would acceler­ate it to 3,000 feet per second (914 mps). As it passed through the subcriti­cal component in the bulge there would suddenly be enough material in one place to make a hypercritical assembly of fissile uranium or pluto­nium, and the resulting reaction would run away explosively.

By May 1943, plutonium was arriving at Los Alamos, and an unfore­seen problem with using plutonium in a bomb became clear. The plutonium-239 supplied from the production reactors had a slight con­tamination of plutonium-240. The Pu-240 had a tendency to fission spon­taneously, and the gun-barrel design could not send the two subcritical pieces of plutonium together fast enough. The spontaneous fissions would set off the assembly as the projectile proceeded down the gun barrel, and the bomb would come apart before it was able to achieve hypercriticality. It was impossible to chemically separate Pu-239 from Pu-240, and an iso­tope separation as was being used to make U-235 was out of the question. The masses of the two plutonium isotopes were too close together.

There was danger of the entire plutonium production effort becom­ing useless, but there was another way to assemble the plutonium into a hypercritical mass. An American physicist from Caltech, Seth Ned — dermeyer (1907-88), had been pushing an idea called implosion, and he presented his first technical analysis of the idea in April 1943, just as the

_______ ESPIONAGE IN THE LABORATORY____________

Secrecy in the Manhattan Project was tight and very well managed. U. S. enemies in Japan and Germany had no idea what was going on in universities, government laboratories, and industrial plants spread all over the country. The Japanese gov­ernment did not know we had an atomic bomb until we dropped one on them, and even then there was skepticism. The German scientists were told after the war that we had developed the bomb. Still thinking in terms of a large nuclear reactor, they found it hard to believe that we had an aircraft big enough to carry such a bomb. Even most people who worked on the bomb project were surprised when the atomic bomb development was announced. Construction crews, lab techni­cians, and industrial workers were all kept in the dark. Laura Fermi, the wife of Enrico Fermi, did not know what her husband had been working on for four years until he gave her a copy of the book Atomic Energy for Military Purposes by Henry DeWolf Smyth in September 1945.

Nobody knew what the United States was working on, with the exception of the Soviet Union. The Soviets, apparent masters of spy craft, had infiltrated the Manhat­tan Project at multiple points and gained enough information to quickly duplicate the methods and designs for which the United States had worked so hard, and they developed similar weapons with minimum effort. We did not know exactly how much information the Soviets had extracted from the bomb program until after their government collapsed in 1990, and the files of their Committee for State Security, or KGB, were opened. The Soviet infiltration turned out to have been deeper and earlier than was realized. The information was sufficiently detailed for the Soviet scientists to build a duplicate copy of the CP-1 reactor in Chicago, but in translation their information was slightly garbled. The CP-1 was built in the abandoned squash courts under Stagg Field. The Soviet documents say that it was built in a "deserted pumpkin patch.”

The primary information leak point may have been Klaus E. J. Fuchs (1911-88). Fuchs was born in Riisselsheim, Germany, to a Lutheran pastor Emil Fuchs and Else Wagner. He attended both Leipzig University and Kiel University and joined the Com­munist Party of Germany in 1932. Finding himself at odds with the Nazi government, he fled in 1933 and was able to land in Bristol, England. Fuchs earned a Ph. D. in phys­ics at the University of Bristol in 1937 and got a teaching job in Edinburgh, Scotland, the same year. Although he was interned at the beginning of World War II for being a

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German citizen, the British needed every nuclear physicist they could find for a pos­sible atomic bomb program, and he was granted British citizenship in 1942, signing the Official Secrets Act.

In late 1943, the small British bomb program allied with the Manhattan Project, and the British scientists were loaned to the United States. Fuchs was first assigned to Columbia University in New York City, but in August 1944 he was moved to Los Alamos Laboratory to work in the theoretical physics division. There, he had access to the difficult problems that were being solved for the bomb design, particularly the implosion method being developed for Fat Man, and he passed all of it to his Soviet contacts.

In 1946, after passing information concerning the secret development of the hydrogen bomb to the Soviets, Fuchs returned to Great Britain, where he was con­fronted by intelligence officers. An effort to crack Soviet ciphers, known as the Venora Project, had implicated him as a spy for the KGB. Finally confessing in 1950, Fuchs was tried and convicted of passing military secrets. His entire trial lasted 90 minutes.

plutonium crisis became evident. Everyone was familiar with the action of a chemical explosive. Set off a spherical bomb, and a shock wave radi­ates out from the point of explosion. If the spherical bomb is made hollow, with a void in the center, then two shock waves are produced. Still the outer shock wave radiates outward, becoming bigger and more diluted as it expands outward. The simultaneously generated shock wave at the cen­ter radiates inward, becoming smaller and smaller and more concentrated the farther it develops, until it is an extremely intense, spherical pressure wave at the center of the bomb.

Neddermeyer suggested that this inner shock wave could be used to shrink a small sphere of plutonium very quickly. The sphere would be so small as to be subcritical, not having sufficient material to form a critical mass. The size of a hypercritical mass depends on several factors, such as the shape of the mass, the number of atoms of fissile material present, and the distances between fissile atoms. The distance between two atoms in a block of plutonium would seem fixed. It is, after all, a solid, incompressible piece of metal. However, in the extreme forces produced by an implosion shock wave, metal can actually be compressed. For just an instant, a piece

of solid metal the size of a softball can be compressed to something the size of a marble. That instant is just long enough for a hyperdense piece of plutonium to become hypercritical and experience explosive fission.

It was a brilliant idea, and Oppenheimer made Neddermeyer the head of a new explosives group to thoroughly study the implosion effect. The implosion was simple in concept, but in application it turned out to be extremely complex. Neddermeyer started out with cylindrical shapes, try­ing to shrink down a rod of metal by putting it in the middle of a cylinder of chemical explosive. The speed with which the shock would develop in the explosive turned out to be very uneven and unpredictable, and his metal rods would end up twisted into odd shapes. After months of unproductive testing, Oppenheimer brought in George Kristiakowsky (1900-82), a Russian-born chemistry professor from Harvard University who was an expert on explosives and chemical kinetics. In mid-June 1944, Oppenheimer read Kristiakowsky’s report on the lack of progress in the explosives research, and he made Kristiakowsky head of the group.

Experiments with the U-235 coming in small batches from Oak Ridge indicated that it was better behaved than initially thought, and the length of the Thin Man was reduced to six feet (1.8 m). The uranium bomb using the gun-barrel assembly scheme was renamed “Little Boy.” The pluto­nium-based implosion bomb would be an egg-shaped device, five feet (1.5 m) around and nine feet (2.7 m) long, with fins on the back to make it fly nose down. It was named “Fat Man.” By 1945, it looked as though both parallel atomic bomb development paths would result in a practical weapon, and the project raced toward completion.

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