Hitler’s scientists might find a way to turn this enormous atomic fission energy into a weapon of terrifying power. A number of them prevailed upon a reluctant Albert Einstein to write a letter to President Roosevelt warning him of the danger and advising him to begin atomic research in America along similar lines.
The result was a scientific and engineering research project unmatched in size and cost by anything in world history. Jointly led by US General Groves and the American scientist Robert Oppenheimer, the project recruited European expatriate scientists, supplemented by the best and brightest young American scientists. Existing American nuclear research projects (such as the powerful atomic accelerator at Berkeley) were melded into the new effort.
The tasks before them were daunting: even to begin, they had to extract usable quantities of the most fissionable uranium isotope, U-235, even though it constituted only 0.3% of uranium in nature. After many attempts and great effort, this was finally achieved at Oak Ridge, Tennessee. Then they had to prove that uranium “enriched” with the U-235 isotope could produce a self – sustaining nuclear reaction. This was done at the University of Chicago, where the world’s first nuclear reactor was built under the seats of the football stadium. These two phases of the project were conducted under the overall supervision of the Italian refugee scientist and Nobel Laureate Enrico Fermi.
It was soon discovered as well that nuclear reactors produced a new, artificial element that fissioned even more readily than uranium: plutonium. In theory, an even more devastating weapon could be constructed from its isotope Pu-239, but plutonium was difficult and dangerous to handle and a plutonium bomb would require even more sophisticated engineering. Realizing that conducting further experiments and production in populated areas entailed serious physical as well as security risks, General Groves supervised the construction of two new facilities: a main research base on a vast, deserted New Mexican mesa near the town of Los Alamos, and a satellite facility at Hanford, Washington to manufacture plutonium.
The main facility at Los Alamos soon became a miniature United Nations of top – ranking scientists, including nearly every past and future Nobel winner in the field. Among those who played central roles in the discoveries to come were Wigner, Bethe, Rotblatt, and Ulam from Germany; Leo Szilard and Edvard Teller from Hungary; Enrico Fermi from Italy; Kistiakowski from Russia; Carson Mark and Louis Slotkin from Canada; and Philip Morrison, Richard Feynman, and Robert Oppenheimer from the U.S.
The project scientists soon realized that the engineering construction of an atomic weapon using U-235 would be a relatively straightforward task; all that was required was to fire two matching hemispheres of the metal towards each other with a detonator, and an explosive fission reaction would take place. But the extraction of U-235 from natural uranium was fraught with seemingly endless and expensive delays. General Groves worried that a prototype might not even be built before the war had ended. (In fact, when Germany surrendered in April, 1945, a number of Project scientists such as Leo Szilard and future Nobel Laureate Josef Rotblatt resigned, believing there was no longer a rationale for the new weapon.)
By contrast, plutonium could be produced and separated quite easily from reactor waste, but was extremely dangerous to handle. More significant for the project were the difficulties of fabricating a plutonium bomb. It does not have a naturally stable metal state, and is highly inflammable. Plutonium spontaneously emits such large numbers of low – energy neutrons that there was a continuous low level of fission in the pieces extracted. (Pieces of plutonium are so radioactive they feel warm to the touch, but this experiment is not recommended).
If the engineers used the U-235 dual - hemisphere design with plutonium, this cloud of “thermal neutrons” produced continuously by the metal would cause the device to “pre-detonate” with little power, much like low – octane gasoline in a racing engine. The challenge was to come up with a new design that would force the metal into an explosive state before its natural fission dampened the reaction.
To solve this problem the scientists and engineers invented the so – called “implosion mechanism”. A hollow sphere of U-235 was coated with a larger sphere of plutonium, with a neutron source such as polonium in the center. The sphere was in turn clad with beryllium, (another very toxic metal) to “reflect” neutrons into the reaction. Outside this sphere were low – and high – velocity chemical explosive “lenses” precisely arranged to collapse the hollow sphere in less than a millisecond. Even so, the complexity of this apparatus led the Manhattan Project scientists to insist on a full – scale test. On July 10, 1945, the first atomic bomb was detonated at a remote area near Alamagordo, New Mexico. Its yield exceeded expectations, proving the effectiveness of the plutonium design, and demonstrating in the most dramatic possible way the horrifying potential of the new weapon. The explosive force was equivalent to the simultaneous detonation of tens of thousands of tons of the most powerful chemical explosive, TNT. Astounded by the unprecedented heat and blast wave he witnessed at Alamagordo, Dr. Oppenheimer quoted from the Hindu scriptures: “I am become death, destroyer of worlds.
The following month, both types of bombs were used on Japanese cities: a uranium bomb (codenamed “Little Boy”) was exploded over Hiroshima on August 6, 1945, and a plutonium bomb “Fat Man” on August 9, 1945, utterly devastating both cities, killing nearly a quarter of a million people.
After WWII ended, nuclear research based on the Manhattan Project went on to develop vastly more powerful nuclear weapons, and the Soviet Union began a crash program to build its own bomb, leading to the nuclear arms race that so terrified the world during the Cold War.
Fortunately, because of what former Secretary of Defense Robert MacNamara calls “an astounding convergence of great fear and fantastic luck” nuclear weapons have not been used in anger since the end of WWII.
- Richard Rhodes, The Making of the Atomic Bomb. NY: Simon & Schuster, 1995.
- The Race for the Bomb. Canadian Broadcasting Corporation – RAI – Telefilm Canada Co production, 1987. (6 hours, 3 parts, in VHS video)