History of Nuclear Weapons - From Los Alamos To Hiroshima

From Los Alamos To Hiroshima

By the beginning of World War II, scientists of the Allied nations were concerned that Germany might have its own project to develop fission-based weapons. Organized research first began in Britain as part of the "Tube Alloys" project, and in the United States a small amount of funding was given for research into uranium weapons, starting in 1939 with the Uranium Committee under Lyman James Briggs.

At the urging of British scientists Otto Frisch and Rudolf Peierls (both from the University of Birmingham)—who had made crucial calculations in 1940 indicating that a fission weapon (which they termed the "super-bomb") could be completed within only a few years. Physicists Frisch and Peierls stated that: "The energy liberated in the explosion of such a super-bomb...will, for an instant, produce a temperature comparable to that of the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city." (this quote is from the Frisch–Peierls memorandum). Early British work on nuclear weapons occurred under the code name 'Tube Alloys' (1941). The "Tube Alloys" project was relocated and amalgamated into the Manhattan Project in 1943. By 1941 (in both the USA and Britain) the project had been wrested into more capable bureaucratic hands, and by 1942 (in America) came under the auspices of a Military Policy Committee led by General Leslie Groves. This was known as the Manhattan Project.

With a scientific team led by the American physicist Robert Oppenheimer, the project brought together the top scientific minds of the day, including many exiles from Europe, with the production power of American industry for the goal of producing fission-based explosive devices before Germany. Britain and the U.S. agreed to pool their resources and information for the project, but the other Allied power—the Soviet Union—was not informed.

A massive industrial and scientific undertaking, the Manhattan Project involved many of the world's leading physicists in the scientific and development aspects. The U.S. made an unprecedented investment in the project, spread across more than 30 sites in the U.S. and Canada. They centralized scientific development at Los Alamos, a secret laboratory that was previously a small ranch school near Santa Fe, New Mexico.

Edgar Sengier was instrumental in securing the necessary quantity of Uranium ore from Belgian Congo. Uranium appears in nature primarily in two isotopes: uranium-238 and uranium-235. When the nucleus of uranium-235 absorbs a neutron, it undergoes nuclear fission, splitting into two "fission products" and releasing energy and, on average, 2.5 neutrons. Uranium-238, on the other hand, absorbs neutrons but does not split, effectively putting a stop to any ongoing fission reaction.

They developed two methods during the wartime project, which both took advantage of the fact that uranium-238 has a slightly greater atomic mass than uranium-235: electromagnetic separation and gaseous diffusion—methods that separated isotopes based on their differing weights. Another secret site was erected at rural Oak Ridge, Tennessee, for the large-scale production and purification of the rare isotope, which required considerable investment. At the time, K-25, one of the Oak Ridge facilities, was the world's largest factory under one roof. The Oak Ridge site employed tens of thousands of employees at its peak, most of whom had no idea what they were working on.

Although uranium-238 cannot be used for the initial stage of an atomic bomb, when U-238 absorbs a neutron, it transforms first into an unstable element, uranium-239, and then decays into neptunium-239, and finally the relatively stable plutonium-239, an element that does not exist naturally on Earth. Plutonium is also fissile and can be used to create a fission reaction, and after Enrico Fermi achieved the world's first sustained and controlled nuclear chain reaction in the creation of the first "atomic pile"—a primitive nuclear reactor—in a basement at the University of Chicago, massive reactors were secretly created at what is now known as Hanford Site in Washington State – using water from the Columbia River in the cooling process – to transform uranium-238 into plutonium for a bomb.

For a fission weapon to operate, there must be a critical mass of fissile material bombarded with neutrons at any one time. The simplest form of nuclear weapon is a gun-type fission weapon, where a sub-critical mass of fissile material (such as uranium-235) would be shot at another sub-critical mass of fissile material. The result would be a super-critical mass that, when bombarded with neutrons, would undergo fission at a rapid rate and create the desired explosion. The weapons envisaged in 1942 were the two gun-type weapons, Little Boy (uranium) and Thin Man (plutonium), and the Fat Man plutonium implosion bomb.

In early 1943 Oppenheimer determined that two projects should proceed forwards: the "Thin Man" project (plutonium gun) and the "Fat Man" project (plutonium implosion). The plutonium gun was to receive the bulk of the research effort, as it was the project with the most uncertainty involved. It was assumed that the uranium gun-type bomb could then be adapted from it.

In December 1943 the British mission of 19 scientists arrived in Los Alamos. Hans Bethe became head of the Theoretical Division.

In early February 1944 the science fiction Astounding Science Fiction published a fiction story Deadline by Cleve Cartmill which described the fission atom bomb in detail. Investigation by the FBI showed that this idea came from non-secret sources and independent invention, not from a security breach.

But in April 1944 it was found by Emilio Segrè that the plutonium produced by the Hanford reactors had too high a level of background neutron radiation, and underwent spontaneous fission to a very small extent, due to the presence of impurities of the Pu-240 isotope. If such plutonium were used in a "gun assembly," the chain reaction would start in the split seconds before the critical mass was assembled, blowing the weapon apart before it would have any great effect (this is known as a fizzle).

As a result, development of Fat Man (the implosion bomb) was given high priority. Chemical explosives were used to implode a sub-critical sphere of plutonium, thus increasing its density and making it into a critical mass. The difficulties with implosion centered on the problem of making the chemical explosives deliver a perfectly uniform shock wave upon the plutonium sphere— if it were even slightly asymmetric, the weapon would fizzle (which would be expensive, messy, and not a very effective military device). This problem was circumvented by the use of hydrodynamic "lenses"—explosive materials of differing densities—which would focus the blast waves inside the imploding sphere, akin to the way in which an optical lens focuses light rays.

After D-Day, General Groves ordered a team of scientists—Project Alsos—to follow eastward-moving victorious Allied troops into Europe to assess the status of the German nuclear program (and to prevent the westward-moving Russians from gaining any materials or scientific manpower). They concluded that, while Germany had an atomic bomb program headed by Werner Heisenberg, the government had not made a significant investment in the project, and it had been nowhere near success.

Historians claim to have found a rough schematic showing a Nazi nuclear bomb. Research was conducted in the German nuclear energy project. In March 1945, a German scientific team was directed by the physicist Kurt Diebner to develop a primitive nuclear device in Ohrdruf, Thuringia. Last ditch research was conducted in an experimental nuclear reactor at Haigerloch.

At the time of the unconditional surrender of Germany on May 8, 1945, the Manhattan Project was still months away from producing a working weapon. That April, after the death of American President Franklin D. Roosevelt, former Vice-President Harry S. Truman was told about the secret wartime project for the first time.

Because of the difficulties in making a working plutonium bomb, it was decided that there should be a test of the weapon, and Truman wanted to know for certain if it would work before his meeting with Stalin at an upcoming conference on the future of postwar Europe. On July 16, 1945, in the desert north of Alamogordo, New Mexico, the first nuclear test took place, code-named "Trinity," using a device nicknamed "the gadget." The test, a plutonium implosion type device, released the equivalent of 19 kilotons of TNT, far more powerful than any weapon ever used before. The news of the test's success was rushed to Truman, who used it as leverage at the Potsdam conference, held near Berlin.

After hearing arguments from scientists and military officers over the possible uses of the weapons against Japan (though some recommended using them as "demonstrations" in unpopulated areas, most recommended using them against "built up" targets, a euphemistic term for populated cities), Truman ordered the use of the weapons on Japanese cities, hoping it would send a strong message that would end in the capitulation of the Japanese leadership and avoid a lengthy invasion of the islands.

There were suggestions to drop the atomic bomb on Tokyo, the capital of Japan, but concerns about Tokyo's cultural heritage forced a reconsideration of this plan. On May 10–11, 1945, the Target Committee at Los Alamos, led by Oppenheimer, recommended Kyoto, Hiroshima, Yokohama, and the arsenal at Kokura as possible targets. On August 6, 1945, a uranium-based weapon, "Little Boy," was detonated above the Japanese city of Hiroshima. Three days later, a plutonium-based weapon, "Fat Man," was detonated above the city of Nagasaki. The atomic bombs killed at least one hundred thousand Japanese outright, most of them civilians, with the heat, radiation, and blast effects.

Many tens of thousands would later die of radiation sickness and related cancers. Truman promised a "rain of ruin" if Japan did not surrender immediately, threatening to eliminate Japanese cities, one by one; Japan surrendered on August 15. Truman's threat was in fact a bluff, since the US had but one remaining uranium-gun type bomb completed.

See also: Japanese nuclear weapons program