History of nuclear weapons

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A nuclear fireball lights up the night in a United States nuclear test.

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The history of nuclear weapons chronicles the development of nuclear weapons. Nuclear weapons are devices that posess enormous destructive potential that uses energy derived from nuclear fission or nuclear fusion reactions. Starting with the scientific breakthroughs of the 1930s which made their development possible, continuing through the nuclear arms race and nuclear testing of the Cold War, and finally with the questions of proliferation and possible use for terrorism in the early 21st century.

The first fission weapons, also known as "atomic bombs", were developed in the United States during World War II in what was called the Manhattan Project, at which point two were dropped on Japan. The Soviet Union started development shortly thereafter with their own atomic bomb project, and not long after that both countries developed even more powerful fusion weapons also called, "hydrogen bombs". During the Cold War, these two countries each acquired nuclear weapons arsenals numbering in the thousands, placing many of them onto rockets which could hit targets anywhere in the world. Currently there are at least nine countries with functional nuclear weapons. A considerable amount of international negotiating has focused on the threat of nuclear warfare and the proliferation of nuclear weapons to new nations or groups.

There have been (at least) four major false alarms, the most recent in 1995, that almost resulted in the US or Russia launching its weapons in retaliation for a supposed attack.

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Fat Man

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Physics and politics in the 1930s

In the first decades of the twentieth century, physics was revolutionized with developments in the understanding of the nature of atoms. In 1898 , French physicist Pierre Curie and his Polish wife Marie had discovered that present in pitchblende, an ore of uranium, was a substance which emitted large amounts of radioactivity, which they named radium. This raised the hopes of both scientists and lay people that the elements around us could contain tremendous amounts of unseen energy, waiting to be tapped.

Experiments by Ernest Rutherford in 1911 indicated that the vast majority of an atom's mass was contained in a very small nucleus at its core, made up of protons, surrounded by a web of whirring electrons. In 1932, James Chadwick discovered that the nucleus contained another fundamental particle, the neutron, and in the same year John Cockcroft and Ernest Rutherford "split the atom" for the first time, the first occasion on which an atomic nucleus of one element had been successfully changed to a different nucleus by artificial means.

In 1934 , French physicists Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles, and in the same year Italian physicist Enrico Fermi reported similar results when bombarding uranium with neutrons.

In 1938 , Germans Otto Hahn and Fritz Strassmann released the results of their finding proving that what Fermi had witnessed in 1934 was no less than the bursting of the uranium nucleus: nuclear fission. Immediately afterwards, Lise Meitner and Otto Robert Frisch described the theoretical mechanisms of fission and revealed that large amounts of binding energy were released in the process. Hungarian Leó Szilárd confirmed with his own experiments that along with energy, neutrons were given off in the reaction as well, creating the possibility of a nuclear chain reaction, whereby each fission created two or more other fissions, exponentially releasing energy.

As the Nazi army marched into first Czechoslovakia in 1938, and then Poland in 1939 , officially beginning World War II, many of Europe's top physicists had already begun to flee from the imminent conflict. Scientists on both sides of the conflict were well aware of the possibility of utilizing nuclear fission as a weapon, but at the time no one was quite sure how it could be done. In the early years of the Second World War, physicists abruptly stopped publishing on the topic of fission, an act of self-censorship to keep the opposing side from gaining any advantages.

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Simple diagram of nuclear fission. In the first frame, a neutron is about to collide with the nucleus of a U-235 atom. In the second frame, the neutron has been absorbed and briefly turned the nucleus into an unstable U-236 atom. In the third frame, the U-236 atom has fissioned, resulting in two fission fragments (Ba-141 and Kr-92), three neutrons, and the release of a relatively large amount of binding energy.

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From Los Alamos to Hiroshima

By the beginning of World War II, there was concern among scientists in the Allied nations that Nazi Germany might have their 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, though, who had made crucial calculations indicating that a fission weapon could be completed within only a few years, by 1941 the project had been wrested into better bureaucratic hands, and in 1942 came under the auspices of General Leslie Groves as the Manhattan Project. Scientifically led by the American physicist Robert Oppenheimer, the project brought together the top scientific minds of the day (many exiles from Europe) with the production power of American industry for the goal of producing fission-based explosive devices before Germany could. Britain and the U.S. agreed to pool their resources and information for the project, but the other Allied power—the Soviet Union under Joseph Stalin—was not informed.

A massive industrial and scientific undertaking, the Manhattan Project involved many of the world's great physicists in the scientific and development aspects. The United States made an unprecedented investment into wartime research for the project, which was spread across over 30 sites in the U.S. and Canada. Scientific knowledge was centralized at a secret laboratory known as Los Alamos, previously a small ranch school near Santa Fe, New Mexico.

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 2.5 neutrons on average. Uranium-238, on the other hand, absorbs neutrons and does not fission, effectively putting a stop to any ongoing fission reaction. It was discovered that an atomic bomb based on uranium would need to be made of almost completely pure uranium-235 (at least 80% pure), or else the presence of uranium-238 would quickly curtail the nuclear chain reaction. The team of scientists working on the Manhattan Project immediately realized that one of the largest problems they would have to solve was how to remove uranium-235 from natural uranium, which was composed of 99.3% uranium-238. Two methods were developed during the wartime project, both of which took advantage of the fact that uranium-238 has a slightly greater atomic mass than uranium-235: electromagnetic separation and gaseous diffusion—methods which 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. It was a massive investment: at the time, one of the Oak Ridge facilities (K-25) was the 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.

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UC Berkeley physicist J. Robert Oppenheimer led the Allied scientific effort at Los Alamos.

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Though uranium-238 cannot be used inside an atomic bomb, when it 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 which does not exist in nature. 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 the state of Washington, using the Columbia River as cooling water, to transform uranium-238 into plutonium for a bomb.

For a fission weapon to operate, there must be a critical mass—the amount needed for a self-sustaining nuclear chain reaction—of fissile material bombarded with neutrons at any one time. The simplest form of nuclear weapon would be 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 which, when bombarded with neutrons, would undergo fission at a rapid rate and create the desired explosion.

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Massive new physics machines were assembled at secret installations around the United States for the production of enriched uranium and plutonium.

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But it was soon discovered that plutonium cannot be used in a "gun assembly," as it has too high a level of background neutron radiation; it undergoes spontaneous fission to a very small extent. If 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). After some despair, Los Alamos scientists discovered another approach: using chemical explosives to implode a sub-critical sphere of plutonium, which would increase its density and make it into a critical mass. The difficulties with implosion were in 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 had ordered a team of scientists—Project Alsos—to follow eastward-moving victorious Allied troops into Europe in order to assess the status of the German nuclear program (and to prevent the westward-moving Russians from gaining any materials or scientific manpower). It was concluded that while Nazi Germany had also had an atomic bomb program, headed by Werner Heisenberg, the government had not made a significant investment in the project, and had been nowhere near success.

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The two fission bomb assembly methods.

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By the unconditional surrender of Germany on May 8, 1945, the Manhattan Project was still months away from 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 sure if it would work before his meeting with Joseph 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 released the equivalent of 19 kilotons of TNT, far mightier than any weapon ever used before. The news of the test's success was rushed to Truman, who used it as leverage at the upcoming Potsdam Conference, held near Berlin.

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The atomic fireball at the "Trinity" nuclear test secretly rang in the atomic age.

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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 which would end in the capitulation of the Japanese leadership and avoid a lengthy invasion of the island. There were suggestions to drop the atomic bomb on Tokyo, the capital of Japan, but concerns about Tokyo's cultural heritage changed the plan. On August 6, 1945, a uranium-based weapon, "Little Boy", was let loose on the Japanese city of Hiroshima. Three days later, a plutonium-based weapon, "Fat Man", was dropped onto 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 die later 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 not completed more atomic bombs at the time

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The atomic bombings of Hiroshima and Nagasaki killed tens of thousands of Japanese civilians.