History of nuclear weapons
Building on major scientific breakthroughs made during the 1930s, the United Kingdom began the world's first nuclear weapons research project, codenamed Tube Alloys, in 1941, during World War II. The United States, in collaboration with the United Kingdom, initiated the Manhattan Project the following year to build a weapon using nuclear fission. The project also involved Canada. In August 1945, the atomic bombings of Hiroshima and Nagasaki were conducted by the United States, with British consent, against Japan at the close of that war, standing to date as the only use of nuclear weapons in hostilities.
The Soviet Union started development shortly after with their own atomic bomb project, and not long after, both countries were developing even more powerful fusion weapons known as hydrogen bombs. Britain and France built their own systems in the 1950s, and the number of states with nuclear capabilities has gradually grown larger in the decades since.
A nuclear weapon, also known as an atomic bomb, possesses enormous destructive power from nuclear fission, or a combination of fission and fusion reactions.
Background
In the first decades of the 19th century, physics was revolutionized with developments in the understanding of the nature of atoms including the discoveries in atomic theory by John Dalton. Around the turn of the 20th century, it was discovered by Hans Geiger and Ernest Marsden and then Ernest Rutherford, that atoms had a highly dense, very small, charged central core called an atomic nucleus. In 1898, Pierre and Marie Curie discovered that pitchblende, an ore of uranium, contained a substance—which they named radium—that emitted large amounts of radiation. Ernest Rutherford and Frederick Soddy identified that atoms were breaking down and turning into different elements. Hopes were raised among scientists and laymen that the elements around us could contain tremendous amounts of unseen energy, waiting to be harnessed. In 1905, Albert Einstein described this potential in his famous equation, E = mc2.H. G. Wells was inspired by the work of Rutherford to write about an "atom bomb" in a 1914 novel, The World Set Free, which appeared shortly before the First World War. In a 1924 article, Winston Churchill speculated about the possible military implications: "Might not a bomb no bigger than an orange be found to possess a secret power to destroy a whole block of buildings—nay to concentrate the force of a thousand tons of cordite and blast a township at a stroke?"
At the time however, there was no known mechanism which could be used to unlock the vast energy potential that was theorized to exist inside the atom. The only particle then known to exist within the nucleus was the positively-charged proton, which would act to repel protons set in motion towards it. Then in 1932, a key breakthrough was made with the discovery of the neutron. Having no electric charge, the neutron is able to penetrate the nucleus with relative ease.
In January 1933, the Nazis came to power in Germany and suppressed Jewish scientists. Physicist Leo Szilard fled to London where, in 1934, he patented the idea of a nuclear chain reaction using neutrons. The patent also introduced the term critical mass to describe the minimum amount of material required to sustain the chain reaction and its potential to cause an explosion. The patent was not about an atomic bomb per se, as the possibility of chain reaction was still very speculative. Szilard subsequently assigned the patent to the British Admiralty so that it could be covered by the Official Secrets Act. This work of Szilard's was ahead of the time, five years before the public discovery of nuclear fission and eight years before a working nuclear reactor. When he coined the term neutron inducted chain reaction, he was not sure about the use of isotopes or standard forms of elements. Despite this uncertainty, he correctly theorized uranium and thorium as primary candidates for such a reaction, along with beryllium which was later determined to be unnecessary in practice. Szilard joined Enrico Fermi in developing the first uranium-fuelled nuclear reactor, Chicago Pile-1, which was activated at the University of Chicago in 1942.
In Paris in 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles; in Italy Enrico Fermi reported similar results when bombarding uranium with neutrons. He mistakenly believed he had discovered elements 93 and 94, naming them ausenium and hesperium. In 1938 it was realized these were in fact fission products.
In December 1938, Otto Hahn and Fritz Strassmann reported that they had detected the element barium after bombarding uranium with neutrons. Lise Meitner and Otto Robert Frisch correctly interpreted these results as being due to the splitting of the uranium atom. Frisch confirmed this experimentally on January 13, 1939. They gave the process the name "fission" because of its similarity to the splitting of a cell into two new cells. Even before it was published, news of Meitner's and Frisch's interpretation crossed the Atlantic. In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction.
After learning about the German fission in 1939, Leo Szilard concluded that uranium would be the element which could realize his 1933 idea about nuclear chain reaction.
In the United States, scientists at Columbia University in New York City decided to replicate the experiment and on January 25, 1939, conducted the first nuclear fission experiment in the United States in the basement of Pupin Hall. The following year, they identified the active component of uranium as being the rare isotope uranium-235.
Between 1939 and 1940, Joliot-Curie's team applied for a patent family covering different use cases of atomic energy, one being the first official document explicitly mentioning a nuclear explosion as a purpose, including for war. This patent was applied for on May 4, 1939, but only granted in 1950, being withheld by French authorities in the meantime.
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, releasing energy and, on average, 2.5 neutrons. Because uranium-235 releases more neutrons than it absorbs, it can support a chain reaction and so is described as fissile. Uranium-238, on the other hand, is not fissile as it does not normally undergo fission when it absorbs a neutron.
By the start of the war in September 1939, many scientists likely to be persecuted by the Nazis had already escaped. Physicists on both sides were well aware of the possibility of utilizing nuclear fission as a weapon, but no one was quite sure how it could be engineered. In August 1939, concerned that Germany might have its own project to develop fission-based weapons, Albert Einstein signed a letter to U.S. President Franklin D. Roosevelt warning him of the threat.
Roosevelt responded by setting up the Uranium Committee under Lyman James Briggs but, with little initial funding, progress was slow. It was not until the U.S. entered the war in December 1941 that Washington decided to commit the necessary resources to a top-secret high priority bomb project.
Organized research first began in Britain as part of the Tube Alloys project, the world's first nuclear weapons project, which also involved Canada. The Maud Committee was set up following the work of Frisch and Rudolf Peierls who calculated uranium-235's critical mass and found it to be much smaller than previously thought which meant that a deliverable bomb should be possible. In the February 1940 Frisch–Peierls memorandum they 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."
Edgar Sengier, a director of Shinkolobwe Mine in the Congo which produced by far the highest quality uranium ore in the world, had become aware of uranium's possible use in a bomb. In late 1940, fearing that it might be seized by the Germans, he shipped the mine's entire stockpile of ore to a warehouse in New York.
For 18 months British research outpaced the American but by mid-1942, it became apparent that the industrial effort required was beyond Britain's already stretched wartime economy.
In September 1942, General Leslie Groves was appointed to lead the U.S. project which became known as the Manhattan Project. Two of his first acts were to obtain authorization to assign the highest priority AAA rating on necessary procurements, and to order the purchase of all 1,250 tons of the Shinkolobwe ore. The Tube Alloys project was quickly overtaken by the U.S. effort and after Roosevelt and Churchill signed the Quebec Agreement in 1943, it was relocated and amalgamated into the Manhattan Project. Canada provided uranium and plutonium for the project.
Szilard started to acquire high-quality graphite and uranium, which were the necessary materials for building a large-scale chain reaction experiment. The Metallurgical Laboratory at the University of Chicago was tasked with the completion of such a reactor, and Fermi moved there, continuing the pile experiments he began at Columbia. After many subcritical designs, Chicago Pile-1 achieved criticality on December 2, 1942. The success of this demonstration and technological breakthrough were partially due to Szilard's new atomic theories, his uranium lattice design, and the identification and mitigation of a key graphite impurity through a joint collaboration with graphite suppliers.