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.

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 (British patent 630,726). 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.^{[citation needed]}