What are nuclear weapons:
A nuclear weapon, also known as an atomic bomb, possesses enormous destructive power from nuclear fission, or a combination of fission and fusion reactions.

A nuclear device no larger than a conventional bomb can devastate an entire city by blast, fire, and radiation. Since they are weapons of mass destruction, the proliferation of nuclear weapons is a focus of international relations policy. Nuclear weapons have been deployed twice in war, both by the United States against the Japanese cities of Hiroshima and Nagasaki in 1945 during World War II.

History:
In the first decades of the 20th century,
physics was revolutionised with developments in the understanding of the nature of atoms, including the discoveries in atomic theory by John Dalton. Around the 20th century, it was discovered by Hans Geiger, 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. This led to the conclusion that the elements around us could contain tremendous amounts of unseen energy waiting to be harnessed.

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 via neutrons. Szilard subsequently assigned the patent to the British Admiralty so that the Official Secrets Act could cover it. 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. Despite his uncertainty, he correctly theorised uranium and thorium as primary candidates for such a reaction, along with beryllium, which was later determined 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.

In December 1938, Otto Hahn and Fritz Strassmann reported detecting the element barium after bombarding uranium with neutrons. Lise Meitner and Otto Robert Frisch correctly interpreted these results as due to uranium atoms’ splitting. Frisch confirmed this experimentally on January 13, 1939. They named the process “fission” because of its similarity to splitting 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 German fission in 1939, Leo Szilard concluded that uranium would be the element that could help him realise his 1933 idea about the nuclear chain reaction.

In the United States, scientists at Columbia University in New York City decided to replicate the experiment. On January 25, 1939, they 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 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. 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 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 utilising nuclear fission as a weapon, but no one was quite sure how it could be engineered.

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.

Organised research began in Britain and Canada as part of the Tube Alloys project, the world’s first nuclear weapons project. The Maud Committee was set up following the work of Frisch and Rudolf Peierls, who calculated uranium-235’s critical mass and found it much smaller than previously thought, meaning a deliverable bomb would be possible. The February 1940 Frisch–Peierls memorandum stated, “The energy liberated in the explosion of such a super-bomb…will, for an instant, produce a temperature comparable to that of the sun’s interior. 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 the Germans might seize it, he shipped the mine’s entire ore stockpile to a warehouse in New York.

For 18 months, British research outpaced American research, 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 known as the Manhattan Project. Two of his first acts were to obtain authorisation 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 U.S. effort quickly overtook the Tube Alloys project, and after the agreement in 1943, it was relocated and amalgamated into the Manhattan Project. Canada provided uranium and plutonium for the project.

Szilard started acquiring high-quality graphite and uranium, the necessary materials for building a large-scale chain reaction experiment. This experiment was successfully demonstrated at the University of Chicago on December 2, 1942. The success of this demonstration and technological breakthrough was partially due to Szilard’s new atomic theories, uranium lattice design, and identifying and mitigating a key graphite impurity (boron) through a collaboration with graphite suppliers.

With a scientific team led by J. Robert Oppenheimer, the Manhattan Project brought together some of the top scientific minds of the day, including exiles from Europe, with the production power of American industry to produce fission-based explosive devices before Germany. Scientific development was centralised in a secret laboratory at Los Alamos.

For a fission weapon to operate, there must be sufficient fissile material to support a chain reaction, a critical mass. Two methods were developed to separate the fissile uranium-235 isotope from the non-fissile uranium-238, which took advantage of the fact that uranium-238 has a slightly greater atomic mass: electromagnetic separation and gaseous diffusion. Another secret site was erected in 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 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 it absorbs a neutron, it becomes uranium-239, which decays into neptunium-239, and finally, the relatively stable plutonium-239, which is fissile like uranium-235. After Fermi achieved the world’s first sustained and controlled nuclear chain reaction with the creation of the first atomic pile, massive reactors were secretly constructed at what is now known as the Hanford Site to transform uranium-238 into plutonium for a bomb.

The simplest form of nuclear weapon is a gun-type fission weapon, where a sub-critical mass is shot at another sub-critical mass. The result is a super-critical mass and an uncontrolled chain reaction that creates 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: 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.

As a result, the development of Fat Man was given high priority since Emilio Segrè found that the plutonium-239 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 unexpected presence of plutonium-240 impurities. If such plutonium were used in a gun-type design, the chain reaction would start in the split second before the critical mass was fully assembled, blowing the weapon apart with a much lower yield than expected; chemical explosives were used to implode a sub-critical sphere of plutonium, thus increasing its density and making it into a critical mass.

General Groves ordered a team of scientists to follow eastward-moving victorious Allied troops into Europe to assess the status of the German nuclear program (and to prevent the westward-moving Soviets from gaining any materials or scientific workforce). They concluded that while Germany had a modest atomic research program headed by Werner Heisenberg, the government had not invested significantly in the project, which was nowhere near successful. Similarly, Japan’s efforts at developing a nuclear weapon were starved of resources. The Japanese navy lost interest when a committee led by Yoshio Nishina concluded in 1943 that “it would probably be difficult even for the United States to realise the application of atomic power during the war”.

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

Dropping it 
On April 12, after Roosevelt’s death, Vice President Harry S. Truman assumed the presidency. At the time of Germany’s unconditional surrender on May 8, 1945, the Manhattan Project was still months away from producing a working weapon.

Because of the difficulties in making a working plutonium bomb, it was decided that the weapon should be tested. 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 energy equivalent to 22 kilotons of TNT, far more powerful than any previous weapon. The news of the test’s success was rushed to Truman at the Potsdam Conference, where Churchill was briefed, and Soviet Premier Joseph Stalin was informed of the new weapon. On July 26, the Potsdam Declaration was issued containing an ultimatum for Japan: either surrender or suffer “complete and utter destruction”, although nuclear weapons were not mentioned.

Despite the arguments, Truman ordered the use of weapons in Japanese cities. Under the clause of the 1943 Quebec Agreement that specified that nuclear weapons would not be used against another country without mutual consent, the atomic bombing of Japan was recorded as a decision of the Anglo-American Combined Policy Committee.

Truman hoped it would send a strong message that would end in the capitulation of the Japanese leadership and avoid a lengthy invasion of the islands. Truman and his Secretary of State James F. Byrnes were also intent on ending the Pacific war before the Soviets could enter it; on May 10–11, 1945, the Target Committee at Los Alamos, led by Oppenheimer, recommended Kyoto, Hiroshima, Yokohama, and Kokura as possible targets. Concerns about Kyoto’s cultural heritage led to its replacement by Nagasaki. In late July and early August 1945, a series of leaflets were dropped over several Japanese cities, warning them of an imminent destructive attack.

On August 6, 1945, a uranium-based weapon, Little Boy, was detonated above the Japanese city of Hiroshima, and three days later, a plutonium-based weapon, Fat Man, was detonated above the Japanese city of Nagasaki. To date, Hiroshima and Nagasaki remain the only two instances of nuclear weapons being used in combat. The atomic raids killed at least one hundred thousand Japanese civilians and military personnel outright, 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 their ability to wage war systematically. On August 15, Emperor Hirohito announced Japan’s surrender

Need for the Nuclear Weapons:
The development of nuclear weapons was a response to several factors, including:

1. World War II: The devastating effects of conventional weapons during World War II led to a search for more powerful weapons to deter future conflicts.

2. Nazi Germany’s nuclear program: The fear of Nazi Germany developing atomic weapons prompted the United States and the United Kingdom to initiate their nuclear programs.

3. Cold War rivalry: The ideological and geopolitical tensions between the United States and the Soviet Union led to an arms race, with nuclear weapons seen as a means to maintain strategic superiority.

4. Deterrence theory: The concept of mutually assured destruction (MAD) posited that possessing nuclear weapons would deter other nations from attacking, as the consequences would be catastrophic.

5. Scientific curiosity: Physicists like Einstein, Fermi, and Oppenheimer were driven by scientific curiosity and the challenge of harnessing atomic energy.

6. National security: Countries sought to enhance their security by possessing weapons that could guarantee their survival and protect their interests.

Thus, the development of nuclear weapons was a complex and multifaceted process driven by a combination of political, strategic, and scientific factors.