The Atomic Bomb: How Nuclear Fission Works, and the History of Hiroshima and Nagasaki

How nuclear fission works

The atomic bomb releases energy by splitting the nuclei of heavy atoms, a process physicists call nuclear fission. When a neutron strikes a heavy nucleus such as uranium-235 or plutonium-239, the nucleus becomes unstable and quickly splits into two fragments of similar mass. Each split releases on average 2-3 new neutrons (fission of uranium-235 releases about 2.45, fission of plutonium-239 about 2.9), along with an enormous amount of energy, about 200 MeV for every uranium-235 nucleus split, the equivalent of 82 trillion joules for every kilogram of material fissioned. About 85% of that energy is heat, carried as the kinetic energy of the fragments.

For the reaction to sustain itself, a minimum amount of material is needed, the critical mass, the threshold beyond which the neutrons produced outnumber those lost to the outside. With a neutron reflector around the material, about 15 kg of uranium-235 or 5 kg of plutonium-239 is enough; without a reflector, Enrico Fermi and Leo Szilard’s early experiments put the uranium-235 threshold at around 50 kg. Shape matters as much as quantity, a sphere has the smallest surface area for a given mass, so it loses fewer neutrons outward than any other shape.

Practical example: once the critical mass is exceeded, the chain reaction behaves like a row of dominoes falling on their own. Each fission triggers 2-3 more, which in turn trigger still more, in a progression that inside a bomb plays out in a fraction of a second. It is the same physical principle that, slowed down and controlled, powers the nuclear reactors used to generate electricity.

Fission should be kept separate from fusion, the opposite process that powers stars, in fusion two light nuclei, such as deuterium and tritium, join into a heavier nucleus, releasing even more energy per kilogram of material (the deuterium-tritium reaction produces helium-4, one neutron and 17.6 MeV). Both bombs used in 1945 relied on fission, not fusion.

The Manhattan Project

Development of the bomb took place during the Second World War, inside the secret program known as the Manhattan Project. It was an operation on an industrial scale, comparable in organization to the era’s automobile industry, bringing together leading scientists, private industry, the military and tens of thousands of ordinary Americans. It cost $2.2 billion at the time and employed as many as 130,000 workers at its peak, spread mainly across three sites: Oak Ridge, in Tennessee, where uranium was enriched; Hanford, in Washington State, where plutonium was produced; and Los Alamos, in New Mexico, where the devices were designed.

On 16 July 1945, in the New Mexico desert, the project reached its decisive moment: the Trinity test detonated the first nuclear device in history, an implosion-type plutonium bomb of the same design that, three weeks later, would be used on Nagasaki.

The U.S. Department of Energy is the direct successor to the Manhattan Engineer District, the Army Corps of Engineers unit that built the bomb. On 10 November 2015 the project’s three main sites became the Manhattan Project National Historical Park, after a national defense law had authorized its creation on 19 December 2014.

Hiroshima and Nagasaki, August 1945

On 6 August 1945, at 8:15 am local time, the B-29 bomber “Enola Gay” (which had taken off from the island of Tinian at 2:45 that morning, piloted by Colonel Paul Tibbets) dropped the bomb “Little Boy” on Hiroshima. Three days later, on 9 August, the B-29 “Bockscar”, piloted by Charles Sweeney, dropped “Fat Man” on Nagasaki at 10:58 am local time, the mission’s original target had been Kokura, scrapped at the last moment because of cloud cover and poor visibility.

	Little Boy (Hiroshima)	Fat Man (Nagasaki)
Fissile material	Uranium-235	Plutonium-239
Design	Gun-type	Implosion
Estimated yield	13 kilotons	21 kilotons
Date and time	6 August 1945, 8:15 am	9 August 1945, 10:58 am
Detonation altitude	About 610 meters	About 503 meters

At Hiroshima, home to between 280,000 and 290,000 civilians plus 43,000 soldiers, the blast damaged or destroyed 70,000 of 76,000 buildings, 48,000 of them razed completely, ground temperature reached 7,000 degrees Fahrenheit, about 3,870 °C, in under a second. Casualty estimates vary with the time frame considered, from 90,000 to 166,000 deaths in the following four months, while the city of Hiroshima itself estimates 237,000 deaths, direct and indirect (burns, radiation sickness, cancer), over a longer span. At Nagasaki, home to about 263,000 people, civilians, soldiers and prisoners combined, an estimated 40,000 to 75,000 people died immediately, with about 80,000 total deaths by the end of 1945, the radius of total destruction was about 1.6 kilometers. According to the World Nuclear Association, most deaths in both cities were caused by blast injuries and burns, not direct radiation.

Practical example: before the bomb was used, a committee of scientists and military officers weighed the idea of a demonstration in an uninhabited area instead of over a city. It rejected the idea unanimously, because there was no guarantee it would convince Japan’s leadership to surrender. The same evaluation weighed the expected human cost of an invasion, the Battle of Okinawa, between April and June 1945, had already cost the United States about 49,000 casualties, and Japan had deployed more than 560,000 soldiers on the island of Kyushu in anticipation of a possible American landing.

Japan’s surrender, and a debate that remains open

Japan announced its surrender on 14 August 1945. The preceding weeks had seen several factors converge. On 8 August 1945 the Soviet Union declared war on Japan, and the next day, 9 August, it invaded Manchuria with over 1.5 million soldiers along three fronts, ending Japan’s hope of a Moscow-mediated negotiated peace.

The relative weight of these factors remains a matter of debate among historians. Some, such as Tsuyoshi Hasegawa, argue that the Soviet entry into the war weighed as much as the atomic bombings, if not more, precisely because it removed Japan’s last diplomatic path toward a negotiated peace. Others assign the decisive weight to the two bombs. There is no single consensus, and it is a simplification to name the atomic bombs as the sole cause of the surrender, the Japanese decision grew out of several converging pressures within the same handful of days.

After the bomb, radiation, rebuilding, non-proliferation

The bomb’s energy split between initial radiation, about 5% of the total, responsible for immediate damage within roughly one kilometer, and residual radiation, about 10% of the total. The latter fell off quickly, according to the city of Hiroshima, 80% of it was released within the first 24 hours, and within a week levels had already dropped to one-millionth of what they were immediately after the explosion. By 2026 the radiation levels measured at Hiroshima and Nagasaki match the natural background radiation found everywhere on the planet. Both cities were rebuilt and became major industrial centers, by 2026 Hiroshima has over one million residents and Nagasaki about 440,000.

Internationally, the nuclear arms race that marked the Cold War, the same technological rivalry between the United States and the Soviet Union that, by a different route, also led to the Apollo 11 moon landing, pushed the two superpowers and other countries toward ever larger arsenals. The Treaty on the Non-Proliferation of Nuclear Weapons, opened for signature in 1968 and in force since 5 March 1970, tried to halt that spread, states with nuclear weapons committed not to transfer them, states without them committed not to acquire them, under verification by the International Atomic Energy Agency over civilian nuclear activity. Extended indefinitely in 1995, the treaty counts 191 states parties, the most widely signed agreement in the world on nuclear non-proliferation.