6 and 9 August 1945: Hiroshima and Nagasaki
The first and only time nuclear weapons were used

6 and 9 August 1945
Hiroshima / Nagasaki

The test site after detonation: a crater of fused soil, also known as “Trinite” radiates from ground zero.

On 16 July 1945, U.S. scientists working on the Manhattan Project successfully detonated the first-ever nuclear explosion in the ‘Trinity’ test at Alamogordo, New Mexico. With World War II still dragging on in the Pacific, preparations moved forward to use nuclear bombs against Japan.

The bombing of Hiroshima and Nagasaki are a reminder of what horrors nuclear weapons can inflict.CTBTO Executive Secretary Lassina Zerbo
Pilot Paul Tibbets (centre) had named the aircraft after his mother.

On 6 August 1945, at 08:15, the first bomb was dropped on the centre of Hiroshima. ‘Little Boy’ was a gun-type fission bomb, using a conventional explosive charge to fire one sub-critical mass of uranium into another. This kind of device had never been tested before, but the scientists were confident it would work.


And it did. The bomb had an explosive yield of around 13 kilotons. At the moment of detonation, a fireball was generated that raised temperatures to 4,000 degrees Celsius, turning Hiroshima – where many buildings were made of wood and paper - into an inferno. The blast created shock waves faster than the speed of sound. This and the radiation immediately killed everything within one kilometre of the hypocentre.

A victim of the Hiroshima bomb (AP Photo/The Association of the Photographers of the Atomic (Bomb) Destruction of Hiroshima, Yotsugi Kawahara)

After the blast, those who approached ground zero searching for the missing were exposed to radiation. Black rain, containing large amounts of radioactive fallout, caused widespread contamination. Estimates of casualties vary greatly. A more conservative estimate by the atomic archive lists 66,000 people killed immediately and a total death toll of 135,000, while the U.S.-Japanese Radiation Effects Research Foundation indicates a range of 90,000 to 166,000 deaths within the first four months.

Hiroshima’s many messages of peace and hope have educated the world about the devastating humanitarian consequences of nuclear weapons, inspiring a global campaign focused on the unacceptability of using these arms under international humanitarian and human rights law. UN Secretary-General Ban Ki-moon's message for Hiroshima Day 2014
'Fat Man' yielded 21 kilotons.

Three days later, on the morning of 9 August, a second U.S. aircraft rose from the airbase at Tinian Island in the Pacific Ocean. The nuclear bomb it carried was code-named ‘Fat Man’. It was a more sophisticated plutonium-based implosion-type bomb which had been tested in the Trinity test. The primary target had been the city of Kokura. However due to a thick layer of clouds, the airplane’s crew reverted to the secondary target - Nagasaki.

Nagasaki before and after

The bomb detonated at an altitude of approximately 500 metres and had a yield of 21 kilotons. Casualty estimates by the Radiation Effects Research Foundation for immediate deaths range from 60,000 to 80,000.


In the weeks that followed the bombings and Japan’s surrender, Hiroshima and Nagasaki became the object of intense studies by U.S. scientists. The Atomic Bomb Casualty Commission was founded to study the effects of radioactive contamination on the victims. Its results, including the extensive film and photo material taken at the time, remained classified for decades.

Boy carrying his injured brother a day after the Nagasaki bombing.

The bombings were officially justified as a necessary evil to end the war and avoid an invasion of Japan, which proponents claimed would have led to a greater loss of life. Critics inside and outside of the United States, however, questioned the morality and necessity of the bombings, arguing that the main reason for Japan’s surrender was the Soviet Union entering the war on the side of the allies and that the diplomatic means were not exhausted. Also the possibility of bombing an unpopulated area as a demonstration has been pointed out.

With nuclear testing, countries can develop even more powerful nuclear weapons. The CTBT bans all nuclear testing. At the CTBTO, we stand ready to implement this Treaty.CTBTO Executive Secretary Lassina Zerbo
The 1951 U.S. 'George' Test was already over 10 times more powerful than the Hiroshima and Nagasaki bombs.

Over 2,000 nuclear tests were carried out after 1945 by different countries. They aided the proliferation of nuclear weapons and helped to develop weapons orders of magnitude more destructive than the Hiroshima and Nagasaki bombs.

Following decades of public campaigning and arduous multilateral negotiations, the Comprehensive Nuclear-Test-Ban Treaty (CTBT) was opened for signature in 1996. It bans all nuclear explosions. The Treaty has yet to enter into force.

To commemorate the victims of the atomic bombs of Hiroshima and Nagasaki, in Vienna, a GENBAKU NO-HI celebration took place in the Rotunda of the Vienna International Centre on 6 August 2015 at 12:00.


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