The science behind the bombing of Hiroshima

Some 135,000 people were killed when, on 6 August 1945, the US dropped an atomic bomb on the city of Hiroshima in Japan. Those who survived suffered radiation sickness and severe burns, and the city was utterly destroyed...

Devastation caused by the atomic bomb that was dropped on Hiroshima in Japan on

Extensive scientific research and development by scientists involved in The Manhattan Project meant the US produced the first nuclear weapons during the Second World War. When the bombs were dropped in 1945, the world had never experienced anything like it before.

It was presumed at the time that the city would not recover for decades after the blast, and nothing would grow for years to come, leaving survivors without the necessary resources for farmland and food.

Here, Jason Goodyer, the commissioning editor of BBC Focus Magazine, reveals the science behind the dropping of Little Boy 70 years ago today…

One second after the bomb struck on the city of Hiroshima, a huge fireball 280m in diameter erupted with a core temperature of more than 1,000,000 degrees Celsius. Heat rays from the explosion raised surface temperatures of everything in their path to more than 3,000 degrees Celsius – more than twice the melting point of iron.

This sudden and extreme rise in temperature rapidly expanded the air around it, generating a blast that travelled faster than the speed of sound. Then, a drop in air pressure in the space behind the blast caused a backdraft powerful enough to burst the eyeballs and internal organs of anyone in its path.

Almost everyone within one kilometer of the hypocenter was killed instantly. Those further out were pelted with bits of the city’s buildings, and badly burnt by the extreme heat, giving rise to the characteristic keloid scars – large overgrown tissue produced when the body creates too much collagen. The scar becomes larger than the initial wound.


A man in hospital shows the keloids on his back caused in the wake of the atomic explosion in Hiroshima. (Credit: Carl Mydans/The LIFE Picture Collection/Getty Images)

Most of the radiation generated by the blast took the form of gamma rays, but 10 per cent was made up of neutron waves. Both are types of ionizing radiation that are capable of causing alterations to DNA, though neutrons are much more dangerous. Around 10 per cent of Little Boy’s 64kg of uranium was eaten up by the initial fission reaction, leaving the remaining 90 per cent of the radioactive material to be strewn all over the city by the blast.

As a result, many of the survivors suffered the symptoms of radiation sickness, which include vomiting, fever, fatigue, bleeding from the gums, thinning hair, diarrhea and, in the worst cases, death. Those that did survive had an increased risk of cancer, though there has been no evidence of abnormalities in their offspring.

Two days after the bombing, Harold Jacobsen, the Manhattan Project Physician, stated that nothing would grow in Hiroshima for 70 years. However, just one month later, red canna flowers began to sprout less than 1km from the hypocentre. Then, on 17 September, the Makurazki typhoon hit, claiming more than 2,000 lives and flooding great swathes of the city. It did, however, bring in a layer of fresh, radiation-free topsoil that would help the city’s flora to return.

The following spring, the city’s iconic cherry trees were blooming. Through further years of slow rebuilding, residents began to grow fruit and vegetables and were gradually able to return to some form of normalcy.

To read more about Hiroshimas recovery after the atomic bomb, check out the August 2015 issue of BBC Focus Magazine, which is on sale now.

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