The US produced the first nuclear weapons during the Second World War following extensive scientific research dubbed the Manhattan Project. When they dropped two bombs on the Japanese cities of Hiroshima and Nagasaki in 1945, the world had never experienced anything like it before.
Here, Jason Goodyer, the commissioning editor of BBC Focus Magazine, reveals the science behind the dropping of ‘Little Boy’ on Hiroshima, which by 1945 had claimed the lives of around 140,000 people.
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 kilometre 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.
Hiroshima: quick facts
- The bomb was dropped on 6 August 1945
- By the end of 1945 the death toll had reached approximately 140,000
- ‘Little Boy’ was the codename for the type of atomic bomb dropped
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.
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