The atomic bombs dropped on Hiroshima and Nagasaki in Japan on August 6th and 9th 1945 forced the Japanese to surrender, ending World War Two.
The atomic bombs dropped on Hiroshima and Nagasaki in Japan on August 6th and 9th 1945 forced the Japanese to surrender, ending World War Two.
They detonated with a force equivalent to 15,000 and 21,000 tonnes of conventional explosives.
Tens of thousands of people were killed instantly or within hours. Many more died later from the radioactivity left behind.
America had been bombing Japanese cities for six months and urging its leaders to surrender - calls which the Japanese government ignored.
US President Harry Truman gave the order for an American B-29 bomber, the Enola Gay, to drop the bomb nicknamed "Little Boy" over Hiroshima. It exploded about 1,900ft above the ground. Six square miles of the city were levelled. Some victims were simply vaporised by the intense heat.The final death toll is estimated at 135,000.
Three days later a larger bomb, dubbed "Fat Man", was detonated over Nagasaki. The final death toll there is estimated at 50,000.
Six days later, on August 15, Japan surrendered and the war was over, Germany having surrendered in May.
While the number of protons in a nucleus defines which element the atom belongs to, the number of neutrons can differ among atoms of the same element.
For example, all uranium atoms have 92 protons in their nuclei – but there are several versions, or isotopes, with differing numbers of neutrons.
The classic example of fission involves the isotope uranium-235, whose nuclei have 92 protons and 143 neutrons (92 + 143 = 235).
When an individual neutron hits a uranium-235 nucleus, it forms uranium-236, which is very unstable.
The new nucleus breaks apart and releases ‘spare’ neutrons as well as energy (as a burst of gamma rays).
Some of the spare neutrons released during fission may go on to hit other nuclei, in which case the process may repeat – a ‘chain reaction’.
In a nuclear reactor inside a power station, the reaction is controlled; in a fission bomb it releases large amounts of energy very fast.
Some applications of fission use other isotopes, notably plutonium-239.
The first sustained chain reaction occurred in a ‘nuclear pile’ in a squash court at the University of Chicago in 1942, as part of the U.S. government’s Manhattan Project.
Led by J Robert Oppenheimer (1904-1967), this was the top-secret mission that eventually developed the bombs used in 1945.
No nuclear weapons have been used in war since 1945.
Nevertheless, thousands of warheads exist with a much greater destructive capacity, and there have been more than 2,000 test explosions in peacetime.
The world’s first nuclear power station to supply the public was Calder Hall, in Cumbria (now Sellafield), which opened in 1956.
Today, about one-fifth of the world’s electrical power is generated by nuclear fission. It is clean and convenient, and costs about the same per unit of energy as other forms.
However, nuclear power comes with a small risk of accidents – such as those at Chernobyl, Russia, in 1986, and Fukushima, Japan, following the tsunami there in March 2011.
Such disasters are extremely rare, but when they happen, radioactive material leaks into the environment.
The other problem is the ‘spent’ rods of fuel which are highly radioactive and have to be encased in concrete, then buried.
They gradually become less radioactive, but only over a long time.
