The First two Atom Bombs in 1945
The Hiroshima bomb was made from highly-enriched uranium-235. This was prepared by diffusion enrichment techniques using the very small differences in mass of the two main isotopes: U-235 (originally 0.7% in the uranium) and U-238, the majority. As UF6, there is about a one percent difference in mass between the molecules, and this enables concentration of the less common isotope. About 60 kilograms of highly-enriched uranium was used in the bomb which was released over Hiroshima, Japan's seventh largest city, on 6 August 1945. Some 90% of the city was destroyed.
The explosive charge for the bomb detonated over Nagasaki three days later was provided by about of 8 kilograms of plutonium-239 (>90% Pu-239), and its preparation depended on the operation of special nuclear reactors. During 1942, under conditions of wartime secrecy, the first human-designed reactor was constructed, in a squash court at the University of Chicago. It used highly purified graphite to slow the neutrons released in fission to enable further fission. This paved the way for more substantial production reactors at Hanford. The plutonium-239 generated in these could be separated by simple chemical methods, with no need for the complexities of isotope separation. The plutonium was first used for the test explosion at Alamogordo in New Mexico on 16 July 1945, ushering in the nuclear age with all its threat and promise.
The Effects of the Hiroshima and Nagasaki Bombs
The devastating effects of both kinds of bombs depended essentially upon the energy released at the moment of the explosion, causing immediate fires, destructive blast pressures, and extreme local radiation exposures. Since the bombs were detonated at a height of some 600 metres above the ground, very little of the fission products were deposited on the ground beneath. Some deposition occurred however in areas near to each city, owing to local rainfall occurring soon after the explosions. This happened at positions a few kilometres to the east of Nagasaki, and in areas to the west and north-west of Hiroshima. For the most part, however, these fission products were carried high into the upper atmosphere by the heat generated in the explosion itself.
In Hiroshima, of a resident civilian population of 250 000 it was estimated that 45 000 died on the first day and a further 19 000 during the subsequent four months. In Nagasaki, out of a population of 174 000, 22 000 died on the first day and another 17 000 within four months. Unrecorded deaths of military personnel and foreign workers may have added considerably to these figures.
It is impossible to estimate the proportion of these 103 000 deaths, or of the further deaths in military personnel, which were due to radiation exposure rather than to the very high temperatures and blast pressures caused by the explosions. From the estimated radiation levels, however, it is apparent that radiation alone would not have been enough cause death in most of those exposed beyond a kilometre of the ground zero below the bombs. Most deaths appear to have been from the explosion rather than the radiation. Beyond 1.5 km the risk would have been much reduced (and 24 Australian prisoners of war about 1.5 km from the Nagasaki ground zero survived and many lived to a healthy old age.)
To the 103 000 deaths from acute radiation exposure have since been added those due to radiation induced cancers and leukaemia, which amounted to some 400 within 30 years, and which may ultimately reach about 550. (Some 93,000 exposed survivors are still being monitored.)
Teratogenic effects on foetuses was severe among those heavily exposed, resulting in birth deformities and stillbirths over the next 9 months. Beyond this, no genetic damage has been detected in survivors' children, despite careful and continuing investigation by a joint Japanese-US Foundation.
The major source of exposure in both cities was from the penetrating gamma radiations, and to a lesser extent from the neutrons (mostly at Hiroshima), emitted during and shortly after fission. There were two further, and smaller, sources of exposure. One, already mentioned, was due to the 'black rain' which fell in some areas, carrying down radioactive materials from within the rising cloud of fission products. The exposures due to these depositions are in general estimated to have been small, but some increased activity from the fission product radionuclide caesium-137 remained detectable for many years in soil and farm products in the Nishiyama district east of Nagasaki.
The second additional form of exposure resulted from the effect of neutrons in inducing radioactivity in various stable chemical elements such as in iron or concrete structures or roofing tiles. The total absorbed doses of radiation from these activities are estimated to be less than one per cent of that from the neutrons which induced them. They could however have caused a significant exposure of people who entered the city within a few days of the explosions.
Hiroshima and Nagasaki since 1945
Both cities were rebuilt soon after the war and have become important industrial centres. The population of Hiroshima has grown to over one million and that of Nagasaki to 440,000. Major industries in Hiroshima today are machinery, automotive (Mazda) and food processing, those in Nagasaki are associated with its international port, particularly Mitsubishi Heavy Industries.
Nuclear energy has come to be an important part of the life of each city in a totally new way: today one quarter of Hiroshima's electricity is from nuclear power and half of that for Nagasaki is nuclear. Both cities are testimony to the positive benefits of a technological society which applies available energy resources to the needs of urban populations and industry.