Hiroshima, Nagasaki, and Subsequent Weapons Testing

Two atomic bombs made by the allied powers (USA and UK) from uranium-235 and plutonium-239 were dropped on Hiroshima and Nagasaki respectively early in August 1945. These brought the long Second World War to a sudden end.

The immense and previously unimaginable power of the atom had been demonstrated. In the following years several governments joined the arms race, while internationally, efforts were focused on constraining the threat of nuclear weapons proliferation.

From the 1950s the power of the atom was harnessed increasingly for peaceful uses, notably electricity generation and medicine. Nowhere is the transition from weapons of destruction to power for good better displayed than Hiroshima and Nagasaki in Japan which have come to depend substantially on electricity from nuclear energy.

Today, as the main nuclear arsenals are being dismantled and a comprehensive test ban treaty is in sight, commercial nuclear power provides a significant portion of the world's electricity. Several factors suggest that nuclear power has a much larger role to play in supplying the world's future energy needs, and this is supported by every reputable projection.

The first two atomic 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 64 kilograms of highly-enriched uranium was used in the bomb which had a 16 kiloton yield (i.e. it was equivalent to 16,000 tonnes of TNT). It was released over Hiroshima, Japan's seventh largest city, on 6 August 1945. Some 90% of the city was destroyed.

The 21 kiloton explosive charge for the bomb detonated over Nagasaki three days later was provided by about 6.2 kilograms of plutonium-239 (>90% Pu-239), and its preparation depended on the operation of special nuclear reactors built for the purpose. During 1942, under conditions of wartime secrecy, the first human-designed reactor* had been 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.

* Several natural nuclear reactors functioned about 2 billion years ago in Africa.

However, the design of a plutonium bomb is very much more complex than one using enriched uranium. Hence the need to test it, and in fact the plutonium was first used for a 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. The majority would have decayed by the time they landed around the globe.

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. (Another figure is 78,500 fatalities, with 5 to 15% of the short-term ones being from radiation.) In Nagasaki, out of a population of 174,000, on the first day 22,000 died and another 17,000 within four months. Unrecorded deaths of military personnel and foreign workers may have added considerably to these figures. About 15 square kilometres (over 50%) of the two cities was destroyed.

It is uncertain what proportion of these 103,000 deaths, or of the further deaths in military personnel, were due to radiation exposure rather than to the very high temperatures and blast pressures caused by the explosions – 15 kilotons at Hiroshima and 25 kilotons at Nagasaki. 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 were from blast injuries or burns rather than the radiation. Beyond 1.5 km the radiation 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).

In comparison, during the period of February 1945 to August 1945, the US bombing of Japanese cities – notably Tokyo, Nagoya, Osaka and Kobe – by B-29s delivered about 100 kilotons of high explosives and incendiaries to urban areas in hundreds of raids, resulting in a large number of deaths and causing widespread destruction. Some 80,000 to 100,000 people were killed in a single 300-bomber raid on Tokyo. About 80 square kilometers of those four cities was destroyed in ten days during March. Overall 67 Japanese cities were partly destroyed, 500,000 people were killed and 5 million more made homeless.

To the 103,000 deaths from the blast or acute radiation exposure at Hiroshima and Nagasaki 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 were still being monitored 50 years later.) There was an increase in leukaemia beginning about two years later and peaking at four to six years later, and other cancers beginning about ten years later.  There was no evident to suggest an increase in leukaemia at less than 500 mSv acute dose. At an acute dose of 100 mSv, an increased cancer risk of 1.05 times normal was calculated. There was concern about ingestion or inhalation of radionuclides, but fires released far higher levels of non-radioactive carcinogens. Additionally, 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 activation products 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.

Subsequent atmospheric weapons tests

The atmospheric testing of some 545 nuclear weapons up to 1963 caused people to be exposed to radiation in a quite different way. The Japanese atomic bombs had caused lethal exposures locally from radiation at the time of the explosions, but very little radiation more than a few kilometres away. On the other hand, subsequent atmospheric tests did not cause any substantial direct exposures of people at the time of the tests. However, the fission products released into the atmosphere caused the whole world population to be exposed to very low but continuing annual doses from fallout. In at least two instances these fission products also caused substantial irradiation to small populations exposed to local fallout close to the site of testing.

The atomic bombs used in Japan in 1945, and the bombs or devices testing during the following seven years, depended on the fission of uranium-235 or plutonium-239, mostly the latter. The explosive effect of each was equal to that of up to a few tens of thousand tonnes of the conventional explosive TNT. On this basis of comparison, the Hiroshima bomb was of about 15 kilotons – that is, of 15 thousand tonnes of TNT equivalent – and that at Nagasaki was of 25 kilotons (ca. 65 and 105 GJ respectively). In addition, the total equivalent of all atmospheric weapon tests made by the end of 1951 was in the region of 600 kilotons.

After 1951, however, devices were being tested which had explosive effects about a thousand times greater, and by the end of 1962 the total of all atmospheric tests had risen from the 1951 value of 0.6 million tonnes of TNT equivalent, to about 500 million tonnes equivalent. This vast increase in scale was due to the testing of 'thermonuclear' weapons or 'hydrogen bombs', which depended, not on the fission of a critical mass of fissile material alone, but on a two or three-stage process initiated by this reaction.

In a thermonuclear bomb, an initial fission, such as occurred in the 'atomic' bomb, momentarily creates conditions of enormously high temperature and atomic disturbance that allows the fusion together of the nuclei of atoms of low atomic number, such as lithium and hydrogen. This fusion liberates further large amounts of energy explosively, such as occurs in the similar reactions in the sun and stars.

In some such bombs, the high-energy neutrons released are used to set off a third stage, making it a fission-fusion-fission process. The third stage consists of the fission of a surrounding 'blanket' of uranium-238 isotope which is fissionable by neutrons of this high energy. This third stage provides about half of the yield of such a weapon.

The release of fission products is approximately proportional to the explosive power unleashed, although fusion as such does not give rise to them. From 1952 to 1962 therefore, with a thousand times more energy released in atmospheric testing than previously, the amounts of fission products discharged into the atmosphere would have increased by a much lesser factor.

To complete this tally of the total fallout to date, all atmospheric tests since 1962 appear to have increased by rather less than 20 percent the total of fission products that had been deposited by previous tests, as judged by the measured deposition of strontium-90 in successive years.

The most important radionuclides remaining from weapons testing are now carbon-14, strontium-90 and caesium-137. The global average dose from these is about 0.005 mSv/yr, compared with a peak of 0.113 mSv average in 1963. Residual dose rates at test sites are mostly low (< 1 msv/yr), apart from at Semipalatinsk in Kazakhstan.

Australian tests

Twelve atmospheric nuclear explosions comprised the main part of UK weapons testing in Australia. Three were at Monte Bello Islands (WA) in 1952 & 1956, two at Emu Field (SA) in 1953 and seven at Maralinga (SA) in 1956-57.

Underground tests and the NPT

Since the 1963 atmospheric test ban treaty, weapons tests have been mostly underground, the exceptions being by France and China. The underground tests have had no immediate environmental effect and are generally seen as relatively benign compared with the atmospheric tests.

In 1970 the Nuclear Non-Proliferation Treaty (NPT) was signed, and now has five weapons states: USA, UK, Russia, France and China. The basis of the NPT was that other states which were signatories eschewed the nuclear weapons option and in return were promised assistance in civil nuclear power development by the weapons states.

Today, 187 states have signed the NPT. The only states with significant nuclear facilities that are not party to the NPT or equivalent safeguards agreements are India and Pakistan , which exploded several nuclear devices in 1998, and Israel, which is generally believed to have nuclear capability. South Africa developed some nuclear weapons but then dismantled them, under international scrutiny, and has joined the NPT. Iraq and North Korea sought to circumvent their obligations under the NPT and this was thwarted by international pressure, but North Korea subsequently resigned from the NPT and then exploded a nuclear device underground in 2006.

From 1957 to 1989 the USA and USSR conducted about 150 explosions underground as part of their peaceful nuclear explosion programmes. A bilateral treaty covering these was signed in 1976. See also information page on Peaceful Nuclear 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, now a major nuclear reactor supplier.


Edward Pochin, Nuclear Radiation: Risks and Benefits, Clarendon Press Oxford (1983)
UNSCEAR, Sources and Effects of Ionizing Radiation (1977 and 1994)
F. Barnaby, The Effects of a Global Nuclear War: The Arsenals, Ambio XI, #2-3 (1982)
International Atomic Energy Agency, Radiation, People and the Environment (February 2004)
US Strategic Bombing Survey – summary report (July 1946)
Jane Orient, Fukushima and Reflections on Radiation as a Terror Weapon, Journal of American Physicians and Surgeons, 19, 2, p48 (summer 2014)
Information on weapons matters and tests can be found on the Nuclear Files project website

Note: Being opposed to the spread of nuclear weapons and their testing, the World Nuclear Association does not normally comment on such. However, due to pubic interest and to complement information pages on safeguards and countering weapons proliferation, this page endeavours to complement the factual information normally provided by the Association on the peaceful uses of nuclear power.

India, China & NPT
Nuclear Proliferation Case Studies
Safeguards to Prevent Nuclear Proliferation