Information Papers

Hiroshima, Nagasaki, and Subsequent Weapons Testing

(June 2008)

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 UF 6 , 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* 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. 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 the blast or 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 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.

See also Radiation Effects Research Foundation (in Japan): frequently-asked questions.

Subsequent Atmospheric Weapons Tests

The atmospheric testing of some 500 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 kilotonnes - that is, of 15 thousand tonnes of TNT equivalent - and that at Nagasaki was of 25 kilotonnes. In addition, the total equivalent of all atmospheric weapon tests made by the end of 1951 was in the region of 600 kilotonnes.

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, the amounts of fission products discharged into the atmosphere were of the order of a thousand times greater than all discharged previously.

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 from 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 more than 0.1 mSv in 1963. Residual dose rates at test sites are mostly low (< 1 msv/yr), apart from 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 who 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.

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.

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.

Sources(much of the paper is taken directly from Pochin's book):
Edward Pochin, 1983, Nuclear Radiation: Risks and Benefits, Clarendon Press Oxford.
UNSCEAR, 1977 and 1994, Sources and Effects of Ionizing Radiation.
F. Barnaby, 1982, The Effects of a Global Nuclear War: The Arsenals, Ambio XI, #2-3.
IAEA 2004, Radiation, People and the Environment.

Information on weapons matters and tests can be found on http://NuclearFiles.org

Note: Being opposed to the spread of nuclear weapons and their testing, the World Nuclear Association does not normally comment on such. However, the 50th anniversary of the Hiroshima bomb in 1995 inevitably focused public attention on weapons, so this paper endeavours to complement the kind of factual information normally provided by the WNA on the peaceful uses of nuclear power.