There are two types of nuclear bombs: the atomic A bombs and the thermonuclear hydrogen bombs known as H bombs. Atomic bombs became the first nuclear weapons to be used in war when the cities of Hiroshima and Nagasaki were bombed at the end of the Second World War. There is enough fissile matter (usually either uranium 235 or plutonium 239 enriched to over 90%) in the core of an atomic bomb to sustain a highly explosive chain reaction.
The far more powerful H bombs are powered by the process of nuclear fusion, where the fusion reactions are sparked off by the heat emitted by nuclear fission. Following experiments carried out in the 1950s and 60s on remote Pacific atolls or Siberian tundras, these terrifying weapons have fortunately never been used in war.
The power of these bombs is measured in the number of kilotons or megatons (millions of tons) of trinitrotoluol (more commonly known as TNT) required to achieve the same effect. The bomb which destroyed Hiroshima on the 9th of August 1945 measured 21 kilotons, whereas the most powerful explosion ever produced by man was a Soviet H bomb dropped in 1961, and which measured 50 megatons.
Nuclear bombs release their energy in the form of a blast, a fireball, visible light and radioactive ionising rays. For a small bomb, 50% of its energy release comes from the blast, 35% through heat, and the remaining 15% in the form of radiation.
During the explosion and a few seconds after, the effects of neutrons and gamma rays, which are long range, is very intensive. Gamma radiation prevails for powerful bombs. Neutrons dominate the small bombs and neutron bombs.
Once the flash of gamma and neutron has passed off, the radioactivity comes from the dispersion of highly radioactive fission products and nuclei made radioactive by the flux of neutrons. The fallout in the area surrounding the explosion, however, is especially intense when the fireball hits the ground.
For bombs of under 100 kT, the radioactivity settles in the troposphere - the lowest layer of the atmosphere - and does not contaminate the ground for several weeks. Bombs of several megatons release 95% of their radiation into the stratosphere (at very high altitudes), where the radioactive particles can remain for up to seven years.
Access to page in french