Nuclear weapons are the most powerful weapons ever invented. The devastating effects from nuclear bombs emanate from the forces that keep atomic nuclei together.
There are two main types of nuclear weapons: Fission bombs, where energy is released through splitting of heavy nuclei (uranium or plutonium); and thermonuclear bombs, or hydrogen bombs, where joining (fusion) of the lightest nuclei (hydrogen) contributes to the release of explosive power, even greater that of fission bombs.
How a fission bomb works
In a fission bomb, energy is released when heavy atom nucleic are hit by slow, so called “thermal”, neutrons and split as a result. During fission, vast quantities of energy are released along with additional neutrons, which can split more nuclei and release more energy in a chain reaction, if conditions are right.
The fission process
The fission bomb is designed to release as much energy as possible before the bomb disintegrates, stopping the chain reaction. The longer a chain reaction lasts, the more powerful the explosion will be. In order to start such a reaction, a “critical mass” is required. The critical mass depends on the properties of the fissile material, its density and geometry. For uranium-235 it is around 25 kg and for plutonium-239 some 5 kg. The nuclei of these elements can be split by thermal neutrons, setting off a chain reaction. The splitting process produces a few hundred different radioactive isotopes such as krypton, barium, iodine-131, cesium-137, and strontium-90.
1. A neutron with a suitable energy level (thermal neutron) hits a nucleaus of U235 (a corresponding sequence is valid for Pu239).
2. The uranium atom is split, which results in release of energy and a number of additional thermal neutrons. These, in turn, may either escape from the material or hit a U238 nucleus so that nothing further happens. But if the neutron hits another U235 nucleus, that one will be split and emit additional neutrons.
3. The number of free thermal neutrons will grow exponentially for the duration of the chain reaction, resulting in release of huge amounts of energy and radioactive fission products as fallout.
How a thermonuclear bomb works
A hydrogen bomb is detonated in three steps: first a plutonium bomb, then a fusion bomb, and finally a uranium bomb (U238). The plutonium bomb goes off first and ignites a fusion process in hydrogen gas (tritium), setting off a large number of fast neutrons. These, in turn, start a fission in a shielding – tamper – of U238, which encloses the bomb.
The primary yield (explosive energy) of a hydrogen bomb is generated by fission of the tamper. The diagram below shows schematically the design of such a bomb.
The explosion will continue over 600 nanoseconds, with the initial fission taking 550 ns. (one nanosecond relates to a second as a second relates to 30 years).
The explosive power of a hydrogen bomb is, in theory, unlimited, which in not the case for a fission bomb. The largest hydrogen bomb that has been tested had a power corresponding to 58 megatons (Mt), which equals about 4 600 Hiroshima bombs. This test was done in 1961 over Novaja Zemlja, in what was then the Soviet Union. A strategic bomb today has a blasting power of some 200 – 500 kilotons, which is frightful, considering the devastating effects caused by the bombs over Japan, which were less powerful by an order of magnitude.
The fusion process
Fusion occurs when two isotopes of hydrogen combine to form a single nucleus of helium, which releases enormous amounts of energy. In order for this to happen, extreme levels of temperature and pressure are required.
Fusion naturally occurs in the interior of stars, which is the source of their energy. On Earth, fusion occurs only in hydrogen bombs or in experiments with nuclear physics (e. g. fusion energy research).
In a hydrogen bomb, two isotopes of hydrogen – deuterium and tritium – are joined to form a nucleus of helium and a neutron is emitted. Fusion of the two isotopes releases large quantities of energy and a shower of fast neutrons.
Last update: February 26, 2015