What are the different types of nuclear weapons?
There are two types of nuclear reactions which are used in nuclear weapons. The nucleus of some heavy elements like Uranium or Plutonium can split into two roughly equal sized nuclei with the release of energy. Such a process is known as nuclear fission. The fission reaction can occur spontaneously in the fissile material. The reaction is also self sustaining. Namely the fission of one nucleus induces the fission of nearby ones and so on, leading to what is called a chain reaction. If a sufficent amount of fissile material is concentrated in one place then the chain reaction proceeds very fast leading to an explosion.
On the other hand certain very light elements like isotopes of hydrogen undergo nuclear fusion. In this process, two light nuclei can combine to form a single nucleus with the release of energy. This is the process by which energy is generated in the core of the sun and other stars. The fusion reaction requires very high temperatures and density before it can get initiated.
All nuclear weapons use these two reactions in different ways. From the military usage point of view, they are classified as either tactical weapons or strategic weapons. Tactical weapons are low yield weapons that meant to be used in the battlefield against military formations. Strategic weapons are high yield weapons to be used against civilian populations in big cities. The basic types of weapons that have been built or conceived of are described below:
Pure Fission Weapons
Weapons in which only the fission reaction takes place are called pure fission weapons or simply fission weapons. The bombs that were dropped at Hiroshima and Nagasaki were both fission bombs. These are the simplest nuclear weapons to design and build. They form the basis for developing other types of weapons. Their yield can range from a few tons to about a few hundred kilotons. They could be either tactical or strategic weapons. The largest pure fission weapon tested is believed to be a 500 kiloton bomb called Mk-18 which was tested by the USA on the 15th of November 1952.
Boosted Fission Weapons
The efficiency of a fission weapon can be increased dramatically by introducing a small amount of material that can undergo fusion. Such weapons are called boosted fission weapons. In boosted weapons, the fission reaction takes place first and produces the required temperatures and densities for the fusion reaction. The fusion in turn accelerates the fission reaction. The fusion only serves to help the fission process go faster and hence make the weapon more efficient. It contributes only about 1% of the yield. Since boosted fission weapons are more efficient than pure fission weapons, they can be made lighter for the same yield. So most of the fission weapons deployed today are boosted fission weapons.
Thermonuclear weapons, also called hydrogen bombs, get most of their yield from the fusion reaction. As in the case of boosted fission weapons, they require a fission explosion to trigger fusion, called the primary stage, to initiate the fusion reaction which is called the secondary stage. However unlike the boosted weapons, thermonuclear weapons contain a substantial amount of fusion fuel and most of their yield comes from fusion. Indeed these are the most powerful of nuclear weapons, often with yields of a few megatons. A third fission stage can also be added to produce very high yield weapons. The most powerful nuclear weapon to have been tested so far is the Tsar Bomba, a 50 megaton three stage weapon exploded by the USSR on 30th October 1961. However it is not necessary for a thermonuclear weapon to have such high yields. The B61(Mk-61) class of thermonuclear weapons deployed by the USA have yields which can be adjusted to be as small as .3 kilotons (300 tons).
Enhanced Radiation Weapons
Enhanced radiation weapons, also called neutron bombs are small tactical thermonuclear weapons which are designed to produce intense nuclear radiation. These weapons are designed to kill soldiers protected by armour (eg. inside tanks). The radiation produced by the neutron bombs can easily penetrate the armour of the tanks and kill the humans inside them.
Salted Nuclear Weapons
Salted nuclear weapons, or cobalt bombs, are thermonuclear weapons which are designed to produce a large amount of long lasting radioactive fallout. This would result in large scale radioactive contamination of the area they are dropped in. The difference between the fallout from the salted weapons and the unsalted one is that the former is much more in quantity and also has a much longer lifetime. The fallout remains radioactive for much longer. The long term effects of such weapons would therefore be much worse. These weapons are called Doomsday Devices since they could possibly kill everyone on earth. Fortunately though these weapons have been conceived of and discussed, most probably none have been tested or built.