Nuclear Bomb is an umbrella term which is associated with a weapon which derives its destructive power from nuclear reactions. It can do this by either fission (fission bomb) or a combination of fission and fusion (thermonuclear weapon). So Atom bomb and Hydrogen Bomb are the two types of Nuclear Bomb. Interestingly people mention word ‘Atom Bomb’ when they are referring to nuclear capabilities of various countries but in reality these countries are now possessing Hydrogen Bomb which is different from Atom Bomb.
An atomic bomb is about fission. You’re breaking up very large atoms (Uranium or Plutonium) in a super-critical chain reaction. When the neutrons, or neutral particles, of the atom’s nucleus split, some hit the nuclei of nearby atoms, splitting them, too. The result is a very explosive chain reaction as the splitting produce large amount of energy. For example bomb that was exploded over Hiroshima and Nagasaki.
A hydrogen bomb, also called as thermonuclear bomb, uses fusion in a chain reaction. The requirement to start a fusion reaction, however is that it requires enormous energy levels (heat) in order to trigger it. The only way to produce such large amount of energy is detonating a fission or atomic bomb. Therefore a hydrogen bomb includes an atomic bomb inside the core. So here atomic bomb is the trigger and when it explodes, it sets off fusion reactions in the nearby deuterium/tritium. It is called a hydrogen bomb because Deuterium & Tritium are simply isotopes of hydrogen. For example in Sun, where two hydrogen are fused to form Helium. Hydrogen Bomb is also called as Thermonuclear bomb. A thermonuclear weapon is a nuclear weapon that uses the energy from a primary nuclear fission reaction to compress and ignite a secondary nuclear fusion reaction.
With this staged design, it is possible to have a thermonuclear bomb which is far more powerful than an atomic bomb. In fact, the hydrogen (fusion piece) of this is thought to be completely scalable (the bomb may be as powerful as desired and it is limited only by practicality/engineering). The majority of the uranium or plutonium in atomic bombs actually goes unused. In a thermonuclear bomb, an additional step means that more of the bomb’s explosive power becomes available. As a result H-Bomb is more powerful than Atom Bomb. Also Hydrogen bombs themselves do not release many radioactive particles but the fission triggers do.
India’s test in 1998 was supposed to be a thermonuclear bomb – Hydrogen based fusion bomb ignited using heat generated due to standard nuclear fission bomb. Director for the 1998 test site preparations, Dr. K. Santhanam, reported the yield of the thermonuclear explosion was lower than expected, although his statement has been disputed.
(Sourced from different sources.)
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