A thermonuclear weapon – better known as a hydrogen bomb – is much more powerful than an atomic weapon, releasing energy several magnitudes higher.
The atom bombs used at Hiroshima and Nagasaki created blasts equivalent to 16 Kilotons.
However the most powerful hydrogen bomb can be 50 Megatons – more than 5,000 times more powerful than a single atomic bomb.
While an atom bomb gets its energy from fission – or the splitting of an atom – a hydrogen bomb uses the fusion of atoms.
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A fusion bomb is more difficult to make because it requires a temperature of millions of degrees centigrade, which is produced by a primary fission reaction.
Fusion occurs when two atoms combine to form a single heavier atom.
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At extremely high temperatures, the nuclei of deuterium and tritum – both hydrogen isotopes – can fuse, releasing huge amounts of energy in the process.
Thermonuclear weapons were developed by the US more than 60 years ago and use the fusion of hydrogen isotopes to create the rapid release of energy.
Pioneers Edward Teller and Stanislaw Ulam created the technology, known as the Teller-Ulam configuration.
The first thermonuclear bomb was tested in 1952, and similar weapons were developed by the Soviet Union, UK, China and France.