Last month North Korea carried out its fourth nuclear test. As with the previous test, three years ago, the yield was equivalent to between six and nine kilotons of TNT. Yet while the first three tests were undoubtedly atomic bombs – the explosive energy came from the fission of the nucleus of plutonium-239 – this time North Korea announced that it had tested a hydrogen bomb. An H-bomb’s energy comes from the fusion of the nuclei of the hydrogen isotopes deuterium and tritium. Whereas A-bombs attain yields measured in kilotons, H-bombs typically attain megaton yields. They are also known as thermonuclear bombs because the nuclei must be heated to a temperature as hot as the centre of the sun in order to initiate the fusion process.