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.

Many commentators immediately dismissed the claim. The Daily Mail, for example, quoted an expert who said:

The seismic data that’s been received indicates that the explosion is probably significantly below what one would expect from an H-bomb test. So initially it seems to be that they’ve successfully conducted a nuclear test, but unsuccessfully completed the second-stage hydrogen explosion.

This assumes that the yield of an H-bomb must be larger than a few kilotons and that an H-bomb must be a two-stage device, in which a primary fission bomb ignites the light nuclei in the secondary, physically separate fusion bomb.

But there is no lower limit on the yield of H-bombs. Nor is there any requirement for a weapon that burns thermonuclear fuel to be a two-stage device. The neutron bombs that ‘only kill people’, leaving buildings and military equipment unscathed, are H-bombs of low explosive yield (a few kilotons at most). And the first Russian H-bomb, exploded in 1953, was a single stage ‘layer cake’, designed by Andrei Sakharov, with adjacent layers of light and heavy nuclei.

So the North Koreans might have exploded an H-bomb, in the sense that the device tested last month might have involved hydrogen isotopes in addition to the fission core used in the previous tests. In the 1956 G2 test, the UK exploded a weapon containing a small amount of lithium deuteride. In the neutron environment resulting from a fission explosion, the isotope lithium-6 produces tritium, which can then undergo the basic fusion reaction with deuterium. This can be observed by detecting a neutron with a specific energy, much higher than the energy of neutrons from fission. In G2, the yield was substantially enhanced by the presence of the lithium deuteride: the expected yield was 60 kilotons, but 98 kilotons was achieved.

The first question to arise from the North Korean claim is whether deuterium, tritium and lithium-6 are produced in North Korea. David Albright and Serena Kelleher-Vergantini of the Institute for Science and International Security (ISIS), which analyses publicly available satellite images to monitor nuclear activity, say they have found some answers. There is little doubt that North Korea can make heavy water (in which deuterium takes the place of ordinary hydrogen). One way to do it is by distillation (deuterium occurs naturally in sea water); another is by electrolysis. Extracting deuterium gas from heavy water is secondary-school chemistry. According to Gary Samore, speaking at the International Institute for Security Studies in London last month, North Korea is more technologically competent than Iran. (Samore was chief nuclear adviser in the first Obama administration and involved in negotiations with both Iran and North Korea.)

As for tritium and lithium-6, Albright told me that North Korea makes both:

It has channels in the 5 MWe reactor that can make tritium and has separation capabilities at an old isotope separation plant at Yongbyon... The lithium-6 plant is not at Yongbyon but it started in the last a few to several years ago. We have recently learned of its location and plan on publishing that result soon.

But what is the point of adding thermonuclear fuel to fission weapons when the explosive yield is not increased? Jeremy Bernstein has pointed out that it would allow a more compact bomb to be built with the same yield as the pure fission device. North Korea’s rocket test earlier this month showed its determination to have the capability to strike the United States. A lighter warhead would help it achieve this aim.

And why does North Korea continue with its nuclear weapon and missile programmes at the expense of its economy and in the face of UN and US sanctions? Samore says that ‘having nuclear weapons is an existential capability’ for North Korea. ‘It is the only way the regime can survive against outside pressure.’

But why explode the nuclear weapon now, when economic and political relations between the two Koreas were improving? According to Samore, the only conceivable reason is that Kim Jong-un felt slighted by the cool reception China gave to the Moranbong Band, the girl band he founded and whose members he reportedly chose himself. On 10 December 2015, Kim said North Korea was ‘a powerful nuclear weapons state ready to detonate a self-reliant A-bomb and H-bomb to reliably defend its sovereignty and the dignity of the nation’. The band was scheduled to give its first performance in Beijing on 12 December. According to the New York Times, ‘attendance by a top Chinese leader could be interpreted as an endorsement of Mr Kim’s government and in particular its claim to a hydrogen bomb’, so no senior official was going to attend the show. Kim therefore told the band to come home and cancelled the performance. The test took place on 6 January.