Not long after the Second World War, the scientists at Los Alamos realised that they could vastly improve the design of a nuclear bomb, making it light enough to fly on a rocket. The idea was to have a hollow sphere which might be partially evacuated. Just before the explosion, a mixture of isotopes of hydrogen – deuterium and super heavy tritium – are injected into the hollow. The fission device is set off, and when a sufficiently high temperature is produced the deuterium and tritium nuclei begin to fuse. This produces helium and a very energetic neutron, which can cause fission in the common, non-fissile isotope of uranium – U-238 – as well as any U-235 or plutonium available, and thus ‘boost’ the fission yield. This improves the efficiency of the device and means it doesn’t have to be so heavy. All of the United States’ weapons are now in one way or another boosted. What is called a ‘hydrogen bomb’ has a fission primary but a different fusion design.

On 2 September 2016, North Korea detonated a nuclear device underground. You can tell something about the strength of an explosion from the earth tremors it generates. In the summer of 1958, I was briefly a consultant for the Rand Corporation in Santa Monica. There were hydrogen bomb tests in the Pacific and there was an office at Rand with a seismometer where the physicists would gather to watch the tremors from the Pacific. It is difficult to estimate the yield this way to high accuracy, and different claims have been made for the North Korean device. But it is certainly the strongest test explosion so far seen from them. It is not known whether the bomb was plutonium or uranium. They produce different fission products and some of these are inert gasses like xenon which can leak into the atmosphere. But the North Koreans have prevented this and do not say what the bomb was made of. Kim Jong-un said that it was a ‘hydrogen bomb’. This is surely nonsense but it may be some sort of boosted device, light enough to fit on a missile. And to judge from last month’s tests, it now seems that North Korea has produced rockets capable of reaching the continental United States.

A sure indication that the North Koreans are interested in boosting – and, eventually, in producing a hydrogen bomb – is their manufacture of lithium-6, which appears to have begun in 2012. The common isotope of lithium is Li-7 but the lighter isotope can be separated from the heavier by essentially chemical means. In the interior of the nuclear device, Li-6 and deuterium are inserted. The neutrons generated from the primary fission interact with the lithium to produce tritium. This saves the injection of tritium which is unstable, expensive and difficult to store. We know that the North Koreans have produced Li-6 because they tried to sell it on the open market in China last year. It has only one use: nuclear weapons.