On the morning of January 6, 2016, North Korea claimed to have successfully tested its first thermonuclear weapon at a site near Punggye-ri, close to where it had conducted its three previous tests. Sensors of the United States Geological Survey picked up tremors in the region of magnitude 5.1. Soon after, North Korea’s leader Kim Jong-un announced that his country now had the ability to make such weapons.
While seismic data point at a human-made source of the tremors – and definitely a nuclear-explosion-like source – the strength of the tremors belied the scale of its cause. For the first time in 35 years, Kim Jong-un will be convening the congress of his Workers’ Party in May this year, and in the absence of any improvement in his people’s welfare, would like something to show for his four-year leadership. For his sake, a thermonuclear weapon holds great significance. However, a magnitude 5.1 quake is pretty close to the magnitude 4.9 quake that followed the 2013 test – which was of a fission weapon, not fusion.
— Jascha Polet (@CPPGeophysics) January 6, 2016
Analysts are already zeroing in on the explosion having only been the next-best thing: a boosted fission reaction.
In the regular version of a fission reaction, a small capsule of plutonium is surrounded by strong explosives. When they set off, the plutonium implodes and becomes supercritical: where a neutron can penetrate and destabilise a plutonium nucleus, causing it to break apart, release energy and more neutrons, which break down more nuclei and release more neutrons, and so on. This was the design of the bomb that blew up over Nagasaki, killing over 40,000 instantly with a yield of 21 kilotons (equivalent of TNT).
In later versions of this weapon, small quantities of deuterium and tritium – the two heavier isotopes of hydrogen – were included alongside the plutonium. When the conventional explosives went off, the plutonium underwent nuclear fission and raised the pressure and temperature (to a few hundred million degrees), a condition in which deuterium and tritium underwent nuclear fusion to form helium. This in turn released a swarm of neutrons that accelerated the rate of the remaining fission reactions in the plutonium. The result was a big boost in the energy burst – so the label of boosted fission reaction.
In a fusion bomb, a boosted fission reaction in turn boosts a larger fusion reaction. In not-excruciating-detail: a ‘trigger’ fissile material and the material to be fused – like lithium deuteride – are placed in a cylinder. The latter is packed in a specifically designed shell. When the triggering fission happens, the reaction releases a copious amount of hot gases, neutrons, a plasma of the surrounding as well as fissile material and X-rays. A well-modelled bomb succeeds in directing these four things toward the deuteride in the right way, especially the X-rays. They heat up the shell and cause it to fly off at great speeds. The resulting recoil, i.e. radiation-implosion, compresses the deuteride and causes it to fuse, releasing 10x more energy than would be in a fission reaction.
In both a boosted fission bomb and a fusion bomb (also called a hydrogen bomb), the biggest problem is to efficiently transfer the energy from the primary triggering stage to the secondary explosive stage. When modelling the former, the principal concern is to understand how the implosion occurs, the factors that influence it, and how the deuterium and tritium gases behave during the implosion. When modelling the latter, the problem is to perfectly channel the output of the primary so that the secondary works at all.
Part of the scepticism directed at North Korea has to do with whether its scientists have mastered these energy transfer techniques enough to have made a reliable fusion bomb – or simply a boosted fission bomb that finally works, an allegation that sits better with the seismic measurements. The numbers point at a yield of 10 kilotons or less, less potent than the bomb that flattened Nagasaki. (It goes without saying that the fact that North Korea possesses a nuclear weapon of any kind is grave enough.)
A similar dispute followed India’s last nuclear test, on May 11, 1998, of a “second generation” nuclear device that the government said had a yield of 45 kilotons. That it was a thermonuclear weapon had been disputed by K. Santhanam, a scientist with the Defence Research and Development Organisation at the time of the test, based on its seismic effects. Another scientist, R. Chidambaram, had then rebutted him with radiological data compiled by the Bhabha Atomic Research Centre that, according to Chidambaram, suggested it had been a fusion bomb.
Just so, the final piece of the January-6 test puzzle is the radionuclide data. When a nuclear weapon goes off, the energy release is also accompanied by the release of radioactive isotopes – like those of xenon after fission and of sodium and manganese after fusion. The worldwide network of sensors run by the Comprehensive Test Ban Treaty Organisation will be able to study the concentration of these isotopes in the atmosphere in the coming days or weeks, based on which scientists will be able to better judge the reactions that produced them.