North Korea’s nuclear test last week is its fourth since North Korea left the Nuclear Nonproliferation Treaty in 2003. Here is the official North Korean statement on the test. It claims that the test was of a hydrogen bomb, conducted in “the most perfect manner.” Early translations mentioned a “miniaturized” bomb, but this statement says
Through the test conducted with indigenous wisdom, technology and efforts the DPRK fully proved that the technological specifications of the newly developed H-bomb for the purpose of test were accurate and scientifically verified the power of smaller H-bomb.
Only the early estimates of quake magnitude, 4.9 to 5.1, are available on the internet as I write. Formulas for calculating yield from quake magnitude also depend on the depth of the detonation, which is another variable that will come from analysis of the seismic signals. Estimates of the yield run between 6 and 30 kilotons. That’s a lot of destructive power: 6,000 to 30,000 tons of TNT, enough to flatten a city. Check it out on Nukemap. It is small, though, for a hydrogen bomb.
Even though the yields of the last two North Korean tests were enough to flatten cities, they were small by hydrogen (fusion, thermonuclear) bomb standards. Alex Wellerstein describes the varieties of hydrogen bombs.
Information to Come – Maybe
The Organization for the Comprehensive Nuclear Test Ban Treaty operates a world-wide network (International Monitoring System) of seismological monitoring stations – and monitors radionuclides, hydroacoustics, and infrasound as well. Their results should appear here when they become available; a briefing from January 7 is here. The radionuclide results will be the most indicative of whether the test was of a fusion device. In addition to the ground-based monitoring stations, the United States and Japan have sent airplanes equipped with air-sampling equipment, and there was a report that the United States had sent drones to acquire samples.
No ground stations have reported increases in radionuclides, and the plume, if there was one, should have passed over some of the stations by now. North Korea has been adept at containing radionuclides in previous tests.
If a representative sample of all of the products of the nuclear reactions can be collected, they can provide information about what was tested: whether the fissile material was uranium or plutonium and its configuration; how much of the yield was from fission and how much from fusion; and what the fusion fuel was.
The analysis of radionuclides is complex. Most of the information is contained in ratios among them. Radioactive xenon indicates whether a test has taken place, but not whether it is fission or fusion. Tritium and lithium have been mentioned as diagnostic of fusion; tritium will diffuse rapidly may be hard to detect. Lithium (which is not radioactive) would be detected in particulates, which would, with any luck, contain radionuclides. Lithium-6 without any lithium-7 would be even more indicative of a test attempting fusion. Lithium isotopes would show only that fusion was attempted, not that it was achieved. That would be deduced from other radionuclides.
Nations collected such data on each other’s tests when those tests were above ground. Here is an example of such an analysis of Chinese and French tests from India. It indicates some of the isotopes that are likely to be analyzed, if they are collected.
The relatively low yields of the tests could suggest a number of things: 1) the tests are small-scale, to acquire data for modeling and technology development; 2) a focus on improving a particular nuclear weapon design; 3) the tests are failed attempts at jumping to a final thermonuclear (hydrogen) design; 4) something else altogether. Contained within the first categories is the idea of boosting; Jeffrey Lewis and others favor that as the explanation, for reasons given here.
Fusion and Fission Fuel
Tritium or lithium-6 is necessary for a hydrogen bomb or for boosting. Natural lithium, in the battery in your laptop, is a mixture of two stable isotopes: lithium-6 and lithium-7. Under neutron bombardment, lithium-6 becomes tritium. Tritium is an isotope of hydrogen that is more susceptible to nuclear fusion reactions than normal hydrogen. It is radioactive, with a half-life of about 12 years. Tritium can be made from lithium-6 in a reactor or in the detonation of a fission bomb.
The Institute for Science and International Security (ISIS) says that the 5-megawatt reactor at Yongbyon may have been refurbished to include irradiation channels in the core, in which lithium-6 might be inserted to produce tritium. The reactor may have been operating recently at low power. Lithium-6 constitutes about 7.5% of natural lithium. Separating it from lithium-7 is simpler than separating uranium isotopes. There are a number of ways to separate them, and the ISIS report notes possible chemical processing buildings at Yongbyon.
The North Koreans have a limited supply of fissile material, although it may be increasing with operation of their reactor and uranium enrichment facilities. Although their fissile material is often expressed in terms of nuclear bombs, it is not clear that they have manufactured any. Those calculations are based on estimates of plutonium stocks and assumptions about bomb designs. But at several tens of kilograms of fissile material, starting with plutonium, would it be wise to go through the work to convert that material to bombs? Or would North Korea wait to build the bombs they seem to want: hydrogen bombs that will fit on one of their missiles?
A test destroys some of that material. They must have calculated that whether the test was a success or failure, what they learned from it would be worth losing the material. That could mean that they have additional material now, or that progress in their design is very important.
What Did They Test?
The tests and their magnitudes were
Any commentary on what this sequence means is mostly guesswork. Here’s mine. Those magnitudes are log-scaled, so the differences are large, almost a factor of ten from 2006 to the most recent tests. And the last two are intriguingly similar.
From North Korean statements, the goal seems to be a hydrogen bomb that can fit on a missile. One of the hypotheses about the first test was that it was a full-up test of a compact hydrogen bomb design. That’s a long way to go in one jump, and they mostly failed.
The later translations don’t use the word “miniaturized,” and leave open the possibility that what is being tested is a part of, or a scaled-down version of the weapon they eventually hope to have. Jeffrey Lewis and others lean toward the idea that what was tested was a boosted fission design. The tests could also be attempts to integrate a fission device with the thermonuclear part of the design. That is a tricky business.
Designing a nuclear weapon relies on extremely complex computer models, easier to develop and run now than while nuclear weapons were being developed in mid-twentieth century. Those models depend on input parameters that can only be obtained by nuclear testing. They also require further work to calibrate them. The latter is called “turning the knobs” and also depends on the results of the tests, but not in a simple way. It would not be surprising if North Korea started with a full-up compact hydrogen device, found it didn’t work, and went back to testing parts of it.
That doesn’t rule out the idea of boosting, which could be one of those parts. But nuclear weapons modeling has always been difficult, empirical, and not fully understood. All nuclear weapons programs have had surprises, larger and smaller than calculated. So the modeling may have predicted something other than what the North Koreans saw, and they are trying to figure out why. One way to do that would be to replay an earlier test, with some things changed: the configurations of materials, their processing, the timing of parts of the mechanism, many possibilities.
The amount of information the North Koreans can get from a test depends on what they spend for diagnostics and how ingenious they are in using them. The instruments measuring the test are vaporized microseconds after their measurement. Things can go wrong there, too, which they might need to correct in a subsequent test.
There is much we don’t know about North Korea’s test. It’s possible we won’t know more than we know now any time soon. What we do know is that North Korea is working toward a nuclear weapon and that they are able to build them at a city-busting size. One way to learn more is to engage them again in talks about their nuclear program.
Photo: People in Pyongyang watching the announcement of the test on January 6.