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The Democratic People’s Republic of Korea (DPRK, North Korea) conducted its sixth nuclear test on 3 September 2017. Preliminary estimates of yield are in the range of 100 to 150 kilotons (kt), although this figure will be subject to revision over a period of weeks. In the end, the degree of uncertainty is likely to be tens of kilotons. Some observers are jumping to the conclusion that this latest test had to be a thermonuclear test (so-called hydrogen bomb). However, this assessment is mostly based on seismic data, which has its limitations.
In contrast, radioactive debris, collected in the air shortly after a test can give unequivocal evidence of whether the test used plutonium or uranium as the fissile fuel. Debris also gives clear evidence of successful true thermonuclear enhancement using lithium and hydrogen isotopes. However, North Korea has spent a great deal of effort to conceal its actual nuclear test results, other than the seismic yield which cannot be concealed. It has had success in preventing the release of radioactive isotopes by carefully sealing the test tunnels in earlier tests (i.e. tests two to five), and thus hampering data collection.
North Korea claimed to have tested a hydrogen bomb in 2016. While it leaked photographs of an alleged fission bomb, North Korea prevented Western scientists from getting the data they needed to verify the claims. Asserting that it now possesses a thermonuclear capability, North Korea has this week released images of what is purported to be a two-stage weapon in the presence of the country's leader, Kim Jong-un. These photographs show details that are consistent with future thermonuclear warheads for long-range missiles, but they are not necessarily the actual state-of-the-art.
It is true that a 100-kt yield is easily achievable in an underground test without any thermonuclear reaction whatsoever. The 1952 Ivy–King event in the US nuclear test program reportedly gave 500-kt yield without any thermonuclear reactions. In fact, a 100-kt yield in an underground test, where size and weight are no constraint, is much easier than designing a so-called miniature warhead with a 10- to 20-kt yield. From seismic yield alone, there is no way to judge whether North Korea’s most recent test was a fission-only test or a test using thermonuclear reactions. Unless radioactive debris is collected, North Korea's claims will have to be taken seriously in the absence of real data.
Without a doubt, North Korea has tested some type of nuclear explosive. But testing underground with strong containment makes it very difficult to accurately judge the true nature of the test. Was it a thermonuclear device or a large, simple fission device? In the absence of significant amounts of radioactive fission products, which are needed for exact verification, the jury is still out.