Sjeltur

Yesterday, only hours after North Korea’s second nuclear test, this blog already published an early estimate about its yield, based on available seismic data. According to Sjeltur’s calculations the yield was about 4.5 kilotons, considerably less than the estimated 10 to 20 kilotons the Russian defense ministry published later in the day. The Russian estimate gave birth to stories in the press claiming the device tested yesterday was as powerful as the bombs dropped on Hiroshima and Nagasaki. New estimates show that the Russian estimate is likely an exaggeration, and Sjeltur’s estimate is probably close to the actual yield. It’s important to know the yield of the device tested yesterday, because it can tell us something about the nuclear capabilities of North Korea.

After North Korea´s first nuclear test in October 2006 the Russians did overestimate the yield of that test as well. Back then they said the yield was as high as 15 kilotons, we know now that it was less than one kiloton. There was some support for the Russian claim yesterday, early South Korean seismic data suggested a yield of 18 kilotons. But now it seems it’s very likely the Russian defense ministry estimate is too high, again. Seismic data from the US Geological Survey and the Geophysical Survey of the Russian Academy of Science suggest a yield between 2 and 6 kilotons. According to professor M. Kalinowski of the University of Hamburg, Germany, the yield was about 3 to 8 kilotons, with a most likely yield of 4 kilotons. Note that this estimate is very close to Sjeltur’s early estimate of 4.5 kilotons.

The yield of the test says something about the nuclear capabilities of North Korea. Shortly before the test in 2006 the government of North Korea informed China that they were about to test a device with a yield of 4 kilotons, but the actual yield was much less. Apparently, the North Koreans had some problems with the design of that device. The test yesterday with a yield close to 4 kilotons suggests North Korea has solved those problems and is capable of detonating a nuclear weapon at full yield of that design. A design like that is technologically far more advanced than the relatively simple devices most other nuclear states started off with in the past. The yield of these simple devices, roughly 20 – 25 kilotons, is the “easiest” to achieve, a device with a lower yield like 4 kilotons is technologically far more challenging.

It’s still possible that the device detonated yesterday was a simple design aiming at a yield of 20 kilotons or so, and it failed. But the yield of the test conducted yesterday is very close to what North Korea already tried to achieve in 2006, so perhaps it’s more likely they corrected the problems with the old 4 kiloton-design succesfully. This is bad news. A simple 20 kiloton bomb uses a lot of high explosives to ensure a good compression of the plutonium pit, and this makes such a device too heavy to be transported by a North Korean missile. But an advanced device, delivering a full yield of 4 kilotons, is probably light enough to be launched with one of those missiles.

Update: Seismologists at Columbia University have published a new estimate of the yield. According to seismologist Won-Young Kim it was between 2.2 and 4 kilotons. The seismic tremor caused by the explosion seems to be smaller than previously thought, about 4.6 on the Richter scale. The yield is in agreement with the model this blog used to calculate the yield shortly after the nuclear test. Using the figure of 4.6 on the Richter scale in Sjeltur’s model, it points to a yield of 3.2 kilotons, nicely between the upper and lower bounds of these seismologists.

The question remains if the test conducted last Monday was an advanced design, or a simple design. If it was a simple design then it’s clear the test was a failure, a succesful detonation of such a design should have given a yield close to 20 kilotons or so. We know that North Korea aimed at a yield of 4 kilotons when they conducted their 2006 test (at least that’s what the North Koreans told China shortly before the test), suggesting an advanced design. Some experts believe North Korea lacks the knowledge and materials to design and build such a device, very well possible, but there’s no way we can verify this, even foreign intelligence agencies have probably a very hard time to find out what’s going on in the Stalinist country. So we can’t rule out an advanced design entirely, the estimated yield is just too close for comfort near that 4 kiloton-figure. Let’s hope it was a simple design that failed, producing a yield coincidently close to the announced yield back in 2006.

It’s very annoying to see how the Dutch press reported about the nuclear test this week. Although it was known that the Russian government exaggerated the yield of North Korea’s first nuclear test, the press took over the Russian estimate of Monday’s test without any criticism, let alone finding out themselves what a plausible yield could be. Tuesday the Russian claim was disputed in the international press, but it took another two days before the Dutch press got it right… The report in Dutch newspaper De Telegraaf about the new Columbia University estimate is not very accurate as well, it claims the yield of the bomb dropped on Hiroshima was 13 kilotons, but according to the latest research it was 16 +/- 2 kt, so at least 14 kilotons. And the report says that the nuclear powers have nukes nowadays with a yield of 50,000 kilotons or so… That’s a big exaggeration, the nuke with the highest yield in the US arsenal that’s in active service is the B83 with a maximum yield of 1,200 kilotons, and it’s very unlikely the other nuclear powers have nukes close to the claimed yield. The most powerful bomb ever detonated was the Tsar Bomba with a yield of 50,000 kilotons, perhaps that caused the confusion, but the Tsar Bomba was nowhere near a ‘typical’ nuke. Read here more about the Tsar Bomba in English, I wrote most of the article about the Tsar Bomba on the Dutch Wikipedia, you can find it here.

Dutch newspaper De Telegraaf has now removed the article with the exaggerated claim about the ‘50,000 kiloton’ nukes… Readers who can decipher Dutch still can read it on the website of the Financieele Dagblad, see here. And newspaper Reformatorisch Dagblad published the same nonsense, see here.