North Korea’s Sixth Nuclear Test: A Two Stage Thermonuclear Warhead

Kim Jong-un knows how to ruin a man’s week. The plan this week was to chill after a hectic one the week before, but Kim Jong-un just keeps bobbing up that rude head of his time and time again.

As we all know North Korea has claimed to have tested a two-stage thermonuclear weapon. Yield estimates vary given that equations for calculating the yield from seismic data vary, nuclear weapons tests make the earth ring like a bell which is what makes the comprehensive test ban treaty doable and we have seen yet another demonstration of the CTBT’s viability. Also, the value of key constants, dependent upon one’s understanding of the test site geology, also varies.

The consensus appears to be converging on about 120KT, which is a figure I had reached soon after the event based on the seismic data, equation, and constants available to me and that I generally employ. Let us adopt that as a guide to shape discussion.

As we also know North Korea earlier in the day showed off what it called a two-stage thermonuclear warhead, it was certainly shaped like one, being placed into a Hwasong-14 ICBM re-entry vehicle. The press release accompanying the photo-op at the Nuclear Weapons Institute stated that the hydrogen bomb warhead would be delivered by a new ICBM. I have commented upon those aspects earlier. I suspect that the hydrogen bomb design displayed will be delivered by a Hwasong-13 ICBM with clustered, Pektusan, first stage engines.

I have one thing to add here. I rather suspect that North Korea will want to conduct an MET test of the Hwasong-14 to demonstrate, both to itself on reliability grounds and externally on credibility grounds, that it has an RV able to protect a nuclear warhead from the vibrations, high G’s, and temperatures of re-entry.

Notice that would also now function as a test of the purported hydrogen bomb RV as well.

There exists the view that North Korea has not really tested a two stage thermonuclear weapon because of the relative low yields of its five previous tests, and the ~ 120KT consensus yield of this test.

I myself do not share this view.

Firstly, I tend to think that North Korea has underdone its nuclear tests quite deliberately. Take the very first test. This is widely seen as a fizzle, it had a yield of ~1KT, however there exists good evidence to suppose that North Korea was aiming for a 4KT yield, low for a first generation device given that 20KT is the nominal yield, and although it was lower than 4KT it was not a fizzle yield relative to the lower than nominal desired yield.

So, I suspect the 120KT yield, North Korea’s first two-stage thermonuclear yield, was underdone just like the very first fission test appears to have been designed to be underdone. North Korea has performed a complex scientific experiment to demonstrate the viability of its physics package not necessarily a Tsar Bomba type of Khrushchevian display. North Korea’s dedication to building a comprehensive strategic nuclear deterrent is a serious one.

The upper bound of a containable explosion in the tunnels under the north portal at the Pungyye-ri nuclear test site is ~282KT.

We also have the earlier imagery from the Nuclear Weapons Institute, and the accompanying statement of variable yield for this weapon of tens to hundreds of kilotons. The position that North Korea has not tested a two stage thermonuclear device confronts this evidence, and it seems to me is mostly based on a generalised incredulity regarding North Korea’s scientific capabilities.

Now there does exist the possibility that North Korea has tested a Super Oralloy type of fission bomb (500KT), designed by Ted Taylor of The Curve of Binding Energy fame, but this is unlikely given its incongruous nature. North Korea seeks the maximum number of standardised warheads, not a bevy of design types and that is the rational way to proceed given its resource constraints. It could be a “layer cake” or “alarm clock” or “Sloika” type of fission-fusion bomb, that is based on differential layers of fission and fusion nuclear material, and there’s interesting history here linked to yesterday’s nuclear test.

When the Soviet Union tested the Sloika on the way to its two-stage thermonuclear weapon Hans Bethe, gathering data from radionuclide particles, was able to deduce its design through a brilliant act of deduction. There was a secondary tremor associated with yesterday’s test, and that’s consistent with a collapse of the test tunnel. That means radionuclides might vent into the atmosphere providing crucial data for nuclear forensics on the design of the device tested, much like with Hans Bethe. However, North Korea claimed that no radionuclides were released following yesterday’s test.

I doubt yesterday’s bomb test was that of a Sloika. The Sloika and Alarm Clock didn’t really go on to become operationally deployed weapons, and they were a critical stage in the development of the two-stage thermonuclear weapon when the very possibility of two-stage radiation implosion was not known to US and Soviet weapons designers.

North Korea is not in the same position. Pyongyang needs to recreate the hydrogen bomb not invent it as if new, thus one suspects that the Sloika intermediate stage isn’t really required.

There’s has also been some scepticism with regard to the compactness of the device tested yesterday. Going by North Korea’s remarks Pyongyang did indeed test the two stage thermonuclear compact warhead seen earlier in the day being placed into a Hwasong-14 RV. One interesting thing about that is that we didn’t see any signs at the Pungyye-ri nuclear test suggesting an imminent test.

North Korea has been primed to go pretty much all year at Pungyye-ri, and there was a lot of discernible evidence of test preparations in the first part of the year, but lately there has been little evidence of an imminent test even though they were ready to go when required on very short notice. One suspects that a large purely experimental two-stage thermonuclear test would have had a more discernible preparation footprint than what was seen.

That’s consistent with the test of a compact device.

I’ve been wanting to write regarding nuclear strategy in the Korean context, but the pace of technical developments have prevented this. I do seek to make one comment regarding this observation at 38North following North Korea’s sixth nuclear test

The significance of this is that it has the potential to dramatically increase the threat posed by its Strategic Force (responsible for ballistic missiles) as individual nuclear warheads potentially now have 10-times-greater destructive power. This would allow fewer missiles to be employed to ensure destruction of a given target, and increase the target set threatened by North Korean ICBMs by allowing a larger number of targets to be engaged with the current missile inventory. If the claim that the device just tested has a variable yield is true (from tens to hundreds of kilotons), then this would also imply a more sophisticated employment doctrine that envisions more limited, flexible and discrete targeting options than would otherwise be needed to implement a minimum deterrence, counter value doctrine

The first half here is important, and spot on I do believe. The second part, regarding escalation control strategies of the type first put into US strategic nuclear war planning with the Nixon administration’s NSDM-242, less so I do believe. To be sure North Korea, it seems, seeks a comprehensive strategic nuclear deterrent able to strike targets in South Korea, Japan, Guam, and the US, however that is consistent with a doctrine of minimum counter-value deterrence.

North Korea won’t engage in limited intra-war deterrence strikes of the type US nuclear strategists are enamoured with. It will be all or nothing at the first hint of a conflict, and the North Koreans have said as much.

Update. The equation that I use is not appropriate because it does not take into effect depth. The better equation for North Korea takes into account available data, based on sound inferences on the Punggye-ri test site, of tunnel depth. The first equation I used was

Mb= 4.262 + 0.973 logY

Where Y is yield in kilotons of TNT.

The better equation is

Mb = 1.0125log (Y) – 0.7875 log (H) + 5.887

Where H is tunnel depth.

Assuming MB ~6.3 and tunnel depth of 600 to 900m leads to a yield range of 370-500KT.

That’s a hydrogen bomb. The points raised above regarding under doing test yields still holds, but is not as salient, because a first hydrogen bomb test has generally been in the MT (megaton) range. Notice that the best upper bound estimate of the contaniability of tunnels at the North portal at Pungyye-ri is 282KT, which is consistent with the secondary tunnel collapse picked up by the United States Geological Survey.

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