James ActonSaving Kim's Blushes

Greetings from Korea.

According to my guide book “North Korea and nukes” is not a socially acceptable topic for conversation here in Seoul. I hope that advice doesn’t apply to participants in a conference on security on the Korean Peninsula because, if it does, I committed a dreadful faux pas over dinner…

According to a recent GSN article, in its June declaration, North Korea declared 30.8 kg of plutonium and announced it had used 2 kg in its 2006 test. While many analysts argue that it is possible to build a first nuclear weapon with less than the 8 kg of Pu that the IAEA suggests is needed, 2 kg really is surprisingly small. David Albright called it “the most puzzling thing in the entire declaration.” And I share his puzzlement. I think it very probable North Korea used at least twice as much.

This evening, however, I had an idea. Maybe North Korea is claiming it only used 2 kg to suggest that the test was not a fizzle but the successful demonstration of a smaller, much more advanced device.

To be clear: I think the test was a fizzle. I can’t imagine that North Korean scientists would have gone for anything other than a simple, easy, reliable device (which would involve lots more than 2 kg of Pu). But the test having failed, North Korea may be trying to hide it by understating the quantity of material it used in the test.

Just a thought. But one that if correct does not bode well for the verification process.

PS As we were coming into Incheon airport, I was able to see North Korea (just). These kind of things are exciting at the end of a 14 hour flight.


  1. Better Idea

    How about a James Bond screenplay. It was really a Russian suitcase device tested by the North Koreans on behalf of the Iranians, with the latter intending to use it against the Knesset building in Jerusalem. The North Koreans tested it because given their backwardness no one would think it wasn’t a fizzle. Also, it needed to be tested as part of a life-extension process.

  2. blowback (History)

    And maybe they just used 2kgs! The North Korean test was most likely not a test in the sense that Trinity was, it was a political demonstration that said to the US that North Korea had arrived as a nuclear power and by its actions, the US has accepted that. So why should the North Koreans waste valuable plutonium on a political demonstration. Futhermore, by creating a degree of ambiguity, (i.e., did it fizzle), they were making it easier for the US to do a deal.

  3. John Field (History)

    In my opinion, the 2 kg is true. I don’t see anything very strange about it at all.

    Look at it this way. Bare critical mass is 9 kg. With a stout reflector of uranium, your bare critical mass is maybe 4.5 or 5 kg. Naturally, if you’re interested in safety of your weapons, you build them just a little below that – like the US does – so one-point safety is pretty easy to get and be confident of. Looking at the shock tables, plutonium pressure goes through the roof past about 2x compression, and it doesn’t seem to me that this is all just single shock entropy pressure – even the cold the curve just plain gets steep down there. Since critical mass goes as the square of density, you will see that with 4 kg it is not so hard to get way over critical.

    Now, let’s replay with 2 kg. The background neutron rate is half AND the buildup time is a lot longer because I’m not going way over critical. Meanwhile, I don’t even need a factor of two compression to get criticality(do the figures above) – in fact, 1.5x probably does it. LASL shock hugoniot data says that happens around 20 million psi, high but not ridiculous given that you get about 6 million psi in a plane wave detonation of RDX – e.g. it is not necessary to make a super thin shell of pu to get the high pressure, perhaps 2 or 3:1 aspect ratio would be enough, not 10:1 like a US warhead. Plus, the uranium tamper is helping to concentrate the shock. Now, even stagnating a single shock will generate this pressure at about 1,600 m/s of material speed. This is just not a big number at all.

    So, consider the advantages of this :

    – the bomb will be half the weight – it will use half the pu – reliability will be good(I have a lot more to say on this topic) – it will require less sophisticated engineering – the yield will be a strong function of achieved compression and therefore be a good diagnostic of performance

    The drawbacks, the yield will be low and variable. Both of these facts can be offset considerably by setting the burst height lower than what would normally be considered optimal – e.g. use the nonlinearity in the damage function to offset sigma yield.

    So, I hate to stand alone, and I have great respect for all the diligent arms control workers, but to me, the mystery is why does everyone think this is a mystery. If I’d been a NORK, this just seems sensible.


  4. Darren (History)

    I am a rank amateur at these kinds of things, but when the yield of the NK test was announced my first thought was “fizzle”, and my second was “successful low-yield fission primary for a staged thermonuclear weapon.”

    Okay, maybe more like my fifth thought, with the first four being “fizzle”. But it would be somewhat disturbing to find out that there is an OTS boosted fission design from Pakistan or somewhere else that needs a lot less testing than is commonly assumed would be required to get to a deployable thermonuclear weapon.

    That would be all kinds of bad if Iran has the plans for that kind of thing.

  5. Lao Tao Ren (History)

    Um… I have been harping about the device being a weaponized device for quite some time. Check the comments on here.

    I also suspect that Kim believed that a successful test would have solved his problems: In other words, a customer willing to pay, cold, hard, gold, was waiting in the wings.

    When are people going to connect the dots?

  6. Andy (History)

    Ok, let’s look at this possibility. Assume 2kg and a final yield of .5-.8kt – what does that say about the design, efficiency, etc.?

  7. Rwendland (History)

    Darren, Lao, you seem to have a worry Iran might be a “customer” for the NK design. Is a plutonium weapon design of much use to someone who only has enriched uranium? I’ve not noticed anyone suggest Iran is planning any industrial scale spent fuel reprocessing to get plutonium any time soon.

    Can anyone say how useful a detailed 2kg plutonium design is in developing an efficient HEU weapon design?

  8. Lao Tao Ren (History)


    I did not mention Iran.

    Please do not put words in my mouth, I already have my feet in there.

    Not being an expert in your question about weaponization of Pu and its applicability to HEU, I would guess that the basic calculations are substantially similar, with a few twists.

    Just read the enriched Uranium covered Declaration from the NORKs!

  9. Hairs (History)

    I am not so convinced as John Field about the plausibility of the North Koreans detonating 2 kg of plutonium. I don’t doubt John’s calculations as they stand, but I think they include some implied assumptions that are not valid.

    As far as I can tell, the LASL data are for planar shocks. But implosion weapons require convergent shocks (either spherical, as in the Nagasaki weapon, or even more non-symmetric geometries such as are used for levitated pits with one-point safety). This non-planar nature introduces (at least) two features that I believe invalidate, or substantially affect, calculations based on the LASL data:

    1) The shock velocity of a convergent shock scales quite closely as 1/(square root of radius). So there is a big difference in applying LASL data to spherical as opposed to planar shocks

    2) Convergent shocks are highly unstable. In particular they are very susceptible to Richtmyer-Meshkov instability, which is bad enough, but R-M instability also “promotes” Helmholtz instability. These instabilities grow extremely rapidly, and if astrophysical calculations are any guide then they are fast enough to have noticeable effects on nuclear weapon calcualtions.

    On top of these two important factors linked to weapon geometry we also have to recall that the LASL data are for “cold” systems. In contrast a weapon is (presumably) designed such that it reaches maximum density around the time of maximum neutron flux – so as to cause the maximum number of fissions. At this point the gases behind the shock front are still feeding energy into the shock, thus we are actually compressing an extremely hot medium. This means that compression calculations based on Gruneisen approximations are not valid; instead a full equation of state should be used, and full equations of state for high-density plutonium are (understandably!) closely guarded secrets.

    Lastly, the neutron population cannot be ignored. An easily remembered approximation is that a 2.39 kiloton yield produces about 6.2 * 10^23 neutrons (with the gross assumptions of 2 neutrons per fission and total yield of 200 MeV/c^2 per fission). Now, 2 kg of plutonium is a bit over 8 moles of the stuff; but in fissioning, one of those moles is going to turn into 1 mole of fission fragments plus another mole of neutrons. Thus we’ve converted 8 moles of particles into 9 moles, which is a 12.5% increase in particle number – something that is not negligible in any equation of state. For a larger weapon, say 2.39 kilotons from 20 moles of Pu, the effect is proportionately lower.

    Overall I believe that producing a nuclear weapon is probably within the capability of most industrialised and semi-industrialised countries; the limiting factor being access to the materials. But producing a small scale weapon is probably an order of magnitude more difficult because of factors such as geomtery, neutron populations, and many others that I’m not even aware of. Even with all the resources available to the USA (with its huge industrial base, economy, and supply of intellectual “firepower”) it took about a decade to go from the successful Trinity Test to minaturised, levitated pit designs. I just can’t see how North Korea could have successfully jumped past all that research and development, and gone straight to the main game without a whole bunch of intermediate steps and mistakes to learn from.

  10. Jeffrey Lewis (History)

    On the 2 kg of NORK Pu.

    I hate to disagree with John. That usually ends up with me looking like a fool.

    But 1/2 the Pu would not be 1/2 the total mass. The overwhelming contribution to mass is from the conventional high explosive. Reducing the amount of HE (as in the Mk 7) requires more sophisticated designs that should allow for substantially more yield and reduced weight. (I’ve made this argument before.

    A couple of years ago there was some defector testimony suggesting that North Korea used 4 kg of Pu in its device.

    At the time, I wrote that was “plausible, though on the low end of the North Korean Pu estimates.”

    I was working from a wonderful paper by Tom Cochran and Chris Paine that calculated the yield from various amounts of Pu at different levels of compression.

    Everyone should check it out: Thomas Cochran and Christopher Paine, The Amount of Plutonium and Highly Enriched Uranium Needed for Pure Fission Nuclear Weapons; Natural Resources Defense Council, August 22, 1994.

    At 4 kilograms, North Korea might be expected (with a medium level of sophistication) to squeeze out 20-30 kt of yield. At 2 kg, the NORKs couldn’t get more than 4 kt.

    I just don’t see the benefit in building a 2 kg Pu device unless that is all of the plutonium they had.

  11. Lao Tao Ren (History)

    @Dr Lewis

    I think you answered your own question:

    “unless that is all of the plutonium they had”

    Speculating here: suppose they had a very small quantity, made even smaller because they have to fill “customer order”, and them being miserly on top, would lead one to your hypothesis.

  12. John Field (History)

    Jeffrey and ‘Hairs’,

    Yes, I know 2 kg is a small number. Clearly, I agree the NORKs could be lying here. Could be, but it seems to me that I don’t have a good reason for thinking so.

    Let me deal with Jeffrey’s comment first.
    You see, the mass of the driver HE scales as the mass of what you are trying to push – which in this case is half the mass. So, all other things being equal, I think the factor of two weight reduction does go more or less to the bottom line. In Cochran and Paine, it is true that 4 kt would be the “medium” technology point, but 0.5-0.8 kt as actually achieved would be well down toward the “low” tech point. Not unreasonable to say maybe they thought it would work a bit better than it did?

    Hairs’s comments deserve attention also because all his observations are reasonable.
    Yes, the shocks are converging and speeding up. But this helps you since the shock pressure is increasing – something which happens to a much lesser degree inside HE where the chemical reactions are searching for thermodynamic equilibrium at high pressure. Furthermore, you get two shocks – one inward and a reflected outward going shock. Maximum compression is achieved as the outward going shock returns to the surface of the pit. This means that the LASL shock data is inappropriate for two reasons – the planar pressure is too low AND it is a double shock situation – but both of these factors improve the compression.

    The converging shock instabilities are more and more of a problem the thicker the original plutonium shell is. The thinner the shell, the faster it can be made to move, but the difficulties increase exponentially with the thinniness – e.g. shell thickness divided by radius. By 10:1, the situation is really difficult – it’s like imploding an eggshell. At 2:1 you could probably squeeze silly putty with your hands and get close enough. 3:1 would take just a little more effort. The lower you choose the ratio, the lower your final pressure will be – again all other things equal. I tried to explain above that I think 3:1 is probably enough for this primitive case.

    Finally, in a normal weapon, the neutron timescale at completion is only about 3 ns and so to build up the last 20 generations or so takes a negligeable amount of time and therefore cold compression is appropriate. In something like a NORK W-2kg-Mk001A, the buildup time might be 15 ns or so and with much smaller final yield, it seems that the implosion dynamics may not be so separable from the neutron dynamics in a preignition situation. So, I think there is something to Hairs comment about a high temperature equation of state here – it seems like it might slightly affect the optimal initiation time, but in any event, it is only an effect at the very tail end of the compression cycle.

    Right or wrong, I think this is an important debate as it is likely to have real policy implications.

  13. Rwendland (History)

    Remember that at the time there were reports that North Korea had pre-warned China that it would be a 4 kt test (info from “a U.S. government official” according to CNN).

    The Tom Cochran and Chris Paine paper says a medium tech 2kg plutonium device would produce 4 kt, so this does fit together.

  14. Lao Tao Ren (History)

    Suppose DPRK were actually working with other powers that are much more advanced technologically and had more horsepower (intellectual, engineering, technical), and access to key materials and components. That would address many of Hair’s objections to DPRK being able to do a 2kg weaponized device.

    Now just suppose that the sources of the knowhow and other kitsch happen to be tightly watched, so cannot be seen to be doing anything, but sure can let things done in their heads and workstations be sent to DPRK….

    What luck do you think US and other intel services will have intercepting the shipment of a single USB thumbdrive or SD card?

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