What Can You Learn From A Weapon Test?

I did an “interview with RFA”:http://www.rfa.org/korean/simcheongbodo/2007/01/24/nk_reported_support_iran_nuke/ a few days ago about the alleged nuclear cooperation between Iran and North Korea (which I blogged about “here”:http://www.totalwonkerr.net/1333/wankery-department-of.)

During that interview, I said:

bq. I don’t know if Iran has nuclear weapons program or not. But if it does that program is using highly enriched uranium for the explosive material. North Korea’s program is based on plutonium. So it’s unclear how much Iran could use whatever information they got from N.Korea.

Because I was unclear on that issue, I asked a couple of physicists. It seems that data from the test of a Pu implosion device actually could be pretty helpful to a weapons program using HEU.

One physicist told me that:

bq. With the caveat that I have never seen classified bomb design info, I would think that the only big difference between the two implosion devices is that the core of the HEU device would be somewhat bigger than the core of the Pu device because about 25 kg of HEU would be needed in comparison to 8 kg of Pu. So, the weapons scientists would have to scale up the HEU device. This would require calculating how to rearrange the conventional explosives that squeeze the HEU core.

Those calculations are apparently not terribly difficult for a competent physicist.

Another said:

bq. The results of the Pu test would validate (or invalidate) the computer models and techniques used to design, manufacture, and test the device. In particular, I would think that a successful Pu test would give a country substantial experience that would apply directly to key components of an HEU device, such as the HE assembly and related electronics and the initiator.

Glad I qualified what I said to that reporter.


Based on a conversation I had with a colleague, I should clarify that I am talking about the extent to which data from testing a Pu-based implosion device could help a state trying to build a similar HEU-based device. Obviously, Iran could simply choose to build a gun-type device out of HEU.

That colleague also pointed out that the design data of an implosion device would be just as important as the test data.

14 thoughts on “What Can You Learn From A Weapon Test?

  1. Robot Economist

    Not to steal Sharam Chubin’s thunder here, but I think the debate over nuclear cooperation between Tehran and Pyongyang speaks volumes about “North-South” biases.

    It seems like the crux of the debate is centered around what kind of weapon the Iranians are actually pursuing. Most analysts see Tehran’s HEU-centered program to generate fissile material and think that they will go for a gun-type detonation mechanism.

    This implicitly assumes that they will settle for the simplier, but riskier gun-type because it is easier for a “Southern” country with limited technical means to pull off.

  2. Paul Kerr

    I’m not sure who “most analysts” are…the fact that Iran has been conducting HE testing has actually fueled fears that Iran is trying for an implosion-style device.

    I don’t understand the rest of your argument.

  3. John Field

    Be careful!

    Uranium has a d-shell electron which will likely shift into the unoccupied f-orbital at high pressure. In Thorium, this happens at around 12 million psi. It’s a good bet that Uranium will have some kind of phase transition someplace on the compression curve up to implosion pressures that probably go to maybe 30 million psi for a fancy weapon.

    That’s bad because it’s going to be tough to get good equation of state data up there, and the effect on shock dynamics may ruin your implosion. In this respect, Pu might be easier to deal with.

    In sum, I think a U weapon is a different design. It seems to me that other than manufacturing cost, there are two reasons to use U.

    1)slow assembly velocity, e.g. primitive design
    2)way over critical to avoid need for tritium boost in igniting secondary.

  4. SQ

    Iran is presumably able to develop a HEU implosion device based on information from the Khan network. The document spotted in Iran by the IAEA on hemipherical casting of U suggests as much. So do the Chinese HEU implosion device blueprints supplied by the Libyans to the IAEA.

  5. Alex W.

    As a lesson from history, the bomb folks were totally caught unawares and impressed by China’s detonation of an implosion U-235 bomb in 1964. My feeling is that if it were a simple case of substitution of materials, they probably wouldn’t have been as impressed as they have been described (in that new book on Spying on the Bomb, for example). Just a thought from a non-physicist.

  6. Haninah

    John, Pu has a seriously complicated f-orbital structure, and undergoes more dramatic solid-phase phase transitions than any other material known to man (see pretty much any article in this issue: http://www.fas.org/sgp/othergov/doe/lanl/pubs/number26.htm). I don’t know if this bolsters or undermines the claim that data from a Pu test might be helpful for an HEU program, but it sure means that Pu is unlikely to be “easier to work with” than HEU – Pu is, after all, “a physicist’s dream, an engineer’s nightmare.”

    On another tack, however, what about the possibility (regularly raised by Jeff over at ACW) that Iran just might be trying to develop a parallel Pu track?

  7. Robot Economist

    By most analysts I mean the technical estimators in Defense Department that I work with (tri-service, not DTRA though). Sure, we’re not nuclear experts, but we do know a thing or two about how third world countries put together weapon systems on a shoe string budget.

    The last sentence in my comment was merely a reflection on how our thinking may be wrong because my little community of estimators frequently assume countries with limited technical means will always seek the simplest technical solution.

    Sharam Chubin argued this “simple solutions for poor countries” assumption was a natural extension of the cultural and social divide between the developed countries of the Northern Hemisphere and the underdeveloped countries of the Southern Hemisphere.

  8. John Field

    I agree, of course it is true that Pu has much more complicated metallurgy at standard conditions.

    The stability of the electronic structure has to do with the state mixing – say for example d-shell with f-shell. As the pressure increases, the energy levels of different states cross and create strong interations that lead to the phase transitions at the crossing pressures.

    Pu is so complex because it is naturally at the crossing point at ambient pressure. This complexity seems to me to be inherent in the actinides generally, and U will manifest similar properties – only at extremely high pressures which are difficult to access experimentally. Therefore, the high pressure equation of state may be MORE uncertain and difficult to obtain than that for Pu, even though the ambient pressure state date might be more complex in Pu.

    Ted Taylor, in his Curve of Binding Energy, remarked that the compressibility of all the actinides were the same – yet, I think this comment needs to be viewed with suspicion at least. He was talking about improvised weapons. Perhaps he meant the same insofar as a low performance bomb could achieve, or maybe he was just being very coarsely approximate.

    It may be possible to alloy the U to move around these phase transitions in pressure space and thereby achieve desirable properties.

    I freely admit this could be much ado about nothing. Maybe this is not a problem of concern, but it seems to me that a potential bombmaker would need to know, not guess, the correct answer to these questions…and a Pu test wouldn’t answer them adequately.

  9. Paul Kerr

    Here’s a comment from a loyal reader:

    Comment: Iran is presumably able to develop a HEU implosion device based on information from the Khan network. The document spotted in Iran by the IAEA on hemipherical casting of U suggests as much. So do the Chinese HEU implosion device blueprints supplied by the Libyans to the IAEA.

  10. yale

    I think much of this discussion misses the real point.

    It is NOT the fissile core (HEU or Pu) that is hard.

    It is the non-nuclear implosion system that is the heart of the technology.

    BTW – that physicist who said 8 kg of Pu or 25 kg of HEU is W..A..Y.. off the mark.

    Only an extremely advanced and experienced and sophisticated design team attempting to achieve either very high yield or a very compact and light (but inefficient) bomb would dare to build with that much material.

    Even Fat Man only used ~6 kg of Pu and it was a pretty touchy and quite unsafe device.

    Anyway, The ability to achieve a converging shockwave is what it is all about. The details of the amount and type of driver explosives, tamper/reflector choices, core configuration, use of different fissiles, etc, are all managable details for early builders.

    The bleeding-edge, very narrow margin, boosted primaries of US missile warheads is not relevant to a nation building an arsenal. Even the US is wanting to robust-up its designs.

    A converging shockwave at sufficient velocity pushing a sufficient tamper will create a city-buster from HEU or Pu.

    The US arsenal was originally designed to use HEU, Pu and varying composites blends along with all sorts of tampers, configurations, etc WITH THE IDENTICAL IMPLOSION SYSTEM.

    The venerable Mk 4 came in a rainbow of flavors for the pits – using whatever the production facilities had available.

    The equation-of-state of HEU and Pu are similar enough for guv’mint work.


  11. John Field

    Yale, oh come on!
    If you don’t know about the equation of state, you don’t even know the shock velocity within a factor of two. You wouldn’t even know when to fire an initiator much less what the final compression is going to be.

    I can see good reasons for using composite pits especially as primary triggers, but that is not the same thing as saying that you can interchange Pu and U at will throughout.

    Jeffrey said, “A job worth doing is a job worth doing right.”

  12. yale

    John, you are missing my points.

    The “killer app”, the sine qua non, is the implosion system and associated non-nuclear parts.

    I neither said nor implied (quoting John:) you can interchange Pu and U at will throughout.

    As I specifically said:

    The details of the amount and type of driver explosives, tamper/reflector choices, core configuration, use of different fissiles, etc, are all managable details for early builders.

    There is no implication that quite important details must not be analysed and designed around. However, these are relatively straightforward.

    If you have the hardest part, the implosion system, then you can (as the US did), use it as a standardized unit, and just vary the specifics of the other components.

    There is no implication in my post that you can just toss a ball of U235 or Pu239 in the center and it will work equivalently.

    BTW – your point about not knowing when to initiate if you don’t bother determining the EOS (something I never said nor implied), it is automatically dealt with if one uses an internal initiator. Just as the US did for it’s entire initial atomic arsenal for over a decade (and as the current proliferators seem to be doing).

    As a second BTW

    The equation-of-state of Pu and U are not as different as you seem to say, just as Ted Taylor who actually KNEW) correctly pointed out.

    As Sublette more recently stated (discussing compression) points out:

    Compression figures for plutonium are classified above 30 kilobars, but there is every reason to believe that they are not much different from that of uranium. Although there are large density variations from element to element at low pressure, the low density elements are also the most compressible, so that at high pressures (several megabars) the plot of density vs atomic number becomes a fairly smooth function. This implies that what differences there may be in behavior between U and Pu at low pressure will tend to disappear in the high pressure region.

    Actually, even in the low pressure region the available information shows that the difference in behavior isn’t all that great, despite the astonishingly large number of phases (six) and bizarre behavior exhibited by plutonium at atmospheric pressure. The highest density phases of both metals have nearly identical atomic volumes at room pressure, and the number of phases of both metals drops rapidly with increasing pressure, with only two phases existing for both metals above 30 kilobars. The lowest density phase of plutonium, the delta phase, in particular disappears very rapidly. The amount of energy expended in compression at these low pressures is trivial. The compression data for uranium is thus a good substitute for plutonium, especially at high pressures and high compressions.


    In any event, the people building bombs will have determined the relevant characteristics of the materials they use, and will tailor the configuration to the implosion system.


  13. John Field

    Yale, flash me an email so we can continue offline. You may be right, but I’d like to explain why I think caution is justified and this discussion is getting too technical for short little blog posts.

  14. Dave

    Minor detail: Fat Man, like any other design using an internal initiator, did not have very much compression of the Pu. While there was a ton of neutron reflectors in the design, you would still need around 7 kg to achieve a criticality of 1.15. More likley, they used 8-10 kilos.
    (Do you suppose that Fat Man’s design will ever be declassified?)


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