James ActonCan We Verify NORK Pu Declaration?

Yesterday’s news that Bush wrote to Kim, reminded me of some back-of-the-envelope calculations I did a couple of months ago on verifying North Korean plutonium production. After all, when (I’ll be optimistic and refrain from saying ‘if’) North Korea submits its declaration, the fun of verifying it will start. One thing the US/IAEA (delete as applicable) will want to make sure is that North Korea has not hidden a stockpile of plutonium. To do this, inspectors will try to estimate the amount of plutonium produced by North Korea and compare it to current plutonium holdings (minus the amount used in the test). The two figures won’t be exactly equal for reasons discussed below but if they’re the same to within, say, 1 kg (less than the amount required to build a bomb) then it’ll be smiles all round. If there’s a difference of 10 kg then things could get awkward.

So, how accurately can North plutonium be verified? I think there are four major sources of error (note that the amount of plutonium currently in weapons or storage can be measured very accurately—although whether North Korea permits it is another story):

1. The uncertainty in the amount of plutonium produced in the 40 MW reactor at Yongbyon. Based on satellite imagery alone, Albright and Brannan put this at 60 ±10 kg (with slight rounding). I’ll be sceptical and assume that North Korean operating records don’t help reduce this error but, in actual fact, they may do.

2. The uncertainty in the quantity of plutonium lost during reprocessing. Most reprocessing plants, even—how do I put this politely?—basic ones, are pretty efficient. In the absence of any more concrete information I think 95 ± 2 % is a fair guess for the extraction efficiency of North Korean plutonium.

3. The uncertainty in the amount of plutonium used in the test. Let’s say that the North Koreans used 6 ± 1.5 kg in their underground firework display last October.

4. The uncertainty in the quantity of plutonium that is/will be in the spent fuel pond at Yongbyon. This is rather hard to measure accurately. I’ll use Albright and Brannan’s number of 11.5 ± 1.5 kg but, again, North Korean records may help reduce this error.

Anyway, adding these errors give a total of error of about 10 kg—remember this number essentially represents the upper limit on the size of a secret North Korean plutonium stockpile. The bad news is that this isn’t good enough to account for one bomb’s worth of plutonium. The good news is that it’s almost entirely due to one source—the uncertainty in the amount of plutonium produced at Yongbyon—and nuclear forensics can help to reduce it considerably.

Specifically, if North Korea permits inspectors to take samples from the reactor’s graphite moderator for analysis then the amount of plutonium produced in the reactor can (hopefully) be estimated to within about 2.5% using a technique called GIRM (Graphite Isotopic Ratio Method) that was primarily developed to account for plutonium production is the former Soviet Union. This, in turn, would allow the US to put an upper limit of about 3 kg on the size of any North Korean plutonium stockpile—probably small enough for comfort.

All of this makes me optimistic that trying to verify the plutonium part of the North Korean declaration won’t derail the process. We just need to hope that (i) the reactor is not being disabled using a material that would interfere with nuclear forensics and (ii) that the North Koreans are ultimately willing to give inspectors rather a lot of access.


  1. Allen Thomson

    It’s interesting, though perhaps not all that relevant, that the total amount of plutonium in question (=< 70 kg) has a volume of about a gallon.

    Not, I hasten to add, that one should try to put the entire amount into a gallon bottle.

  2. SQ

    The above comment merits immediate submission to “Hints from Heloise.” Is she syndicated in Pyongyang?

  3. Lao Tao Ren (History)

    Could some of the PU be used in the Pakistani test that was suspected to be a DPRK PU weapon?

    Or is that suspected Pakistan PU bomb test another tall tale from the arms control community?

  4. AHM (History)

    Two points:

    1)The plutonium “lost” during reprocessing should show up in the waste streams (or in the process lines)

    2)95% may be high; the North Koreans claim that they lost 30 percent; see Albright and O’Neill 2000, p.88

    (p.s. Your links are all borked.)

  5. Yale Simkin (History)

    A very nice intro doc to the Graphite Isotopic Ratio Method is available from our friends at PNL

    The statement in the post:

    …1 kg (less than the amount required to build a bomb)…

    is not correct. 1 kg of Pu is most certainly sufficient for a bomb. The statement would better read as:

    1 kg (less than the amount that NK, with its present capability, requires to build a bomb)

    I also see a problem with:

    …3 kg on the size of any North Korean plutonium stockpile—probably small enough for comfort…

    Assuming that NK corrects the issue they may have had with either pre-initiation or poor geometry in the compression in their bomb design, then 3 kg is more than enough for them to build a ferocious atomic weapon with current capability.

    btw.. the links in the post need fixing. The urls are being prefixed with the URL of ACW

  6. James Acton (History)

    AHM: I hadn’t seen that before. Thank you.

    Yale: Yes and no. I agree your wording is better—I was being a bit sloppy. As for whether 3 kg is enough for the North Koreans—it’s hard to know (unless you’re a bomb designer).

  7. Yale Simkin (History)

    Extremely oversimplifying (but sufficient to make the point):

    If an uncompressed 5 kilograms of Pu in a good reflector is one critical mass, then 3 kg is 0.6 of a critical mass.

    Since the critical mass is inversely proportional to the square of the density,

    at an implosion compression of 2x (the first generation Fat Man achieved that) then 5 kg forms 4 critical masses, and bang

    In the identical implosion assembly the 3 kg Pu would create 2.4 critical masses (about the same as the Little Boy HEU bomb) at 2x compression.

    If you examine Cochran and Paine’s chart under Low technology, 3 kg yields 1 kiloton – a formidible and fearsome weapon. Under the Middle technology (which I see NK possibly at), 3kg yields 10 kt.

    Amount of Plutonium and Highly Enriched Uranium Needed for Pure Fission Nuclear Weapons

  8. Lao Tao Ren (History)

    It begs the question whether the DPRK test was as much of a failure as it is made out to be.

    The inspectors really need to get a look at the tested weapon’s design.

    Could it be possible that DPRK went ahead and tested a far more sophisticated device (and hence, harder to build, more prone to fizzle)than a typical first test?

    Could it be that they went directly to building a weaponized, deployable weapon that their existing platforms are capable of carrying?

    Could they have had outside help in tweaking the design and hence, had the confidence to go directly to weaponization and bypass the ‘proof of concept’ laboratory device?

    Inspectors: answers, please.

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