“There is no question that the political and security implications of the [recent North Korean nuclear] test are huge and almost entirely negative,” writes Ivan Oelrich, over at the Strategic Security Blog. “The technical significance of the test is somewhat less than meets the eye.”
[A week ago,] the outside world knew that the North Koreans had plutonium available from fuel rods that had been removed from the reactor at Yongbyon. We knew that at least some of the plutonium had been separated out of the fuel rods and, since separation is a fairly straightforward process, it was a fair assumption that most or all of the plutonium had been separated. So we knew about their plutonium supply (and the test tells us nothing more about that), but another key question remained: Could they fashion the plutonium into a bomb?…Before the test, we did not know whether the North Koreans could build an implosion bomb or not. Had the test been successful, we would now know that they could, although we would still not know how close they were to a useable weapon; their test device might have weighed tons and been a once off, rigged up, laboratory experiment. But the test was not successful, so we still don’t know whether the North Koreans can build a workable implosion bomb. Presumably the North Koreans learned something from the test so the probability of the next test being successful is somewhat higher than the probability that the first test would have been successful. This is not much of difference, leaving us in pretty much the same position we were in before the test…
Why might the test have failed? An implosion bomb uses conventional high explosives to compress plutonium until it becomes “critical,” that is, it will sustain a run-away chain reaction. The pressure from the conventional explosives has to be carefully controlled, for example, it must be symmetric or else it is like squeezing a ball of putty: pressure on one side doesn’t compress the plutonium, it just squirts it out the other side. The most likely reason for the failure is some problem with the compression and there is any number of reasons why the compression might not be adequate. If the test were carefully instrumented (which is not necessarily the case), the North Koreans should be able to narrow down the cause, which will give them a much improved chance for success with their next test.
UPDATE 10/14/06 11:20 AM: “Initial environmental samples collected by a U.S. military aircraft detected signs of radiation over the Sea of Japan, possibly confirming North Korea’s nuclear test,” the Washington Post reports.
UPDATE 10/15/06 7:06 PM: “The proposition that the apparently low yield of the test is a failure is not self evident,” says Defense Tech pal John Pike, pointing to this Weekly Standard piece. After all, Pike notes, the yield on the American B61 nuke can range anywhere from a third of a kiloton to more than 350 kt.
UPDATE 10/15/06 7:23 PM: No excerpt will do justice to this epic retelling of North Korea 50-year quest for the Bomb. So just go and read the whole thing.
I wouldn’t think getting enough Pu out of old fuel rods is that easy, as implied here. The percentage of optimal isotope for a decent bomb is lots higher than the percentage for fuel, but I’m more familiar with the older case of U235/U238. What about all those centrifuge cycles they keep talking about in the case of Iran? (Well, that’s UF6, no?) Also, that issue is just as important as implosion efficiency (plus, the Norks could have tried the old “gun” two-chunk approach, as for “Little Boy”?) Remember than even a big cheesy low-yield bomb would cause lots of trouble, say in a boat. In any case, Ill-boy is even more rone-ry than before…
I am sorry, but I just no longer trust any intelligence facts released by this President
or his Administration. They squandered their
credibility with America long ago……
To suggest that the Norks were trying to test a low-yield, possibly muffled bomb ignores the political context of the situation.
They wanted to prove they had nukes. They wanted to make it inescapably clear. They’ve only been blustering and threatening about them since I was in middle school (am now in graduate school). That they couldn’t even manage a kiloton of yield suggests strongly that their technical capabilities are not up to par.
They may be able to manage it next time — but I think the results of this test are not nearly as ugly to us as it might have been if they’d been successful. It doesn’t look good that we didn’t do a single worthwhile thing to stop them, but we get another chance nonetheless.
Neil wrote:
‘The percentage of optimal isotope for a decent bomb is lots higher than the percentage for fuel, but I’m more familiar with the older case of U235/U238.’
The plutonium would almost certainly have come from the 50MWe Yongbyon reactor which is a copy of the British Magnox reactors of the 1950s. It is probably a copy of the first large reactor at Calder Hall, the plans of which were declassified a long time ago.
The Magnox stations allow on-line refuelling which not only boosts their up-time, but allows fuel elements to be irradiated for relatively short periods of time; ensuring that there is relatively little Pu240 in the recovered nuclear fuel which spits out neutrons and ups the critical mass.
The alternative is that NK has got a uranium cascade up and working which could spin uranium up to bomb grade. Technically that is more complicated than separating plutonium from uranium which is relatively straightforward – especially now that the Purex process has been declassified for many years now.
A cannon-style Little Boy bomb is possible, but only if NK has made bomb grade U235. Whilst a plutonium cannon is theoretically possib, it requires some insane muzzle velocities to overcome the risk of predetonation from stray neutrons coming out of Pu240.
The second drawback is that a cannon needs more than a critical mass of material; if NK has as little fissile material as we suspect, than a uranium cannon seems very wasteful; it’d be better spent putting a subcritical mass into an implosion device. Only two countries have ever built them – the US with Little Boy and South Africa which ‘needed’ a number of bombs in a hurry.
What makes me suspect that it is an implosion bomb is that it didn’t work. The uranium cannon, whilst terribly inefficient, is a dead cert. Fire it and you end up with more than a critical mass – after that Dr. Einstein is in control and you end up with 12 kT.
Here? Hmmm it looks like a case of predetonation in an implosion bomb. We should know the fissile material fairly soon now that the US is reporting that it has found fission products in atmospheric samples.
It is typical for a partisan rag like the Weekly Standard to argue the threat upward. The triggers (or primaries) to a Teller-Ulam thermonuclear generally need to be a few kilotons to produce the kind of pressure needed to set off a secondary.
The potential for a really low yield test is also relatively small. Regardless of how much Pu-239 one uses for a fission bomb, it ultimately has to be compressed into a critical mass. If you decrease the initial mass of Pu-239, you have to increase the amount of force compressing the pit.
Neither scenario is complete impossible, but seems highly unlikely in context. Getting a sub-kiloton yield on a pre-announced test just leaves too much ambiguity as to technical capabilities. I think it would be safer to chalk this one up to failure, but remain suspicious.
I honestly wouldn’t put too much stock into the Norks transfering their nukes to anyone, especially (*gasp*) terrorists. No states have transfered nuclear weapons since 1945 because they are just too much a liability in another person’s hands.
As a nyekulturnii, and assuming they only got 200 t (the latest I have heard) or so, perhaps there are the following broadly logical possibilities:
1)It was a basic 20 kt Pu device that went sour, or more like the late 40s stuff, which is a bit trickier.
2)They are trying to miniaturize (minimum imploder, reflector) it to get it in a warhead and didn’t get it quite right. That might not be to hard to fix.
3)They are trying to minimize Pu cause they ain’t got much and they shaved it a little too fine.
4)It is a thermo trigger, so was pretty small and they didn’t quite get it right (but close?)
5)That some of their Pu has too much 240 and they decided to use it up this way.
Some reports have the intended yield at 4 kt — which may be compatible with 2, 3, or 4.
Anybody know if this makes sense?
Bernard