Saturday, October 14, 2006

Federation of American Scientists put North Korean nuclear test into perspective

VP of Strategic Security, Ivan Oelrich provides a technical assessment of N. Korea:

North Korea’s Bomb: A technical assessment.



There was early confusion about how large the explosion actually was, with U.S., French, and South Korean seismologists reporting a yield equivalent to about 500 tons of high explosive, that is half a kiloton, while the Russians reported that the yield was in the range of 10 to 15 kilotons, or twenty to thirty times larger. From the beginning, the source of this huge discrepancy was difficult to understand. Soon, the Russian seismic data were released and it became clear that even their own data did not support the Russian claim. Most reports as of yesterday had settled on the lower yield figure of about half a kiloton.

The assumption is that the low yield indicates a major failure of the test. It is actually easier to build a medium-sized nuclear weapon than a small one. The simplest Manhattan Project style weapons will naturally have yields in the ten to twenty kiloton range. It is quite difficult to design and build a nuclear weapon that reliably produces a limited yield. Sophisticated nuclear powers have done it, with some bombs and nuclear artillery shells, for example, having yields of a fraction of a kiloton. That was most likely not the aim of North Korea, nor is the country likely to have the technical sophistication to build a very low-yield nuclear weapon. It follows that the low yield was a mistake, a test failure. (It is fair to ask how the North Koreans could accomplish by accident what only the most sophisticated nuclear powers can do by design. The difference is reliable yield. If the North Koreans repeated their test, they might get a yield of several kilotons, or a yield of zero. Making a bomb that will sometimes be a dud is easy, making a bomb that is exactly the same “dud” each time is difficult.)

So how has the situation changed with this test? From a technical perspective, less that we might first think. 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 except that now they have a little less), but another key question remained: Could they fashion the plutonium into a bomb? We did not know, although the U.S. intelligence community concluded as far back as in the early 1990s that North Korea probably had built a couple of nuclear weapons.

There are two basic routes to a nuclear weapon, either using plutonium or enriched uranium. Enriching bomb-grade uranium is more difficult than producing plutonium in a simple nuclear reactor (although this is becoming less true with the continuing developments of gas centrifuges). Once a bomb-builder has the uranium, however, the construction of the simplest uranium bomb, a so-called “gun-assembled” bomb, is relatively easy. Plutonium is the opposite: producing the material is the easier step (especially if one does not worry about radioactive environmental contamination) but plutonium demands a more sophisticated and challenging bomb design, an “implosion” bomb. The Iranians seem to be on the uranium route (like Pakistan); the North Koreans have followed the plutonium path (like India).

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. So the political implications of the test are huge but the technical implications are quite limited.

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 in your hand: 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. Assuming the test was carefully instrumented (and given North Korean technology, this is not certain), 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.

We might be able to learn something ourselves about the test if radioactive debris escaped from the test site. It is not easy to completely contain an underground nuclear explosion. Russian tests often leaked. The US was much better at containing tests but even US tests leaked in a couple of cases. Of course, it is much easier to contain a half kiloton test than a ten kiloton test but some radioactive material might have leaked out. Detecting that would, first, confirm that the test was, in fact, nuclear and analyzing it might provide some limited information about the design of the weapon and the source and age of the plutonium.

What does this mean about possible responses? First of all, there is something to be accomplished by responding. When the North Koreans broke out of the safeguards on their reactor, making several bombs’ worth of plutonium available, it was a disaster for the control of their weapon program. The outside world could keep an eye on the reactor and account for the materials there but once the material left the site, trying to track it was hopeless; the volume of the plutonium is small, any of thousands of buildings could house it. But there was one remaining important hurdle that would generate a clear signal if jumped by the North Koreans: a test. Had the test been successful, then the cat really would be out of the bag. There would be no way to track some uncertain quantities of plutonium and the North Koreas would have a design that, in principle, could be replicated to produce more bombs in any of hundreds of nondescript light industrial facilities. (I say “in principle” because a more sophisticated nuclear power would require more than one test, but the North Koreans may have substantially lower reliability and confidence requirements.)

But the test was not successful. The North Koreans no doubt learned a great deal from their test but they have not proven to themselves, or the world, that they have a design that works. If the first test were successful, the marginal value of subsequent tests would have been relatively much smaller but with an unsuccessful test, the value of the next test will be as great or greater than the last test. We should not give up and say this is now a lost cause. There is much to be gained by using threats of sanctions and other tools to stop follow-on tests.

I didn't see this in the mainstream media. All I saw was fear, fear, fear and commentary by the usual suspects. Where have all the journos gone, long time passing........


At October 16, 2006 5:50 PM, Anonymous Anonymous said...

Great info, Julie. Thanks.


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