Monday, October 16, 2006

What's in a Yield?

After careful analysis, the U.S. intelligence community has confirmed that North Korea did, indeed, test a nuclear device last week. Conclusive evidence came in the form of air samples, collected by our intelligence platforms after the test. Those samples contained traces of at least two radioactive gases associated with nuclear blasts. It would be virtually impossible for Pyongyang to "fake" that sort of evidence, indicating that North Korea did conduct a nuclear test.

And, if that weren't enough, there are signs that Kim Jong-il may be preparing for a second test. Both ABC News and NBC News reported Monday that suspicious vehicle activity and personnel movements had been observed near the site where the first test was conducted. The activity may represent the early stages of preparations for a second nuclear blast, although the preparations detected are far from conclusive. However, activity observed with the initial test provides a baseline for future events, providing analysts with tip-offs that be a predictor of additional tests.

But the real story from last week's test is the surprisingly low yield from the blast. According to U.S. estimates, the device detonated by the DPRK had an explosive force equal to 1,000 tons of TNT, or roughly one kiloton. By comparison, the atomic bomb dropped on Hiroshima in 1945 had a yield of six kilotons; nuclear devices detonated by India and Pakistan in 1999 (a more useful yardstick) were in the 6-13 kiloton range, with the Indian weapon toward the higher end of that scale, and the Pakistani device toward the lower end.

The comparison to India and Pakistan is important, since North Korea (allegedly) had access to Pakistani bomb designs, thanks to the A.Q. Kahn proliferation network. A North Korean device based on proven Pakistani designs should have produced a bigger blast--at least in theory. Credible reports from the late 90s suggest that the Pakistani bomb detonated after the Indian test was only partially successful. If the North Korean weapon shared critical design features with its Pakistani counterpart, it may have inherited some of the problems associated with early devices built by Islamabad, and resulting in a smaller blast, like the one detected last week At this point, Pyongyang might be described as "barely" a member of the nuclear club, with significant hurdles that must be overcome before North Korea can produce smaller, higher-yield weapons, cable of fitting atop a ballistic missile.

Of course, there are other explanations for the small bang detected last week. Richard Miniter, a respected writer on security matters, believes the low-yield explosion is proof that Pyongyang may have perfected a "suitcase" nuke, an ideal weapon for client states (and terrorist supporters) in Syria and Iran. Miniter's theory is within the realm of possibilities, but I'm not quite prepared to climb out on that limb, for a couple of reasons.

For starters, a nation's first nuclear device tends to be a bit larger than follow-on versions. Downsizing a nuke to fit atop a missile remains a complex proposition, despite the availability of outside help, and 60 years of accumulated nuclear know-how, much of it readily available in scientific papers and on the internet. Prototype nuclear devices-like the one detonated in North Korea last week--tend to be larger weapons, best encapsulated in an oversized gravity bomb. While the DPRK clearly wants smaller warheads for its ballistic missile force, it likely lacks the technology to produce those weapons, at least for now.

In fact, there is general consensus in the U.S. intelligence community have long viewed the "size" issue as a major limitation of North Korea's current nuclear program. However, this obstacle will be eventually overcome, if history is any indicator. For virtually every member of the nuclear club, smaller weapons typically come a bit later in the development process, after the nuclear technology has been perfected. For example, U.S. tactical nuclear weapons in today's arsenal are much smaller than the Hiroshima or Nagasaki bombs, but deliver a yield that is many times more powerful. Working with much more limited resources, it will take Pyongyang a while to develop smaller, more tactically viable weapons. Put another way, there is no reason to believe that Pyongyang has deviated from the normal developmental cycle, and achieved some sort of technical breakthrough that would allow it to begin mass production of small nukes for ballistic missiles, or other purposes.

Indeed, for a regime that demands the world's attention, it would seemingly be in Kim Jong-il's interest to produce the biggest possible bang, underscoring the threat posed by North Korea's nuclear arsenal. That requirement would (seemingly) dictate a bigger blast than what was observed last week. While we may never know exactly what transpired at that test site in the DPRK, available evidence hints at a blast that may have been only partially successful. While that development is bad enough, it reminds us that Pyongyang's nuclear program is still in its infancy, and the world community has an opportunity--no, a responsibility--to halt these efforts before it becomes more advanced.


To give you some idea of how far North Korea has to go in building "better" nuclear weapons, consider these statistics. According to Jane's (and other authoritative publications), a single nuclear warhead from a Minuteman III ICBM has a yield of 330 kt. Warheads on a Trident D-5 SLBM (sub-launched ballistic missile) have a yield of up to 6 mega-tons (MT) each. Tactical nukes actually have a "selectable" yield, allowing them to deliver an explosive force ranging from relatively small, to fairly substantial. By that standard, the device tested by North Korea last week was a veritable pop gun.


CERDIP said...

>>By comparison, the atomic bomb dropped on Hiroshima in 1945 had a yield of six kilotons;

Trintity was ~20 KT,
Hiroshima ~16 KT,
and Nagasaki ~20 KT.

(Hiroshima was a Uranium bomb, the other two were Plutonium).

I haven't heard what traces the sniffers found in the Pacific, but I would bet that the Norks fired a Plutonium bomb. They're harder to get to work right, thus more likely to "fizzle" if the design or implementation is not up to the job, and Plutonium is "easier" to obtain in fissionable quantities, via breeder reactors, whereas fissionable uranium takes months of painstaking refinement.

Big Foot said...

This is what Steven DenBeste, (the knower of most things) said as to why the blast was so small and the problems the North Koreans are grappeling with.

The two most popular and easiest things to make atomic bombs out of are Uranium 235 and Plutonium 239.

The Manhattan Project had parallel development efforts to create both kinds. The uranium bomb was considered to be simpler, and ultimately the design was considered to be reliable enough that they didn't think they needed to test it. The first one, code named "Little Boy", was dropped on Hiroshima.

But the design for the plutonium bomb was much trickier, and it was decided that before attempting to use it on an enemy target that they should set one off as a test. So the world's first nuclear explosion at Alamogordo, code named "Trinity", was the first plutonium bomb. The second one, code named "Fat Man", was dropped on Nagasaki.

The reason they were different has to do with a fundamental difference between uranium and plutonium: plutonium absorbs neutrons much more easily than uranium does. After a critical mass of U-235 is formed, it takes milliseconds for a chain reaction to build to the point of full-scale energy release. So the design for "Little Boy" consisted of a ring of U-235 at one end of the bomb, and a plug of U-235 at the other that would fit into the hole. Explosives drove them together to form a critical mass.

That design wouldn't have worked for Plutonium. The problem is that during the process of forming the critical mass, it can go off early, with much less energy than it is supposed to, and blow the critical mass back apart again.

"Little Boy" was long and narrow; "Fat Man" was round. The reason is that it consisted of a series of wedges of plutonium around the outside of that big sphere. Explosives would then force all the wedges into the center at the same time, and if it all went correctly the critical mass would form fast enough to be completed before the plutonium really started responding, in microseconds.

Iran's bomb program is concentrating on U-235. Enriching the isotope sufficiently is painful, but the bomb design once you have an adequate quantity of highly-enriched U-235 is quite straightforward.

NK's bomb program is based on Pu-239. Producing the plutonium is easier, but the bomb design is much harder to pull off.

If you screw up the design, what you get when you set it off is a misfire, with far less yield than you should have gotten. There's still a "bang", but not an "earth shattering kaboom", as it were.

So there are reports that NK just set off a nuclear test. North Korea has announced that they were successful, but they also reported those missile tests as successful even though it turns out they weren't. (Everything that NK does works perfectly, you know.)

USGS reports a Richter 4.2 event in North Korea. Is that a successful detonation or a misfire? Wikipedia's page on the Richter Scale has a chart converting different readings into approximate real-world explosion-equivalents. Richter 4.0 is listed as 1 kiloton. Richter 4.5 is 5.6 kilotons. Richter 5.0 is 32 kilotons, and the example they use for Richter 5.0 is the Nagasaki bomb. That's "Fat Man", the second plutonium bomb.

If I did my math correctly, then Richter 4.2 is 2 kilotons, and that suggests that the NK bomb misfired. (The Pakistan nuke test was 12 kilotons. India's first test was also about 12 kilotons.)

Apparently NK is announcing that the target yield was 4 kilotons. I don't believe it. Whenever anything strange happens to one of their weapons tests they tend to announce, "That's what I meant to do."

By the way, whenever I write about nuclear weapons I always deliberately include technical errors and deliberately leave out relevant details. You don't need to write to me to point them out.

UPDATE: The South Korean government is saying Richter 3.6. If so, the yield was a quarter kiloton. It's looking increasingly like it was a misfire, which is really easy to do with Pu-239.

UPDATE: The man I feel most sorry for right now is Abe, the Japanese Prime Minister. I'm sure this was just what he wanted for breakfast during his first week on the job.