by Cheryl Rofer
A seismic signal from northeastern North Korea seems to indicate another nuclear test. I haven’t seen a specific statement that the signal had the double peak characteristic of nuclear tests, but the location was the same as North Korea’s previous nuclear test, and preparations had been going on at the test site for a few weeks.
We’ll hear more about the yield later, as more of the seismology labs across the world weigh in with their estimates. Right now, Jeffrey Lewis and Geoff Forden are working their spreadsheets, which are changing rapidly, although they seem to be converging (along with others they link to) on a yield of less than 10 kilotons. The Hiroshima and Nagasaki weapons were in the 15-20 kiloton range.
That’s higher than the first North Korean test in 2006, which was less than 1 kiloton, which amounts to a fizzle in nuclear weapons circles, although 500 tons of TNT could be pretty destructive.
The Technical Strategy
It’s embarassing to announce a nuclear test to the world and then have it show up not so well. The Indians and Pakistanis dealt with this problem in 1998 by setting off multiple tests simultaneously. This tended to mask the seismic signals from the tests, and it took the rest of the world quite a while to figure out that some of the tests were probably less than the testers expected.
When you don’t have a lot of fissionable material, however, that’s harder to do. The Pakistanis set off five devices. That’s almost equivalent to North Korea’s total store of plutonium.
So if you want a sure success, you can go back to the thinking of Robert Christy. In the Manhattan Project, enough was known about uranium-235 that a gun-type weapon (Little Boy) could be designed, and the designers were sure enough of the design that it didn’t need to be tested. Plutonium was a new element, and they didn’t have enough of it to measure the crucial properties until June 1944. Seth Neddermeyer had been designing explosives that would provide a much faster assembling of a critical mass than the gun method. His work was a sideline until those measurements were made. The plutonium that was available could not be made into a gun-type weapon.
Implosion is very difficult to do uniformly, which is essential for a nuclear weapon. “Lenses” of two different explosive compositions, with different detonation velocities, shape the explosive wave. But all kinds of things can go wrong, like imperfect casting of the explosives and lack of proper timing for the multiple detonators. So that weapon (Fat Man) had to be tested.
I’ll let Richard Rhodes* take it from here:
Seth Neddermeyer’s shell-configured core had been abandoned even though thin-walled shells give the highest compressions in implosion. Designing out their hydrodynamic instabilities required calculations too difficult to accomplish by hand. Berkeley theoretician Robert Christy designed a more conservative solid core, two mated hemispheres totaling less than one critical mass that implosion would squeeze to at least double their previous density, shortening the distance that fission neutrons would have to travel between nuclei and rendering the mass supercritical.
So it’s likely that North Korea, wanting a sure success as the Manhattan project scientists did, used a solid plutonium core. This, however, results in a big bomb. There was a reason the Nagasaki bomb was called Fat Man. And Fat Man’s yield was about 20 kilotons, whereas this North Korean test may have been less than 10.
However, they have much more computational power than Robert Christy had, so they may have improved a more sophisticated design, sticking to internal priorities rather than trying to impress the world.
Siegfried Hecker, former director of the Los Alamos National Laboratory and co-director of Stanford’s Center for International Security and Cooperation, was there in February for his sixth trip, although the North Koreans were so irritated at that time that they didn’t take him to Yongbyon nuclear complex. His assessment of their program is here.
The Political Strategy
The North Koreans left the six-party talks, kicked the IAEA inspectors out and have been threatening to restart their nuclear weapons program since early this year. Their motives, as usual, are unclear. According to the New York Times, North Korea has said
The study of the policy pursued by the Obama administration for the past 100 days since its emergence made it clear that the U.S. hostile policy toward the D.P.R.K. remains unchanged…There is nothing to be gained by sitting down together with a party that continues to view us with hostility.This doesn’t add a lot to understanding what they are reacting to, or what their goal is. So here are some speculations.
Siun at Firedoglake has turned up more in this vein from Asia Times.
Analysts say Pyongyang was irate over criticism by the U.S., Japan and South Korea of its April rocket launch and has grown impatient at the lack of attention it has received from the Obama administration -- both possible motives for today's action. North Korea had threatened to test another missile, or another nuclear device, unless the United Nations apologized for condemning that launch, which Pyongyang said was a peaceful satellite.
The North Korean news agency said the test had been "safely conducted on a new higher level in terms of its explosive power and technology of its control. The test will contribute to defending the sovereignty of the country and the nation and socialism and ensuring peace and security on the Korean peninsula and the region."
[Koh Yu-hwan, a professor at Dongkuk University in Seoul:] "North Korea had been expecting the new US administration to mark a shift from the previous administration's stance, but is realising that there are no changes. It may have decided that a second test was necessary. [It] seems to be reacting to the US and South Korean administrations' policies."
Analysts believe the North Korean leader, Kim Jong-il, hopes to use the test to shore up support from the military amid mounting speculation that he is about to name one of his three sons as his successor.
According to a leading expert on North Korea, the British academic Aidan Foster-Carter, a developing fight for supremacy is the most probable explanation for Pyongyang's aggressive behaviour. "North Korea is snarling more. That suggests an internal power struggle," Foster-Carter told a seminar at the Chatham House thinktank in London last week. "The dog barks loudest when it's feeling vulnerable. And maybe it's safer to be a hardliner than a softliner when there's a power struggle going on."I’ll add one more observation, that of the Kris Kristofferson song. North Korea, partly on its own and partly because of the actions of other countries, is isolated in a way that no other country is. It can be reasonably sure there will be no military retaliation because of Seoul’s proximity and China’s fear of a flood of refugees. So it is free to do as it pleases. And, whatever its objectives may be, it can freely pursue them. This is where strategies of isolating and sanctioning countries leads. The rest of the world has little leverage.
Jim Hoare, a former British ambassador to North Korea, said a second explanation should be considered: that North Korea was reacting to what it perceived to be threatening and destabilising external events, notably the ending of South Korea's "sunshine policy" that had encouraged deeper engagement.
The third possible explanation for North Korea's action today is the least palatable: the possibility that, increasingly, nobody is really in charge in Pyongyang and that the country is beginning literally to run out of control. If North Korea suddenly imploded, the US and South Korea might come in from one side and China from the other, Foster-Carter warned. The danger of history repeating itself was, he said, a "baleful prospect".
I’ve seen a couple of mentions that what North Korea wants is to be accepted as a nuclear power, although I’m having a hard time finding them just now. If that is indeed their goal, it could dovetail with an objective that has recently been enunciated by both Barack Obama and Gordon Brown: to bring all nuclear powers into discussions on reducing nuclear weapons.
* The Making of the Atomic Bomb, 1986, Simon and Schuster, Touchstone Press, New York, p. 655.