how well did the North Korean test really go?

(apologies for the use of the word “crap” in 2 quotes)
It’s beginning to look as though the North Korean test was a bit of a failure. In all the (quite proper) outrage at the incident it seems that things went a bit wrong.

The explosion was picked up by seismic recording stations around the region as something inbetween 3.6 to 4.2 on the richter scale. As DefenseTech and armscontrolwonk put it,

even at 4.2, the test was probably a dud.

they go on:

Estimating the yield is tricky business, because it depends on the geology of the test site. The South Koreans called the yield half a kiloton (550 tons), which is more or less — a factor of two — consistent with the relationship for tests in that yield range at the Soviet Shagan test site:

Mb = 4.262 + .973LogW

Where Mb is the magnitude of the body wave, and W is the yield.

3.58-3.7 gives you a couple hundred tons (not kilotons), which is pretty close in this business unless you’re really math positive. The same equation, given the US estimate of 4.2, yields (pun intended) around a kiloton.

The device that North Korea said they were testing is made from plutonium so the device would have been an Implosion weapon (see the link at wikipedia). This typically results in a 10-20kt weapon – somewhere between 10 to 40 times the power of what was actually recorded.

So what could have happened? There are two alternatives.

1. It failed. Here’s all the techie stuff:

First and most important, Pu-240 has a high rate of spontaneous fission, meaning that the plutonium in the device will continually produce many background neutrons.

In a nuclear explosive using plutonium, the plutonium core is initially “subcritical,” meaning that it cannot sustain a chain reaction. Chemical high explosives are used to compress the plutonium to higher than normal density (so that the neutrons released in each fission have a higher probability of hitting other atoms and causing more fissions).

In a well-designed nuclear explosive using weapons-grade plutonium, a pulse of neutrons is released to start this chain reaction at the optimal moment, but there is some chance that a background neutron from spontaneous fission of Pu-240 will set off the reaction prematurely.

With reactor-grade plutonium, the probability of such “pre-initiation” is very large. Pre-initiation can substantially reduce the explosive yield, since the weapon may blow itself apart and thereby cut short the chain reaction that releases energy.

Calculations demonstrate, however, that even if pre-initiation occurs at the worst possible moment (when the material first becomes compressed enough to sustain a chain reaction), the explosive yield of even a relatively simple device similar to the Nagasaki bomb would be of the order of one or a few kilotons.

While this yield is referred to as the “fizzle yield”, a 1-kiloton bomb would still have a radius of destruction roughly one-third that of the Hiroshima weapon, making it a potentially fearsome explosive.

Regardless of how high the concentration of troublesome isotopes is, the yield would not be less. With a more sophisticated design, weapons could be built with reactor-grade plutonium that would be assured of having higher yields.

That sounds about right. It is reported that the Russians and Chinese were given some warning of the test. The Russians have reported that the yield was about 15-20kt which seems a bit strange for them to say given the seismic data but perfectly understandable if that’s the information they were given by North Korea.

There is another alternative, though:

2. The test was a fake.

This is somewhat more implausible but, as commenters on the Washington Monthly site put it:

Can you say Bluff?
Posted by: greenchilecheeseburger on October 9, 2006 at 12:29 PM | PERMALINK

KJI wanting some more attention perhaps? World press spending too much time on Iraq? Take a hollow in the ground, fill it with 1kt of conventional explosive, pop it off?

I do agree the whole thing is odd; it is worse than useless to have fewer than 5 nukes that you are sure will work (which is why the Pakistanis tested 3 in a row). Firing off one that doesn’t even work leaves you in a worse position strategically.

Posted by: Cranky Observer on October 9, 2006 at 12:30 PM | PERMALINK

It seems to me that if you only have a limited amount of plutonium, like NK has, you might want to use as little as possible for a test.

Could have been a dud, I suppose, but I don’t think we can assume that, and I don’t see that it matters all that much in a practical sense- even a “dud” nuke capability is a pretty good deterrent against the sort of military adventurism that got us into Iraq.
Posted by: pdq on October 9, 2006 at 12:32 PM | PERMALINK

Either way, Kim Jong-Il has got egg on his face today. Back at DefenseTech/armscontrolwonk:

But, from the initial data, I’d say someone with no workable nuclear weapons (Kim Jong Il, I am looking at you) should be crapping his pants right now.

First the missile, then the bomb. Got anything else you wanna try out there, chief?

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