It beggars belief to accept uncritically Pyongyang’s claim that the recent North Korean nuclear test was a 2-stage H-bomb.
There are so many reasons to be skeptical. But look at it this way. This was their fourth test. We know that the design they are using is spherical plutonium implosion. They got it from Pakistan, who got it from China, who got it from the Soviets, who stole it from the U.S. via Klaus Fuchs and the Rosenbergs. Basically, copies of Fat Man all.
Fat Man (and Trinity, which was Fat Man without an aerodynamic casing) each yielded about the same (Trinity 20 kT, Fat Man 21). That’s about right for that design which–while by today’s standards is very “first generation”–is still much more sophisticated than uranium gun-assembly (Little Boy) and quite hard to do.
In four tests, North Korea has never cracked 10 kT and are probably well below that. I read all the government and agency estimates of this latest test and the lowest I saw was 4.8 (South Korean government) and the highest was 6.7 (CTBTO). That is not great for that sort of bomb. You could almost call it a “fizzle.” If so, it would be their fourth, making them oh-for-four.
All the five declared nuclear powers cracked 20 on their first try, including the first ever test in human history.
If that was an H-bomb (no way) it was a pathetic showing. But it wasn’t an H-bomb. I don’t believe they know how to do it and I don’t believe they can produce enough lithium 6 or tritium to make it viable. So far as we know, neither the Indians nor the Pakistanis have gone thermo, despite far larger economies and deeper scientific and engineering benches. Even the Israelis haven’t for certain (that’s clouded in mystery, though). But North Korea has? The North Koreans had to buy the Fat Man design from Pakistan, but they came up with Teller-Ullam on their own? Their fission primary is still Fat Man–1945 technology, which, again, they had to buy–but they mastered two-stage radiation implosion on their own? Even the Russians probably didn’t manage that (though Sakharov always said he did, but I doubt it; see Reed and Stillman, The Nuclear Express). Not credible.
The other theory flying around is that it was a “boosted” weapon. Boosted means that you add a lot of tritium (but much less than you need for an H-bomb) so that the extra neutrons greatly increase the fission efficiency, upping the yield. Well, if that’s what it was, then the North Koreans are really sub-par. If you can only get, at most, 6.7 kT from a boosted 239pu core, you just aren’t very good at this.
Here’s what I think, though admittedly this is pure speculation: North Korea has managed to make some tritium at Yongbyon. They are very proud of this. But they don’t quite know what to do with it. The half life is approximately 12 years; use it or lose it. They can’t use it in a boosted weapon because they don’t know how. They can’t use it in thermo because they don’t know how and they don’t have enough. But what they CAN do is put a little into yet another Fat Man and test it close enough to the surface to ensure that it gets into the atmosphere so that US and Japanese surveillance planes will detect it. Aha! That will terrify the imperialist capitalist pigs and their running dogs! Except, it won’t. Because we’re not that stupid.
As to the timing, late last year they made a public claim of having an H-bomb and the U.S. and others basically laughed at them. This “test” was “we’ll show them!”
If they really had an H-bomb and they really wanted to show us, why not test it above ground? Whatever the yield, there could be no ambiguity about that. What’s stopping them? They withdrew from the Non Proliferation Treaty. They never even signed the Limited Test Ban Treaty or the Comprehensive Nuclear Test Ban Treaty. Are they worried about international public opinion? Greenpeace? If you’ve really got one and you want us to know it, light it above ground! Maybe that would be too much for China, I suppose. But a more likely explanation is that they don’t have it and they don’t want that to be too obvious.
