And then for a number of very interesting but very complex political reasons in the early 1990s when the North Korean world first came apart that was the first time they actually worked directly with the United States in order to take this nuclear program that they were building up and essentially freeze the bomb component of that program. The reason they reached across the United States was that, as you remember, in the end of 1991 the Soviet Union came apart. When that happened here was a very strong and supportive ally of North Korea. Not only the Soviet Union but the Soviet Bloc and you find today strong connections from Eastern Europe to North Korea. And the Russians essentially deserted them overnight and so the financial help, the technical help, everything that was there beforehand went away.
The second major bloc that dealt with North Korea was China, but this was also a time when China was worried much more about its own economic rise than it was about the ideology around the world and the Chinese actually hooked up with the South Koreans because they felt that that was a better way to go economically. So North Korea at that point felt it really had no friends left and it actually reached across the United States in order to try to strike a deal.
Well, that was difficult to do but eventually a deal was struck in 1994. The essence of that deal was that the North Koreans would freeze their plutonium program – and again I’ll tell you more about that. In return the US would actually make sure the North Koreans get two light water reactors, the ones that are better for electricity than for bombs and that we would normalize relationships with them. Well, it turns out that was a rocky marriage from 1994 to 2002, and in 2002 the marriage came apart because that’s the first time the Bush administration actually sent representatives to Pyongyang and instead of sort of holding out the peace branch, they accused the North Koreans of having cheated on this agreement. North Koreans walked away and in 2002 actually began bomb production. And then that continued for the rest of the Bush administration and indeed, along the way, the North Koreans not only were able to extract plutonium from their reactor products but they actually conducted a first test in 2006 and a second test in May of 2009.
So, the North Korean nuclear story is a story of 50 years in the making. It’s not something that you do overnight. It’s 50 years in the making and it vacillated back and forth as to whether it was more civilian or a more defense, and I’ve written a paper on this subject published in the Journal Dædalus in the winter of 2010 which actually looks at the simple connectivity between technical capability and between political intent.
So, let me just give you a little premiere on the bomb because in order for you to appreciate what plutonium means, what highly enriched uranium means, you need at least to know these essentials and if I insult, some of your technical – sorry – but I thought I should give you that as background. There are essentially two paths to the bomb and if you look at the periodic table, the two practical fissile materials are uranium and actually only one isotope of uranium, that is 235 that occurs only seven tenths of a percent of natural uranium, and the other one being plutonium which is principally man-made from reactors. And so the two paths are — let’s look on the right hand side first. And that is, you take the natural fissile material, uranium-235, and you essentially throw away all of the rest of what you find in mother nature, that is the 238 isotope of uranium. You do that by a process called enrichment, essentially just concentrating one isotope.
As you might imagine, that’s somewhat difficult to do because they’re both uranium, they both have the same chemical properties, in essence. So you have to take advantage of one being just a little heavier than the other. And that’s what you do in a centrifuge. So you turn the uranium into a gas, you spin it very, very fast. The light stuff stays on the inside. The heavy stuff goes to the outside. And you just keep doing this over and over and over and that’s where the term centrifuges cascades come from. And so, that was what was done. Actually doing the Manhattan project days in the United States, we did it by a different technique called gaseous diffusion, but today the technique of choice is the centrifuge. And what you see on the right hand side are just rows of these centrifuges.
The second path to the bomb is you actually start with the natural uranium or slightly enriched uranium and you put it in the reactor and you start the neutron reaction, and you fission the uranium-235 atoms that makes a bunch of fission products, and it makes a bunch of neutrons. 238 picks up a neutron and through a few decays becomes plutonium. And it turns out that plutonium is even a better bomb material than uranium-235, except now you not only have to make it in the reactor, which is the top diagram on your left hand side but then you have to extract it from the reactor products, from the fission products. And that you do chemically and that’s what we call reprocessing. You do that in a reprocessing facility.
Now, it turns out for the uranium-235, you can make a rather simple bomb. And that is you just take two sub-critical hemispheres and you put them in a gun and you shoot them together very rapidly. Okay, when you do that, in essence, if you do it right, that was the Hiroshima device, about 13 or so kilotons, 13,000 ton TNT equivalent. It destroyed a city. One plane, one bomb, destroyed a city. That’s because of this nucleus stuff. When you split the nucleus, you get a factor of millions in the energy gain compared to all the other chemical techniques. And so that’s why nuclear is so special. And not only do you get that in nuclear energy but also you also get it in the bomb if you do it right, or if you do it wrong, depending on your point of view. So this gun assembly works for highly enriched uranium for a bomb that’s typically 80% to 90% enriched. It doesn’t work for plutonium for very good nuclear physics reasons I won’t get into. So for plutonium instead, you have to use what we call the implosion device. And that is, you have a sub-critical mass of plutonium, pack explosives around it like a [soccer pole], you put detonators around and you try to implode it all very symmetrically to drive it to super-criticality and then it blows up, that’s Nagasaki. It takes about six kilograms for the implosion device of plutonium, takes a few tens of kilograms for — in highly enriched plutonium. You can also use the implosion device with highly – use highly enriched uranium in the implosion device but a much simpler way to go, and this would be the nuclear terrorism lecture, would be to use this gun assembly.