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How many nukes in North Korea's arsenal? (Part II)
Lee Wha Rang
[Part I was published on February 17, 2003.]
There are two basic designs of nuclear bomb: gun-type assembly and implosion. As shown in the figure below, in the gun-type design, two blocks of uranium enriched to about 80% U-235 are shot into each other by conventional high-explosives.
The bomb dropped on Hiroshima, the Little Boy, was of this design. It had 64.1 kg of uranium enriched to about 80% of U-235. It had the explosive power of 15,000 tons of TNT and killed about 200,000 people. “Tampers” are made of U-238 blocks that hold in high pressure and temperature and reflect neutrons back to the fissile blocks. Other than the United States, South Africa is the only nation that has built gun-type nukes.
The implosion type uses a spherical geometry – see the figure below. High explosives or other firing means are placed evenly on the surface and fissile materials – uranium and/or plutonium – are placed at the center. The trigger implodes the sphere and the surface collapses squeezing the fissile matter into a state of high density, high pressure and temperature. If things are set right, nuclear explosions occur.
The bomb dropped on Nagasaki (the Fat Man) was an implosion type. It had 6.2 kg of plutonium and the destructive power of 22,000 ton of TNT. About 70,000 residents of Nagasaki were killed.
The US CIA says North Korea may have made 1 or 2 “crude” bombs. The basis of this estimate is as follows. A crude plutonium bomb requires 35.2 lbs (16 kg). The US CIA estimates that North Korea has at least 70 lbs (31.5 kg) of plutonium and so it could theoretically have made 1-3 (31.5 kg / 16 kg) plutonium bombs.
The ‘critical mass’ of fissile matter drops sharply with the fissile density as the inverse square of the density - that is, even the tiniest amount of fissile matter can be made critical if squashed hard enough. The Nagasaki bomb had 6.2 kg of plutonium and North Korea could have built as many as five Nagasaki bombs. Modern plutonium nukes of China, Russia and the United States contain as little as one kg of plutonium. Using this figure, North Korea may have or could produce as many as 32 nukes from the Reactor I plutonium alone. If the report of North Korean acquisition of plutonium is true, then this figure goes up by a factor of 2 or 3 - in another word, North Korea may have or could have one hundred or so nukes..
The picture gets much more complicated when you take into account the possibility that North Korea may know how to make thermonuclear bombs – the H-bomb. The basic physics of the H-bomb is that high-energy neutrons can break apart the abundant U-238, and so the basic design principle is to produce high-energy neutrons using nuclear fusion. Since the probability of explosion increases with particle density, extremely high pressure is created inside the bomb. In brief, a small fission bomb is used to triggers nuclear fusion, which creates high-energy particles, which in turn, creates high pressure and temperature that lead to nuclear fission of U-238.
A simplified diagram of the H-bomb is shown. A small amount of LiD (Lithium-6 deutride) placed in the inner core of an implosion bomb can significantly boost the bomb yield. LiD powder turns into Li, D, and tritium gases that undergo fusion releasing fast neutrons, which in turn enhance nuclear fission of plutonium, U235 or U-238. Any nation that can make implosion bombs can make fusion ‘booster’ bombs.
...One can see that in the H-bomb design, an implosion bomb is used as a trigger, which ignites fusion on a larger scale than in a booster bomb. Fusile substances surround the implosion bomb trigger and another much larger implosion bomb, and the whole thing is placed inside an explosion bomb made of a uranium-238 casing. U-238 nuclei fission if bombarded by high-energy neutrons, photons and alpha particles.
The secondary stage is made of a hollow lithium-6 deutride cylinder or ellipsoid case in by a layer of U-238. At the core of the cylinder is a Pu-239 or U-235 rod about one inch in diameter. The casing is wrapped in a layer of plastic foam and a plug of U-238 separates the secondary from the trigger.
The Teller-Ulam bomb is often called a "2-stage bomb" because the fission trigger ignites the fusion stage. Since the shock wave dies out in a few microseconds, a 2-stage bomb has a limit on the bomb yields and additional stages are required for super bombs.
The rule of thumb is each stage can be 10-100 times the previous stage in explosive power. It is believed the Soviets had a design for the Dooms Bomb – a large freighter stuffed full of fissile materials. Such a super-bomb would have destroyed much of the world, as we know it.
It should be clear by now the significance of North Korea’s enriched uranium program. It is most likely that North Korea has the technical expertise to manufacture the H-bomb. Using enriched uranium in the nuclear trigger, North Korea would not need any plutonium. Furthermore, the spent rods – in the thousands – in the storage pool may be used in the H-bomb with any reprocessing for plutonium extraction.
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