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Comment by znowi | Hacker News Reader

There's a catch. While it's trivial to design a gun-type bomb (Little Boy), it's very difficult to produce weapon grade uranium for it. As for implosion-type bomb (Fat Man), it's relatively easy to get a hold of plutonium, but the design is extremely complex.

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> As for implosion-type bomb (Fat Man), it's relatively easy to get a hold of plutonium, but the design is extremely complex.

Relatively easy as in, you need a chemistry lab and a working breeder reactor. To elaborate a bit, for Uranium the main task is isotope separation, this is quite complicated since different Uranium isotopes are chemically identical and have a mass difference of just 1%. On the other hand, more or less all plutonium isotopes are fissile, and the task is to chemically separate them from other elements. ( And that one needs to work with spend nuclear fuel.) On the other hand, Plutonium does not occur naturally, so one needs a breeder reactor to produce it first.

But Pu240 undergoes a lot of spontaneous fission, and if you have too much of it your bomb will fizzle. Pu238 generates a lot of decay heat, which makes it great for NASA missions but not so great for bombs. For a while it was used in pacemakers.

Bombs are generally made from fairly pure Pu239, which can be made by bombarding U238 with neutrons. But if you leave the plutonium in the reactor, you get a mix of isotopes that's pretty much useless for bombs. Separating the isotopes is harder then enriching uranium.

For these reasons, nuclear waste from reactors, even with reprocessing, isn't actually much of a proliferation concern.

These problems are discussed in considerable detail in [1], specifically:

    While reactor grade plutonium would probably be of no 
    interest to a nation with access to better grade 
    material, it could be effectively used by a nation 
    capable of good weapon design, but without access to 
    better fissile material. Even a low technology nation 
    could fashion powerful weapons from it, after all even 
    a 1 kt device greatly exceeds the destruction of any 
    conventional weapon.

So yes, recognized nuclear states are using very pure Pu239, but this is done for engineering convenience and not an absolute necessity. And the calculation probably changes for a state with a clandestine nuclear program, since it is much easier to hide the additional computing capacity to make a working reactor grade plutonium design, than it is to hide a breeder reactor.

[1] http://nuclearweaponarchive.org/Nwfaq/Nfaq6.html (In section 6.2.2.10 Reactor Grade Plutonium)

From that section of your link: "Using this material in a bomb would be a challenge. Continual active cooling would be needed to prevent deterioration and damage to the core, explosives, and other components. The high rate of neutron emission means that predetonation is inevitable, even with a very efficient implosion system. However, even the relatively primitive Fat Man design would have produced a 0.5 kt or so yield with this material. With optimal implosion design yields in the range of at least several kilotons are possible. If fusion boosting is used, then the adverse effects properties of reactor grade plutonium can be completely overcome"

Yield restricted to a couple kilotons is generally considered a fizzle. It's big compared to chemical explosives but only about a tenth as big as Hiroshima. And the experiment in OP's article only talked about making a working bomb, not an optimal one.

The point on fusion is interesting, though. It might be worth repeating the experiment for thermonuclear bomb designs.

Except the Nth country experiment participants designed an implosion-type bomb. The people running the experiment understood that designing a gun-type bomb would have been too easy. Nonetheless, you're right that obtaining the fissile material is much harder than the bomb's design.

Good point - Little Boy's design was not even tested prior to its detonation.

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