Some thoughts on Nuclear Proliferation
I present myself as an expert here, but may be confused. You should probably stick to Wikipedia.
I’m not sure the centifuges in North Korea are all that important.
North Korea doesn’t use centrifuges to make bombs. Like almost everyone (all except Pakistan and the thin boy made by the US& UK & dropped on Nagasaki) they use a nuclear reactor to make Plutonium then extract it using ordinary chemical processes.
Similarly, the Iranian Arak reactor (once under construction & whose reactor vessel is now full of concrete –thanks Obama) was a much worse threat than their centrifuges.
The idea that enriching Uranium is about as easy as extracting Plutonium is deadly. It is the reason Bush ended the Agreed Framework which stopped N Korean Plutonium extraction (or at least the excuse). They had intelligence showign N Korea bought centfigues from Pakistan (which I am sure is much more reliable than the intelligence showing Iraq had WMD and an active Nuclear program and was assisting al Qaeda). This was, in his own word, the hammer John Bolton needed to smash the agreement.
Since then no-one has heard of those centrifuges (which snark aside I guess are in N Korea). After Bush abbrogated the Agreed Framework, N Korea extracted Uranium from spent fuel and made bombs.
The crucial threat is graphite (or I think heavy water) mediated nuclear reactors. The speed of Neutrons is crucial. If slow they are absorbed only by Uranium 235 (causing fission) if faster by Uranium 238 which becomes Plutonium 239. That is the fissile material in all US nuclear weapons (it is much easier to make than highly enriched Uranium 235 & one needs much less of it). If the neutrons go even faster, some are absorbed by Plutonium 239 making Plutonium 240. This is an odd one unlike any nuclide found in nature. It fissions just on its own (doesn’t need to be hit by another neutron). So the spontaneus fission of Plutonium 240 means that one can’t make a bomb of mixed Plutonium 239 & 240 as the neutrons released by the 240 cause pre-detonation of both the 240 and the 239.
Plutonium 240 is the reason that the Osiraq reactor (bombed by the Israelis) couldn’t be used to make bombs. That was clearly Saddam Hussein’s plan when he bought it from the French, but they tricked him making a reactor which made Plutonium 240. Since he couldn’t admit he was trying to make a bomb, they could pretend he wanted electricity and build a reactor which generated electricity but whose spent fuel could not be used to make bombs. All of this was explained to Begin (also I assume by the Israeli’s who make atomic bombs) who had the reacto bombed anyway (Abu Nidal Abu Smidal a reactor is a reactor).
The conflation of Uranium 235 (not used for bombs except as first used in reactors to make Plutonium 239) and Plutonium 239 (the stuff in all our bombs) has caused great trouble. It is related (as an excuse not the true cause) to Bush’s decision which lead to North Korea having nuclear weapons. It confuses people about the Iran deal (they look at the minor quickly reversible part not the key less reversible part). It means people believe aluminium* tubes are a reason to go to war (not close even if they weren’t rocket casings).
I will now google to check how many centrifuges are believed to be in North Korea (I see quite a few — I didn’t know that when I began typing)
* I will not type “Aluminum” without quotation marks until our tariffs are eliminated.
The U235 was used to make the gun-model nuclear weapon dropped on Hiroshima, nicknamed “Little Boy.” The scientists determined the critical mass of U235 needed to detonate and split the amount between a target piece and an aimed piece. They determined how much area needed to be swept by the two pieces in order for the neutron flux to be high enough to detonate and decided a modified 75mm cannon would suffice to shoot the “bullet” fast enough into the target before U235mass accidentally went sub-critical and released only a fraction of its potential energy.
The implosion bomb, nicknamed “Fat Man,” dropped on Nagasaki used Pu239, as had the test at Alamogordo, NM. It was round because it used high explosive lenses to compress a subcritical mass of Pu239 until its density reached criticality. That’s why Pu239 remains the preferred material of choice as well since the implosion approach is used for almost all nuclear weapons these days. You can minimize the amount of Pu239 you use by increasing the density you can crush the fissile material to. The DPRK would likely be working on only their fifth or sixth nuclear weapon now if they had gone the uranium enrichment route while their reopening their reactor, enabled by Bush’s rash actions, has given them a flood of fissile material.