It’s not certain if NK has any actual delivery mechanism for their nuclear weapons, other than putting it in the back of a truck and driving it to the DMZ. (yes they have medium range missiles but it’s not known if they have made their nuclear weapons small and robust enough to be put in a missile and survive the firing and re-entry to still be viable when it reaches it’s target).
Anyway, unlike Japan, South Korea does not have any enrichment facilities. So they’d have to build the enrichment facilities, enrich enough Uranium, then build a bomb. They certainly have the industrial manufacturing capacity to build gas centrifuges quickly. So assuming they were doing this openly, throwing their entire national resources at it and not being disrupted or sabotaged by outside influences… I’d think it would be achievable in less than a year.
To create a weapon based upon ownership of electrical power reactors involves the creation of plutonium[sub]239[/sub]. Despite a lot of misunderstandings, this isn’t the only isotope of plutonium in spent fuel rods. Pu[sub]240[/sub] (amongst others) is also produced, and it has the effect of contaminating the plutonium so that it can’t be used in a gun type weapon. In order to create weapons grade Pu, rather than reactor grade, you need to deliberately place Uranium in the reactor core to be irradiated by neutrons, and converted to Pu, but remove it before the concentration of Pu[sub]240[/sub] gets too great - which is much earlier than a simple spent fuel rod. This limits the rate at which you can create weapons grade Pu, and results in an easily detectable activity.
Once you have your mostly Pu[sub]239[/sub] and limited amounts of Pu[sub]240[/sub] intermixed with the Uranium and other actinides, you need to extract and concentrate it. This is a non-trivial, dangerous, and expensive process. It requires significant infrastructure and again, is not something it is easy to cover up. But you still need the technology to build an implosion device. This isn’t so difficult for a modern technological society, but it is a lot more work than a gun type device.
The alternative is a gun device, and that means U[sub]239[/sub] and the whole world of gas centrifuges and painstaking, expensive, atom by atom accumulation of material. Again, being an advanced industrialised nation, South Korea could build all the technology it needs internally, but we are still talking large scale industrial efforts.
Building a nuclear weapon is ‘up the alley’ of ROK industrial and technological capability more than almost any other country that doesn’t already have them, certainly within a small handful. However I think certain unavoidable lead times would make the process at least a few years, assuming it’s completely ‘in house’.
Note that some advanced indigenous ROK weapons of recent years are based in part on technology purchases from Russia (the ‘Cheon-gung PIP’ ABM capable SAM, active defense systems on ROKA tanks, etc) so while that would be an obviously bigger and more sensitive political issue it might not be ruled out either for aspects of an ROK nuclear program. Although the US would seem the more likely source of assistance if there was to be any outside assistance, and especially if assistance on very sensitive aspects.
Another very ‘quick fix’ though assuming the US supported ROK nuclearization would be to provide so called ‘dual key’ nukes like the many 100’s at one time and still a couple 100 now positioned for use by NATO allies. The weapons are held in US custody in the Allied country but intended to be delivered by the ‘host’ country’s a/c (German and Italian Tornado’s, Belgian, Dutch and theoretically at least Turkish F-16’s) in nuclear war, requiring both countries’ approval.
The US had its own nuclear weapons in the ROK until 1991, that could also be reversed.
Are we sure SK doesn’t have battlefield nuks from the US already, or at least the US troops stationed there don’t have them? The US just needs to park a few subs off the coast of NK and there’s your Cuban Missile Crisis.
Assuming the common definition of entering ‘space’ at 100km altitude an 800km range BM is going to leave the atmosphere on a ‘normal’ (not ‘depressed’) trajectory. NK Scud-B copies and ‘stretched Scud’ types with ranges of 300 and 500km respectively are estimated to have actual apogees of 85 and 120km respectively. A longer range missile like a ‘Nodong’ launched on a lofted trajectory v a target in ROK, as opposed to one at its much greater maximum range, might reach 400km.
Anyway it’s true that the lower speed inherent in a ‘normal’ trajectory BM the shorter the range makes it easier to build one, though also easier to shoot it down. ICBM’s are harder to make work as well as much harder to shoot down because they are going so much faster as they come down. Most discussion of NK BM capability from US POV is about ICBM’s of theirs eventually threatening CONUS, though also to some degree medium/intermediate range ones threatening US bases in the Pacific outside ROK, or threatening Japan. NK’s delivering a nuke v targets especially in northern part of ROK is a lot less daunting challenge. For one thing in between ox cars and ballistic missiles, they could be delivered by a/c and there could be no gtee of shooting all the attackers down. But in the time frame it would take the ROK to develop its own nuke I think it can be assumed the NK’s will have a reasonably reliable nuclear BM to strike the ROK, if they don’t already.
Some info here, various media articles have discussed the Nodong-loft scenario
Why would that be suicide for South Korea?? It wasn’t suicide for the North to do the same thing after all. And it would depend on what you mean by ‘passively watch’…they would scream, rant and threaten. And maybe they would do more testing, perhaps even attempt to sink another SK warship or shell another island. But they do all those things already.
And if the ROK really wanted nukes back as a deterrent I’m fairly sure they would go this route.
If they REALLY wanted to make a home grown nuclear weapon then I agree with this…there would be lead time, but they certainly have everything they need to do it. Might take them a year or two to spin up to where they are ready to test, then a year or two to refine their design and system and produce a reliable weapon. In parallel they would probably develop a delivery system which would be about the same, with maybe an additional year to marry the two systems into a fully operational weapons platform.
Assuming you believe official pronouncements on US/ROK side, yes. The US forces in Korea did have nukes in country from 1958 until 1991 when they were removed by a directive of Bush I, part of post-CW nuclear draw downs. And unless there’s been a highly leak resistant conspiracy to conceal a nuclear weapon sharing arrangement between US and ROK like the ones US has with nominally non-nuclear allies like Germany and Italy (see above), then there isn’t and has not been one with the ROK. Though again, to provide a nuclear capability ‘in’ the ROK, US nukes could quickly be move back there, or a nuclear sharing arrangement quickly arranged, if both countries wanted it.
You could make a uranium implosion device, but why would you want to? The gun design is a lot easier to make work. The only reason implosion is used at all is because plutonium is easier to produce, and it doesn’t work with the gun design.
Even MRBMs go high enough to exit most of the atmosphere - in some cases well over 300km. Hell, even SRBMs like the V-2 could easily clear 100km, and picked up speeds over 5700km/h at parts of their trajectory. Thermal issues are present. Edit:
One catch is that plutonium can pretty much only be used with implosion weapons due to Pu-240 contamination (too many spontaneous neutrons)- uranium is suitable for either gun or implosion types because it doesn’t emit spontaneous neutrons like Pu-240.
The other catch is that plutonium is drastically easier for most reactor-owning states to get enough of, as it’s a relatively simple process to extract it from spent reactor fuel rods. The isotopic composition can be varied by the reactor conditions. Contrast this with HEU, which takes mountains of raw material, and ends up with a very small amount of finished product after multiple steps of enrichment.
So you have easier to get fissile material counterbalanced by harder bomb engineering.
Arab countries never bombed Israel’s nuclear facilities at Dimona to put an end to Israel’s nuke program. And I think the North Koreans would understand - realistically - that South Korean possession of nuclear weapons means that Seoul has a deterrent - that’s all - and that, in fact, trying to put an end to Seoul’s nuclear program by violent means, is far more likely to be suicidal for Kim and his regime, and cause far greater calamity for the North, than simply letting the South Korean nuke program be.
I’d thought, from reading Dr. Sublette’s nuclear weapons archive, that implosion weapons were favored over gun-types, as they utilized fissionable material much more efficiently. Moreover, I thought that most US weapons were constructed from HEU, and were either single or double-end plate implosion designs, due to weapons maintenence and safety reasons?
In any event, I’d look at sources like NTI’s profile of South Korea, and the size of any HEU-requiring research reactor, for determining a bound for how large the size of any immediately-capable arsenal would be. They don’t have a naval reactors program, which is otherwise a giant source of HEU requirements. They agreed to abandon some moratorium on pyro-processing waste in 2015. No idea if that would help them hide a “Green Run” or three, sufficient to obtain enough Pu-239, very low in pre detonation susceptible isotopes like Pu-240, to develop a credible deterrent.
Really, IMHO, they made a serious error in abandoning their persistent chemical weapons deterrent. All of the “10,000 artillery tubes shelling Seoul” scenarios pre-suppose that those guns: work, their crews can operate with the efficiency of NATO gun bunnies, and that those crews can keep up that firing rate through US/SK counterbattery and air strikes. Using VX against those suspected sites is a great way to either kill the crews outright, or force them into MOPP 4 to where their 4 rd a minute fire rate turns into 1 rd a minute.
Ultimately, NK will continue doing what they do so long as it costs less to put up with Kim, than it costs to deal with the damage they may cause, in addition to the WAG, half a trillion USD to several trillion USD it’ll cost to re unify the Korean peninsula. Assuming the PRC decides not to substantially interfere.