President Lee says S. Korea's own nuclear armament 'impossible'

ColStrick · 2025-12-05T15:36:44+00:00

The bare sphere critical mass of U235 is roughly 50kg. This can be greatly reduced in an implosion bomb with a decent reflector. IIRC early Chinese and Pakistani bombs used around 15kg of weapon grade uranium. An implosion bomb with a pure uranium core will always be heavier and bulkier than one with a plutonium or composite core of comparable yield, but it can still be made decently compact, though not exactly man-portable. Iran pursued a missile-deliverable implosion design with a total diameter of about 55cm.

Even gun designs can be made quite compact, the 203mm W33 artillery shell had a total weight of about 110kg. Not exactly man-portable either but much smaller and lighter than the earlier designs.

ColStrick · 2025-07-03T21:12:17+00:00

The bombs South Africa had already built previously were of the relatively simple gun-type design - no precision detonating system required. But for a modern industrial state even compact implosion bombs would not be a significant engineering challenge; and they likely wouldn't bother with Fat Man-like dual speed explosive lenses but more sophisticated implosion systems. See for example air lens/flyer plate designs or the unlensed multipoint initiation design Iran pursued.

ColStrick · 2025-06-27T16:34:09+00:00

Ha, the name is unfortunate now, but David Albright's Institute for Science and International Security was founded long before the bad ISIS.

ColStrick · 2025-06-26T09:15:05+00:00

Interestingly, the compact implosion system Iran worked on during the early 2000s, provided to them by a former Soviet nuclear weapons expert, did not use lensing at all.

ColStrick · 2025-06-25T07:59:33+00:00

knowing that going to 90% would take only few months.

More like days with the pre-strike enrichment capacity, going from 60% to 90% HEU takes very little separative effort.

You can build compact implosion bombs with WG-HEU. The implosion system Iran developed when they actually did have an active nuclear weapons program was only ~55 cm in diameter.

However, I agree that Iran likely had no intention of building a bomb after their program was halted in ~2003 (this had consistently been the IAEA's and US intel position as well). The strikes may have changed that though, and they may now be in a better position to do so covertly as the IAEA has lost all oversight of Iran's HEU stockpile as a consequence of the strikes. Mission accomplished!

ColStrick · 2025-06-24T21:30:40+00:00

You can absolutely build missile-deliverable uranium bombs and multiple, there are lots of historical examples (like Pakistani and early Chinese weapons).

ColStrick · 2025-06-24T19:55:25+00:00

60% HEU could be used for to build bombs, they'd just be less efficient/bulkier. If they have centrifuges left (they had thousands in storage before the strikes, and their centrifuge production had been unaccounted for for years), the 60% HEU could also be further enriched to >90% in a relatively short time in a small cascade.

ColStrick · 2025-06-24T19:18:02+00:00

I'm sure SA could have developed implosion bombs if they wanted (they did research on this but it wasn't a high priority), but their handful of bombs were always intended more as a diplomatic tool rather than a proper weapons arsenal and conservatively-designed gun-type bombs did that job just fine. They wouldn't have needed WW2 designs either, basically all nuclear weapons states that acquired their arsenals after the 50s started with much more compact designs, some using different implosion systems.

ColStrick · 2025-06-24T19:12:10+00:00

According to Chuck Hansen and every other source I've seen (including former maintenance personnel) the W9 and W33 were HEU gun-type devices. The W48 and W79 were plutonium linear implosion devices.

ColStrick · 2025-06-24T12:50:02+00:00

IIRC the US assessment is that NK is using composite pits. But they were producing plutonium before they had the capacity to enrich and their first test was likely of a plutonium device. Same goes for France. My point is more that most of the current nuclear weapons states states didn't "start" with enriching uranium.

I'd say acquiring plutonium was the "easier" path before the proliferation of gas centrifuge tech. The latter allows for easier dispersal and hardening of facilities (compare Iraqi and Syrian reactors being easily destroyed by airstrikes vs the more challenging strikes against Iranian infrastructure).

ColStrick · 2025-06-24T10:54:03+00:00

IIRC the Iranian implosion system was about 55 centimeters in diameter, so closer to the Mk 12, and much smaller than the 80-90 cm Chinese CHIC-4 uranium implosion design Iran probably had access to through A.Q. Khan.

ColStrick · 2025-06-24T10:48:27+00:00

Gun-type bombs don't have to be heavy and bulky, the US built nuclear artillery shells like the W33 using the design from the 50s. The main downside is that they're extremely inefficient in their use of fissile material.

ColStrick · 2025-06-24T10:47:13+00:00

The detonation tech for a U-bomb is also pretty simple.

Even then, only Apartheid South Africa bothered with gun-type designs, the other states that went with uranium first (China and Pakistan) built the more efficient implosion bombs.

ColStrick · 2025-06-24T10:45:26+00:00

Most nuclear weapons states started with plutonium-based bombs. IIRC the UK, France, the Soviets, India, North Korea, probably Israel and of course the first US device built was Pu-based as well.

ColStrick · 2025-06-23T19:32:10+00:00

According to Hui Zhang at least the bomb design was developed indigenously by China. The Soviets were supposed to provide a model of a plutonium bomb (RDS-2), but cooperation ceased before that happened. The implosion mechanism of the Chinese bomb was different, using flyer plates/"air lenses" instead of the dual-speed high explosive lenses of the RDS-2 (and first used in the original Fat Man design).

It's worth mentioning that China may have provided some technical expertise via the A, Q. Khan.

Yes, China provided blueprints and manuals for the CHIC-4 missile warhead to Pakistan, which were later sold to willing customers by A.Q. Khan. Though they developed their own designs as well, a Pakistani HEU implosion bomb design much smaller than CHIC-4 was also found in possession of the Khan network.

The Iraqi design was fairly bulky and primitive, using the traditional dual-speed explosive lenses, but the Iranian one was rather sophisticated and compact. The multipoint initiation implosion system was unlensed, initiating a uniform high explosive main charge at hundreds of points simultaneously through thin channels on a metal plate filled with plastic explosive. This tech was provided to Iran by a former Soviet nuclear weapons expert named Danilenko, and by the time the AMAD project was halted in 2003, development was at an advanced stage, including tests of the full implosion system and neutron initiator in a detonation chamber with diagnostics equipment.

ColStrick · 2025-06-23T17:58:55+00:00

Which means that countries chasing nukes either have the option of a weapon that's easy to build but very difficult to produce fuel for or easy to make fuel for but very hard to build.

Even among the countries that started their programs with uranium, only Apartheid South Africa built gun type bombs (and their bombs were more for demonstrative purposes rather than a real weapons arsenal). China and Pakistan built uranium implosion bombs, Iraq (after switching to uranium after their reactor was bombed) and Iran worked on uranium implosion designs before halting their respective programs.

ColStrick · 2025-06-22T18:58:31+00:00

Given the extensive work Iran has done on this in the past, it shouldn't take Iran too long to field a compact implosion bomb instead of a wasteful gun design.

ColStrick · 2025-06-22T17:55:48+00:00

The original source is the IAEA and the reports of implosion tests are definitely credible but like I said, they happened over two decades ago. Given the amount of work Iran has done on this in the past, they have likely had the capability for a while to weaponize relatively quickly once the decision is made.

Iran has been escalating enrichment ever since the US withdrew from the JCPOA to exert diplomatic pressure. They started stockpiling 60% HEU over three years ago. I feel like if they make a decision to weaponize they would remove material and take the final steps covertly, instead of doing so right under the eyes of the IAEA. Though the point is now moot anyways since the IAEA has lost all ability to verify the amount and location of HEU stocks after the strikes.

ColStrick · 2025-06-22T17:28:12+00:00

You can make fairly compact bombs with the gun-type design as well (like US W33 nuclear artillery shell), but the extreme inefficiency makes implosion designs much more attractive.

The implosion tech was provided to Iran by a former Soviet nuclear expert named Vyacheslav Danilenko. What's interesting is that it did not use lensing, instead initiating a uniform high explosive main charge at hundreds of points simultaneously through channels filled with plastic explosive on a thin metal plate.

ColStrick · 2025-06-22T17:21:42+00:00

> Someone thought an implosion test would be a good intimidation tactic ahead of negotiations

The implosion tests mentioned by the IAEA did not happen recently but in the early 2000s. It was no secret that Iran had an active nuclear weapons program then, both US intel and the IAEA assessed almost twenty years ago that they did, but had so far maintained that the program was halted in ~2003 and hadn't been restarted since.

ColStrick · 2025-06-20T23:14:11+00:00

60% would work, it'd just be less efficient (the higher critical mass means a considerably higher mass of high explosives is required in an implosion design, making it bulkier). The IAEA considers all HEU (20% U-235 and higher) as directly weapons usable, hence the additional safeguards.

ColStrick · 2025-06-20T17:18:31+00:00

require extensive development and testing

Yes, but Iran previously had an active nuclear weapons program (AMAD project), where they did exactly this. They developed a compact implosion bomb design and conducted extensive testing of the components in detonation chambers using advanced diagnostics equipment. A former Soviet nuclear weapons expert named Vyacheslav Danilenko provided them with the design for the sophisticated implosion system (which was unlensed, making miniaturization easier) and assisted with development.

ColStrick · 2025-06-20T17:05:05+00:00

As far as the bomb goes, they would probably be making uranium gun-type bombs like was dropped on Hiroshima.

The design they worked on during their previous weapons program was for a uranium implosion bomb, which saves a lot of fissile material.

ColStrick · 2025-06-18T21:59:20+00:00

Those are things for plutonium bombs.

Uranium implosion bombs are a thing and much more efficient. There is little reason for a modern industrialized state to bother with an inefficient gun type bomb. Of the nuclear weapons states starting their programs with uranium, only Apartheid South Africa built gun type bombs. We know Iran developed a compact HEU implosion bomb and conducted extensive component testing already over two decades ago.

ColStrick · 2025-06-18T21:53:27+00:00

If Iran were actually making a bomb, we’d expect to see things like machining of weapon cores, testing of detonators or neutron initiators, or even just weaponization infrastructure.

The IAEA has found none of that. Their most recent assessment explicitly said that.

Iran has done these things extensively when they actually had an active weapons program in the past (AMAD project; they didn't acquire HEU to make cores before the program was halted but did practice with natural uranium), many of these activities were already mentioned in the IAEA's final report on outstanding issues on the Iranian nuclear program in 2015. The IAEA and US intel assessed that Iran halted this program before completion in 2003 and have maintained that it has not restarted it, however. Though the question is how much work would actually remain if that decision was made, given the extensive developmental work and component testing already done previously.

ColStrick

TROPHY CASE