Thanks for this, Zog. I appreciate these thoughts. And I also appreciate your admission of financial interest. A lot of thoughtful people also echo many of these sentiments.
I don't expect to convince anyone here that Silex should be stopped. But I also know that's somewhat beside the point because that's not possible. The separation concept is well known (more well known than that for centrifuges, probably). As I said, I think it is more likely than not that something related to condensation repression (possibly in combination with some aerodynamic process if that helps improve the isotope separation) will dominate the uranium enrichment market in the future.
So, I would agree that you cannot uninvent something that's already been invented. But that's also exactly the point. How do you then deal with it? I understand that very few people on this forum are interested in this kind of analysis, but I've been in the policy world for enough years to understand that the world is shifting in a direction that could be quite dangerous with regard to nuclear weapons, and there are reasons to be skeptical that the way this problem has been managed will continue to work. Nothing is certain here, of course. It is, however, fairly easy to imagine a lot of accumulating dangers.
So far, we have three ideas for how to manage this, and you touched on all three. Nuclear deterrence, improving detection of clandestine nuclear activities, or arguing that proliferation risk with Silex or any kind of emerging technology is small because other routes to weapons are more likely and that the benefits outweigh the risks.
I think there are good reasons to be skeptical of all three.
I came to this topic initially somewhat like you. My grandfather was training as a paratrooper and living in what is still today Ohio State's football stadium when the atomic bomb was dropped on Hiroshima. Had this not happened, I likely never would have been born because my grandfather was scheduled to head to the Pacific theater and my father had not yet been born. There is also plenty of evidence to suggest that WWII had no foreseeable end without the bomb being used. Japan was not interested in any guarantee that the United States could give before the bomb was dropped. See Evan Thomas's new book, "Road to Surrender."
This again, however, is somewhat the point. Nuclear weapons loom in the background, and due to a a series of stumbles in a war (which is always unpredictable), we could find ourselves in a situation where using them again becomes the only option.
It's difficult to argue that Russia would have still invaded Ukraine had they not possessed nuclear weapons. Without them, they would have likely ended up in a more direct conflict with NATO that would have included NATO directly attacking Russia and destroying much of the country's industrial and military capacity, not to mention many people's lives. This could still happen eventually, but the presence of nuclear weapons creates more caution about how to escalate. Would this war have still happened in the first place if there was no possibility of undertaking it without the cover that nuclear weapons provide? It's doubtful. We're already well into an area that most people here are likely unaware of, but Russia has been very clear about what it cares about for twenty years. It is concerned about the ability of the United States with its long-range precision conventional weapons to begin destroying strategic nuclear targets. This prospect combined with US missile defense to intercept whatever remaining nuclear weapons Russia may fire at the US after a conventional attack has been the stated rationale for Russia not wanting to reduce its nuclear arsenal. It would then just be fewer targets for the US to hit.
This is actually the big project that Snyder is working on now. It's the question of how a conventional weapon would be used in a war to destroy a nuclear missile housed inside a silo, which is what Russia worries about. The physics is apparently insanely complicated and no one in Washington really understands whether this is possible or what all the implications are. But it has the potential to turn all debates about nuclear strategy upside down. If a country's nuclear deterrent is not survivable due to the capabilities of conventional weapons, what does this mean for the future of arms control and global stability?
Now, this would be an incredibly stupid thing for the United States to do, but no one should doubt that the range of people Americans may elect to the presidency may differ in how attractive such a move might be. Also, this could happen without presidential authorization, so this comes down to people in the U.S. military.
Russia has been complaining about this for a long time without anyone listening. The is partly why we are in the situation we are in with Ukraine. That has become a conflict over a small amount of territory, but the military context and how it has evolved is something Americans working in foreign policy have little appreciation of. There is too much history to unpack here, but Russia is concerned about U.S. capabilities, and a combination of ignorance, U.S. allies, and U.S. domestic politics prevents the U.S. from being able to address Russia's concerns. If it is true that Russian strategic missile silos could be destroyed by the United States' conventional weapons before Russia even knows that it's happening, their concerns are worth taking seriously.
Russia feels like they have no choice but to rattle their nuclear weapons, and they will continue to do so. Of course, not attacking Ukraine in the first place would be better, but as I've said these concerns have gone unaddressed for a couple decades. This broader conflict will continue even after the fighting in Ukraine stops. Namely, Russia will continue using its nuclear weapons in an attempt to get the United States to pay attention to what it cares about. From their perspective, nothing else has worked. Their thought is if the U.S. genuinely fears for its own security, then maybe the US will listen.
How does this end? Or how does any conflict between the U.S. and Russia end? Some people could argue that the threat of using nuclear weapons could make a non-nuclear war less likely or that their presence would hold down the scale of non-nuclear conflict. But I would assess the trade-off of an increased chance of non-nuclear war against the possibility of thermonuclear war as advantageous. Obviously not all would, but I would. How can you possibly identify the mutual red lines here? You simply can't. A nuclear war is likely to happen by simply stumbling into it.
When it comes to proliferation, nuclear weapons could be attractive for the reason just given: as a cover under which a conventional conflict could be fought. Or alternatively, doubts about U.S. security guarantees could spur nuclear proliferation to U.S. allies or to other countries concerned about their own deteriorating security environment. This is why the U.S. feels stuck. It's trying to hold onto the status quo by defending and assuring allies who might otherwise choose to acquire nuclear weapons, while at the same time wanting to stop their spread to others (e.g., Iran). This status quo probably cannot hold. And the longer the US and Russia are not able to resolve their differences, the more attractive nuclear weapons may become for all sorts of reasons: great power disagreements tend to lead to a deteriorating security environment, and there are already plenty of non-nuclear countries upset about the inequities in international nuclear politics.
A technology like Silex points very clearly in the direction of viewing the nuclear proliferation problem as a problem of whether states want them or not. This is also true with centrifuges today, but Silex may be significantly harder to detect and easier to master. While centrifuges may be the route of choice today, this does appear to be changing, and there is no guarantee that centrifuges will remain as attractive a choice. I think there are several reasons to suspect that if countries want nuclear weapons, they will opt for Silex. Of course, this all depends on what one particular country imagines to be true. There is no physical principle that can be applied to assess this, but a fuller understanding of Silex can help make the case that the way this problem has been managed needs to change. This is not to say that centrifuges should not already make that case possible, but it seems there is still a significant engineering challenge compared to where Silex may be already and where it is going. Centrifuge engineers will be in drastically short supply compared to laser expertise. This is why Argentina opted to enrich uranium with the condensation repression approach with lasers: the global expertise was far easier for them to tap into with lasers than it was with centrifuges. And Iran already captured a physicist several years ago to work on what was likely a Silex-type program:
https://www.nature.com/articles/nature.2013.12884. And it's important to remember that all of this laser technology continues to change, with more people and more applications, unlike centrifuges which is a niche area for a smaller more specialized group of people. It's difficult to assess all of this, but this is just what is going on.
Also, thorium doesn't make a lot of sense as a technological route to nuclear weapons (although it's not one you can rule out) because you need to start with fissile material to irradiate the thorium. If you need fissile material anyway, why not just build one with that fissile isotope instead? Why worry about using a nuclear reactor with thorium fuel (with a more complex reactor operation that a light water reactor or a breeder) to make a weapon? Just go build it. A debate about thorium seems to hinge on whether uranium or plutonium is more attractive, but almost everyone agrees that the uranium route is more likely to avoid detection. So, which uranium route still comes down to centrifuges or lasers. And uranium is plentiful just like thorium. Getting access to natural uranium can happen from all kinds of sources that are not uranium mines, and there are over 10 different techniques that can be used to extract uranium from seawater. So, access to uranium is not a constraining barrier to acquiring nuclear weapons.
As you say, a bare critical mass is about 50 kg, but with very rudimentary weapons designs today you can do it with far less. The IAEA assesses a significant quantity to be 25 kg of uranium-235 enriched to 90%. So, that's about 27.8 (25 divided by 0.9). All calculations about Iran's nuclear capability are based on this number, but almost everyone knows that 25 kg is too high. You could build a nuclear weapon with far less than that if you just have a bit more sophisticated design. I would say 8 kg of 90% HEU is adequate; you at least can't rule that possibility out.
As far as detection of fissile isotopes in the air, you would need to be very close to the source of the fissile material, which means you would essentially need to know where the clandestine plant was anyway. Detection of krypton gas for reprocessing of spent fuel to extract plutonium may work better, but you would need to have a low krypton background. Detection of krypton is how the US learned that North Korea was reprocessing its spent fuel back in the early 2000s. But with uranium, air sample monitoring appears almost impossible from what I hear. It just can't be detected from very far away.
The problem with arguing that Silex has taken awhile to develop and is therefore less of a concern is that development always takes a while before the recipe becomes well known and the technology and expertise spreads. Much of the development time has been due to laser technology and how to get the separated product out of the gas. But those issues are getting resolved. The idea that it would also take a possible proliferator a long time may not be true. At least with centrifuges, there is engineering expertise required to assemble the machines and balance them while spinning (this is no easy task). It's possible that such knowledge increases the time required to master centrifuge technology and that no such barriers may exist with Silex (but, of course, there may be other barriers with Silex). Also, the two countries that have successfully built undetected uranium enrichment plants are South Africa and North Korea. Iraq had one too that might not have been discovered had the United States not fought the Gulf War in 1990 (Iraq had help from German engineers and at the time of the Gulf War they knew what they were doing). So, it's questionable to argue that some of this laser technology is out of the reach of a non-state group if a country like North Korea can build a full centrifuge plant (which they did and showed it to a prominent U.S. nuclear scientist - Sigfreid Hecker. Here's a review of his latest book:
https://www.tandfonline.com/doi/full/10.1080/08929882.2023.2184563?src=). It might be true, but what physical principle can that judgement rest on?
All of this is highly uncertain, but what I hear you saying Zog is that you would be surprised if a country used Silex to clandestinely build a nuclear weapon. And I wouldn't. Certainly not in the too distant future. That seems like where this rests, and I just think if people had more concern about this (not only about Silex but about the easy to anticipate coming demand for nuclear weapons), more could be done about the broader geopolitical context on which all of us depend. Instead, the United States seems to think that more pressure will maintain the status quo when in reality its military advantages are now already in a place that scare Russia and China (at least in the strategic weapons area–Taiwan is a different story). This is why those countries feel they have no choice but to do things that get the United States' attention. That all of this is happening while Silex appears to be taking off should at least prompt a moment's pause of where this is all heading. I understand that nothing can be done about Silex to stop it, but that's the point. The question is what does that mean?
I'm sure this is more on this topic than people here would like to hear from me, but I do enjoy the debate and appreciate the knowledge and expertise that exists here.
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