1964, Physics PhD who knew nothing about nuclear physics designs a bomb: https://www.theguardian.com/world/2003/jun/24/usa.science
Physics junior in the mid-1970s designs a device good enough to impress Freeman Dyson: https://en.wikipedia.org/wiki/John_Aristotle_Phillips#%22A-B...
As search engines continue their trend of considering your search term just a suggestion, I can't pull it up, but there's also a case where a high school physics class decided to try to design one and also came adequately close.
The hard thing that is actually the stopper is the enrichment of the relevant materials. The other hard part is getting the best possible yield; there's huge variances in what you get from the same amount of fuel depending on how well you can put it together before it blows itself apart, but that's not a stopper for a terrorist group. Getting to Hiroshima levels is apparently not that difficult, as evidence by the fact it was done so many decades ago.
Delivery is another major challenge, but I'll consider that separate from the task of creating one at all.
In the real world, we have classification, but by-and-large those are about very specific elements of very specific things (i.e. the exact shape/location of that secondary, not that the secondary exists or that Sandia does modeling of that sort of thing). No one's really the gatekeeper of knowledge of things like nuclear engineering or biological gain-of-function. There's not really a litmus test for someone to attend to a microbiology graduate program or take a chemistry class that would enable them to develop synthetic drugs.
Same thing with martial arts; no one's hiding some secret martial technique. A BJJ purple belt will, in a fistfight, toy with just about anyone else on the planet not trained in jiujitsu like they're a toddler. And you can just, like, walk into many strip malls across the North America, pay your $200/mo, and a few years later of consistently showing up, you're there. No secret death touch or spiritual clarity needed.
I think secrets are largely against the ideals of the enlightenment. There are certain temporal operational concerns. Democracy dies the death of a thousand cuts when unelected bureaucrats classify the most mundane things by default “just in case”. 9/11 happened because of these silos. Our foreign adversaries often know more about what we are up to than US citizens, etc.
More often than not, comparatively simple chemical reactions are hard to reproduce reliably just by reading the research papers.
I think enough is known now to really narrow down the problem to something a nation state can do.
That said, if it was that easy, I'm sure we would've had terrorist attacks with nukes already. Or if terrorism was that big an issue. I don't know if it hasn't happened yet because technology and three-lettered agencies are doing their job right though.
The whole thing is a giant, high profile, and dirty mess.
There are two problems with that statement. Let’s examine them
Firstly, does majority of terrorists want to nuke NewYork? If you gave 9/11 bombers a 5 megaton warhead, would they use it? You have to remember that many of them imagine they have a just cause.
Second, imagine you are could make a nuke at home and were completely immoral, who would you sell it to?
There are many evil governments and organisations that could pay more and be better clients than terrorists.
Assembly of the actual warhead could be aided by the OP diagram.
The hardest part of building nukes is acquiring weapon-grade enriched uranium, because it's controlled as hell and you will get bombed if you try to make your own.
If you spend hundreds of millions of dollars on enriched uranium, paying salaries for team of engineers is the easy part.
There is a rumor that the USSR flirted with the idea of a pre-emptive strike on Mainland China to decapitate their nuclear program after the Sino-Soviet split. This did not happen obviously.
Iran didn't get bombed, although that may just be because other forms of sabotage were available.
Syria & Iraq on the other hand, yeah those got bombed. But it's not 100% a guarantee.
