A couple of bright physics grad students could build a nuclear weapon. Indeed, the US Government actually tested this back in the 1960s - they had a few freshly minted physics PhDs design a fission weapon with no exposure to anything but the open literature [1]. Their design was analyzed by nuclear scientists with the DoE, and they determined it would most likely work if they built and fired it.
And this was in the mid 1960s, where the participants had to trawl through paper journals in the university library and perform their calculations with slide rules. These days, with the sum total of human knowledge at one's fingertips, multiphysics simulation, and open source Monte Carlo neutronics solvers? Even more straightforward. It would not shock me if you were to repeat the experiment today, the participants would come out with a workable two-stage design.
The difficult part of building a nuclear weapon is and has always been acquiring weapons grade fissile material.
If you go the uranium route, you need a very large centrifuge complex with many stages to get to weapons grade - far more than you need for reactor grade, which makes it hard to have plausible deniability that your program is just for peaceful civilian purposes.
If you go the plutonium route, you need a nuclear reactor with on-line refueling capability so you can control the Pu-239/240 ratio. The vast majority of civilian reactors cannot be refueled online, with the few exceptions (eg: CANDU) being under very tight surveillance by the IAEA to avoid this exact issue.
The most covert path to weapons grade nuclear material is probably a small graphite or heavy water moderated reactor running on natural uranium paired up with a small reprocessing plant to extract the plutonium from the fuel. The ultra pure graphite and heavy water are both surveilled, so you would probably also need to produce those yourself. But we are talking nation-state or megalomaniac billionaire level sophistication here, not "disgruntled guy in his garage." And even then, it's a big enough project that it will be very hard to conceal from intelligence services.
> The difficult part of building a nuclear weapon is and has always been acquiring weapons grade fissile material.
IIRC the argument in the McPhee book is that you'd steal fissile material rather than make it yourself. The book sketches a few scenarios in which UF6 is stolen off a laxly guarded truck (and recounts an accident where some ended up in an airport storage room by error). If the goal is not a bomb but merely to harm a lot of people, it suggests stealing miniscule quantities of Plutonium powder and then dispersing it into the ventilation systems of your choice.
The strangest thing about the book is that it assumes a future proliferation of nuclear material as nuclear energy becomes a huge part of the civilian power grid, and extrapolates that the supply chain will be weak somewhere sometime, but that proliferation never really came to pass, and to my understanding there's less material circulating around American highways now than there was in 1972 when it was published.
The other thing is the vast majority of UF6 in the fuel cycle is low-enriched (reactor grade), so it's not useful for building a nuclear weapon. Access to high-enriched uranium is very tightly controlled.
You can of course disperse radiological materials, but that's a dirty bomb, not a nuclear weapon. Nasty, but orders of magnitude less destructive potential than a real fission or thermonuclear device.
And this was in the mid 1960s, where the participants had to trawl through paper journals in the university library and perform their calculations with slide rules. These days, with the sum total of human knowledge at one's fingertips, multiphysics simulation, and open source Monte Carlo neutronics solvers? Even more straightforward. It would not shock me if you were to repeat the experiment today, the participants would come out with a workable two-stage design.
The difficult part of building a nuclear weapon is and has always been acquiring weapons grade fissile material.
If you go the uranium route, you need a very large centrifuge complex with many stages to get to weapons grade - far more than you need for reactor grade, which makes it hard to have plausible deniability that your program is just for peaceful civilian purposes.
If you go the plutonium route, you need a nuclear reactor with on-line refueling capability so you can control the Pu-239/240 ratio. The vast majority of civilian reactors cannot be refueled online, with the few exceptions (eg: CANDU) being under very tight surveillance by the IAEA to avoid this exact issue.
The most covert path to weapons grade nuclear material is probably a small graphite or heavy water moderated reactor running on natural uranium paired up with a small reprocessing plant to extract the plutonium from the fuel. The ultra pure graphite and heavy water are both surveilled, so you would probably also need to produce those yourself. But we are talking nation-state or megalomaniac billionaire level sophistication here, not "disgruntled guy in his garage." And even then, it's a big enough project that it will be very hard to conceal from intelligence services.
[1] https://en.wikipedia.org/wiki/Nth_Country_Experiment