However, there is another side, which is the opposite of subtle: the creation of the bomb. And, of course, you have to do this at the atomic scale, so lots of easy mass based techniques fall through. Zippie type centrifuges spin so fast that the only way to keep the friction from turning the entire device into slag is magnetic bearings! Why all of this? The art of enriching is subtle: you have to separate two groups of atoms whose only distinguishing factor is that one is 1.2% heavier than the other. The spinning parts of a gas centrifuge are spinning so fast that they are approaching the tensile strength capabilities of single crystal titanium. He mentions gas centrifuges are out of the question due to aluminum not being available, but it goes even deeper than that. King-Ink pointed out the subtle task: enrichment. The creation of a nuclear bomb requires an astonishing dynamic range of skills.
The ability to make a nuclear bomb is sufficiently far from the technology of swords and shields that they really don't fit. You will need to stretch a lot to make this a cohesive book.
