Given that the Nagasaki bomb was a plutonium device the mathematics and physics involved are 57 year old technology much of which has been in the public domain for over forty years. I presume any Phd in physics could get enough information to figure out the forces involved in generating critical mass/density in a short period of time in a good library. Likewise the solutions to each of the mechanical and electrical engineering problems has been published and at least a few years back was in the public domain. Of course given the proper casing of U238 the proper mix of hydrogen isotopes and the proper medium to turn the X-Ray pressure into physical pressure one can advance to the megaton range.
Stay well - Stay safe - stay armed - yorktown
In theory, there is no difference between theory and practice. In practice, there always is. Having the published "solution" to an engineering problem is at best just a leg up on actually solving the problem.
Frankly, anyone capable of building a lensed-implosion plutonium bomb is more than capable of refining his own plutonium—and even if he can't, he can readily acquire plutonium from Iraq or North Korea, since Hussein and Kim both have plenty and would gladly sell their own mothers for working bomb designs. So either way, theft from the former Soviet Union is a sideshow; the clear and present WMD danger is from the Axis of Evil.