Do you have, say, ten years to spare? Then you could probably design a functional nuclear weapon. We know that, because of a very odd experiment conducted in the 1960s by the United States government.
In the 1960s World War II was well over, and the United States and the Soviet Union were both settling into a nice, long, cold war. Both countries were nervous, knowing that each had designed and built nuclear weapons. At least, though, they were the only two countries that could manage it. But how long would it take until the next country built a weapon? This set the stage for a strange experiment, conducted by the United States.
It was called the Nth Country Experiment. The idea was, the United States was the first country to design nuclear weapons. The Soviet Union was the second. What country would be the Nth? There was no available data on the idea yet. There couldn't be. The United States' development of the atom bomb happened under extraordinary circumstances - including the circumstance of not knowing whether the idea was even feasible. The Soviet Union wasn't going to talk about how they developed their bomb - although by that time the US knew they'd had spies in the Manhattan Project. The Nth Country Experiment was meant to estimate what the probability that any other country could develop such weapons given basic knowledge.
In 1964, the US picked three young physics students who had gotten their PhDs in physics but hadn't had any specific education on weapons. They were given a salary, a basic support staff, and all publicly available information on nuclear weapons. They were then asked to design a nuclear bomb. The United States wanted to know if a small number of bright people, with sufficient motivation and the knowledge that such bombs were possible, could come up with the right design. No one had to wait too long. In 1967, the PhDs presented the officials of the Nth Country Experiment with a design for, what all the established bomb designers agreed, was a working atomic bomb. The design was a little over-fussy, but it was functional and, with a larger staff, or a little experimentation, it wouldn't take long to refine.
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The results of the Nth Country Experiment were not comforting, but they did shed light on what delays countries in building nuclear weapons. Clearly, it's not technical expertise in the design of the bomb.
You can read the full report from 1967 here.
Image: Nuclear Weapons Archive