Over the past few weeks, I've been working on a design for a gun-type fission weapon. However, I'm not sure if it would be legal to do so in the US. I source all my information from publicly available sources (usually sources from Wikipedia or anything I google)/declassified documents, and I don't ever plan on actually building the design I'm making. I'm still worried that what I'm doing might be illegal, though.
I'm pretty sure the basic thrust of the matter is "you're protected by the first amendment protections of speech and journalism", though I think formally speaking the exact legal status of nuclear weapons publications are not entirely clear. Cases about it have tended to fall apart and get dropped before precedents got set.
The Atomic Energy Act of 1946 ostensibly automatically classifies and controls any and all information about nuclear weapons, regardless of the circumstances of their creation. United States v. Progressive, Inc. was a case about a publication purporting to reveal the secrets of the H-bomb, and based on unclassified/public information, that was challenged under this provision. Ultimately the case was dropped by the government, and many legal pundits believed it likely that SCOTUS would have ruled the provision an unconstitutional prior restraint on speech. But it otherwise appears to be valid law, at least in the sense of not having been struck down by a court, to this day. The government has tried to censor publications even within the last decade, though with limited success, in part because of the Streisand effect: aggressively trying to prevent people from looking at it just gives it free publicity and makes more people look at it. A more successful attempt by the government appears to be the case of "The A-bomb kid", who wrote a college paper with a design for a nuclear weapon, and made a mockup of the design, both of which were confiscated by the FBI, but this was back in 1976 (and said kid later wrote a book about the whole thing).
That said, there are quite a number of ways of easily finding varying levels of nuclear weapons designs online already. Including on wikipedia. Government policy across the world has usually been to never acknowledge any purported leaks in any way; the US Department of Energy calls it their "no comment" policy. But such purported leaks do exist and can be found with relative ease, and many of the weapons designs you can find are based both on such leaks, as well as reverse engineering from non-weapons nuclear technologies such as nuclear reactors. The government is far more likely to try to restrain those who have actually encountered actual governmental nuclear weapons information, and have signed NDAs, but even then it rarely succeeds or pushes too hard. Most of the scientists who worked on developing the first nuclear weapons have published or publicly stated information concerning such endeavors, after all (though usually not terribly deep and technical; scientists have self-censored on nuclear weapons pretty much since German scientists first made public their possibility in the early 1930s).
As such, you are most likely just fine, and any attempt by the government to stop you is probably just going to result in more publicity and potentially more sales and money for you. But this is technically still a bit of a grey area, and consulting with a lawyer may be worthwhile.