Nuclear fission with the radioactivity and decay products is indeed pretty easily verifiable. In my opinion definitely a real phenomenon.

But that does not prove that nuclear bombs exist or are even viable.

By the way, now that I think of this. Many years ago I looked into this topic out of personal interest.

It seems that when nuclear material warms up, it absorbs less neutrons, thus stabilizing the reaction at a certain temperature.

(Also explains reactor meltdowns not resulting in an explosion like a nuclear bomb, but instead some kind of blob of "elephant's foot")

But I am not an expert :)