Erhm... The U.S. is a pretty big place. Like... Really big. You can be so remote that no one hears the blast except people you want to hear.
I know, I went to school in the Mojave desert.
Doesn't stop the radiation from being blown away, but I really think keeping it a secret from 1945-1955 wouldn't be that hard to do if you're only blowing up the desert.
Even so you are talking about a super mega huge project which ties up countless of people. They drafted most of the top physicists into the Manhattan Project, and they kept their mouths shut while the war was going on, and set up gigantic industrial plants to develop and produce the bombs, which they could also keep obscured or secret, while the war lasted. But once the war is over you have to release a lot of the workers and scientists, and they're not all going to keep their secrets. The Manhattan project was larger than any of the other top-secret armaments projects of the cold war, and the end of the war will mean many people (especially the top scientists, many of whom were not the quiet personality type) won't feel as obligated any more to keep their mouths shut.
And then there's the political side. What reason would there be for the Soviets, to keep their own project secret? Imagine the PR coup if they made their atomic tests public, before the US made theirs. The world would be in awe of the commies, and uncle sam would look like follower, not like a leader, in atomic weapons. They'd have to detonate a dozen bombs all at once in a huge public test, just to one-up the commies and restore the confidence of the nation.
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