I've started reading Dark Sun, Richard Rhodes' award winning history of the development of the fusion (H) bomb. (Hat Tip, Fernando). I've barely started, but I have to say that Rhodes is a compelling writer.
One thing that caught my eye was that key Russian scientists were aware of the possibility of a uranium bomb in 1939, and some were already advocating a strong program to try to build it. Two fundamental problems prevented it: the uncertainty as to whether a bomb would actually work, and the enormous expense required to find out. In the end it was decided that the necessary resources could be more usefully spent preparing for the coming war with Germany. The government did not trust the scientists enough to go for broke, and the scientists, with intimate knowledge of Stalin's terror, did not trust the government enough to go all out.
Rhodes sums it up:
Trust would not be a defining issue later, after the secret, the one and only secret—that the weapon worked—became known.
Rhodes, Richard. Dark Sun: The Making Of The Hydrogen Bomb (p. 42). Simon & Schuster. Kindle Edition.