Iran 1946

* note: this is a bit off topic

I heard that Kennen threatened to nuke USSR in 1946 if they did not withdraw from Iran, but I couldn't find any confirmation on this. Could U.S. have nuked USSR in 1946? Did they already build more bombs after the first two dropped on Japan? And if they did, did US have the capability to safely deliver these bombs on USSR?

I wouldn't know, and it's off topic anyway. The rest of my replies will relate to US nuclear potential at the very end of WWII and following.

Could U.S. have

Yes. The capability was there.

Did they already build more bombs after the first two

There was another bomb ready to go at the time, that wasn't used. I don't know all that much about the immediate postwar production, but it did exist.

And if they did, did US have the capability to safely If a bomber based on Tinian can hit Hiroshima, the US could have found some airfield close enough to hit the Soviet Union (not anywhere in the Soviet Union, but plenty of important points). The state of Soviet defenses is speculative; they had absolutely no experience at stopping anything like a B-29, although they had a few fighters that would operate well at those altitudes.