The USSR would have had nukes, too, by then. Stalin knew about our program thanks to the Rosenbergs and Klaus Fuchs. He would have pressed ahead with his bomb project regardless.
And what target in North Korea would have been worth an atomic bomb?
Chosin reservoir after we had evacuated?
The idea was to irradiate the Yalu river from one end of the Peninsula to the other, to keep out the million Chinese ground troops.
Of course it probably would not have kept the Soviets from sending in their air forces and pilots.