Soul = Seoul
The NorK’s - starving their people for 50 years in their insane hatred of SKorea and the US and a long racial war with Japan - don’t care about the radiation “returning” from the blasts that hurt their enemies.
The Russians - and Chinese for what it was worth then - in the Cold War were rational. They wanted to defeat and conquor the US and West. They did not want to destroy it. (They DID need to destroy its Christian culture. And succeeded in that very very well.)
Pyongyang is 121 miles from Seoul, and upwind. Any fallout would most likely be blown toward Japan.
But I don't think North Korea would waste a nuke on Seoul just yet. More likely targets are US island military bases further away, like Guam or Okinawa, allowing to Kim to claim he was "deterring aggression" rather than attacking civilians.