A person can only safely stay in Chernobyl for a few hours from what I understand.
Compared to Nagasaki and Hiroshima it apparently contained far more radioactive material.
The problem with radioactive material leaking into the sea is that currents slowly transfer it across long distances, far from the region.
I think Putin is talking about "Tactical Nukes" which will hit a planned area & of course will cause serious problems for an advancing army.
I believe OneMoreRep is an ex Gunner in the Marines so he will know this.
Back in the day we were tought to fire a Nuclear round on the M109 Gun as did the other American Guns the M107, M110 which we British used in Germany.