Maybe it’s the same logic as the dam distraction, it makes it harder for Ukrainians to advance. Instead of Ukraine having to ford a river due to the dam destruction they will make Ukraine cross nuclear fallout to get them to leave. It feels like it’s just terrorism logic at this point where Moscow said they would set off a nuclear bomb if their demands aren’t met and Ukraine saying no to the demands…thus the terrorist just detonates the bomb as they run away.