How to get immediately notified in case of nuclear attack on Ukraine?

> so I can book a flight

If you're counting on flying away from Ukraine's neighbour after an attack like that, that's already not going to work. Flights were cancelled for over 2 days and only slowly restarted after 9/11. And that was basically nothing in comparison. In reality airports close to the Ukraine borders are already used by the military flights which take priority over civilian traffic. Main roads leading that way see regular military traffic.

The extra problem with booking the flights is that if there's a nuclear attack, the available options will not be updated immediately - you may book a ghost flight and then again have to fight with everyone else for a place on the new schedule.

Sorry, I don't have a good positive suggestion for transport. But if you really want to make a good plan, I'd check: where to get food directly from the farms in the area (there will be a run on the shops like we've never seen with covid), and where the local underground shelter is (most towns around that area have buildings from the times when bomb shelters were common - you can even see the signage for them sometimes), and... I guess figure out what you'd do in case of temporary martial law.

I think any such attack is likely to set off a snowball effect which would make any destination potentially hazardous.

Post-attack is way too late. There will be total chaos. Since you're nearby, just listen for the radiation hazard sirens and turn on the radio/TV when you hear it. Any country around you has a very developed warning system since the Cold War.

(I'm just one more border away, already bought my iodide...)

If this happens, it will be minutes before the media covers it, so any media subscription will probably do.

As a matter of fact, it's likely the attack would be pre-announced too.

So maybe move even closer to Ukraine and listen carefully.

If it happens it'll be on all news channels.