Radiation, nuclear warning and bridges?

[PincerDK]

Hello!

When do you know when the enemy is using nuclear weapons?

I was playing as WP in AFNorth in Danube Front 85' (solitaire) and sometimes I see "Radiation" marked on a hex. I haven't used nukes before in the game, so it would seem to be the AI who using them.

1. How do you know when it is possible to use nuclear weapons?
2. Which types of units is capable of firing tactical nukes? WP mostly?

Another question, what is the difference between ordinary bridges and pontoon bridges? do they have different advantages/disadvantages?