After a nuclear attack, fallout will contaminate every tile affected by the blast. Any units that end their turn in a contaminated tile take 50 damage each turn. Production cannot be applied to anything in tiles containing contamination, such as repairing buildings and districts. Tiles that are contaminated cannot be worked by the city until the contamination timer expires or until the tile is "cleaned."
Nuclear contamination can be "cleaned" from affected tiles by Builder and Military Engineer units. Cleaning a contaminated tile takes 1 build charge. Builder, Military Engineer, and Roman Legion units that are able to clean contaminated hexes will take less damage if they end their turn in such a hex. Any city tiles not directly affected by the nuclear blast will still function normally and contribute to the city’s yield.