Columbia river for our secondary system
Never happened. People have know to keep their boiler water clean for at least one and a half centuries. On the other side of the condensors it can even be sea water but that boiler water is kept clean.
Nukes have the extra loop that exchanges heat with the boiler water - thus cooling water that goes through the cooling towers is the third loop. If anything from the first loop has made it into the third it's either a string of failures or a huge catastrophic event obvious to anyone within miles.