This is a all a too common problem. The neck of the radiator breaks from age, heat, elements,etc. This is a common problem and kind of a design defect so, don't be alarmed. The radiators are now plastic vs. metal older cars. Mine broke about a month or so ago. The bad news is you'll have to get a new radiator or have that one rebuilt. Check the links above for Partshop or Fastlane. Ask them if they have the necks with the metal sleeve inside. Rebuilding the hot side of the coolant tank with new tank and gaskets on both cold and hot tanks. It may not be worth it depending, on the cost. I paid like $215 at Reseda Radiator only, to find out I could've gotten a new one for about $50 more at another place. Make sure the new one has the metal sleeve inside the neck of the radiator so, this problem does not occur again.

-mike