No, it's not hazardous. Just take your radiator off and flush it out with water. Wrap it in several layers of bubblewrap and, if you can, put some foam blocks around the inlet and outlet. Then I'd suggest putting the whole thing in a garbage bag before putting it in a box. I only do this so if any left voer water spills out of the radiator it won't weaken the box. Then just ship it off like you normally would.
Again, it's not hazardous and there's nothing to declare.