Some cars these days come with plastic radiators instead of metal, which makes them pretty easy to crack over time due to aging and wear and tear. Once it does crack, you'll now face a coolant leakage issue that can sometimes be rectified as easily as sealing in the crack. You can find dedicated radiator repair kits in most auto parts stores. It usually entails mixing some epoxy resin together and spreading that over the cracks. With a bit of heat, the epoxy would soon harden and seal off the cracks on the radiator. You might even consider sanding off excess epoxy to help smoothen the surface of the repaired area against the rest of the radiator.