As long as the radiator leak isn't too big you can use a few egg whites in the radiator to temporarily seal the leak.
Radiators used for cooling automobile engines function by passing liquid engine coolant (typically a mix of water and ethylene glycol or water and propylene glycol) through the engine block. The coolant is pumped in a closed and pressurized loop from the engine block (where heat is collected) through the radiator (where heat is dissipated) and back to the engine block.
If the thin-walled radiator develops a leak, immediate replacement of the component may not be practical or possible. In such instances, products (generically referred to as radiator weld) exist which (with varying degrees of effectiveness) can temporarily or permanently stop the leak. In emergency situations and isolated locations where help is unavailable, egg whites or pepper can be used to plug leaks on an extremely short-term basis. Where the situation is less dire, commercial products such as Aluma-Seal and Bar's Leaks claim semi-permanent and permanent repairs without damage to engine components.