concerns about the repeated small explosions, the atmospheric venting of radioactive gasses, and the possibility of larger explosions led to a -radius evacuation around the plant. during the early days of the accident, workers were temporarily evacuated at various times for radiation safety reasons. at the same time, seawater that had been exposed to the melting rods was returned to the sea heated and radioactive in large volumes for several months until recirculating units could be put in place to repeatedly cool and re-use a limited quantity of water for cooling. the earthquake damage and flooding in the wake of the tsunami hindered external assistance. electrical power was slowly restored for some of the reactors, allowing for automated cooling.