Authorities scrambling to find a safe resolution after a tank containing a hazardous chemical at a Southern California aerospace plant was damaged believe the risk of a catastrophic explosion has been eliminated. But they said Monday that the evacuation order affecting tens of thousands of residents remains. The tank overheated last week and began venting vapors into the air around Garden Grove, a city in Orange County. Officials said Monday that a crack that  developed in the tank relieved pressure and cooled the chemical. No injuries have been reported but more than 50,000 residents were under an evacuation order over the Memorial Day weekend.