READ HERE: Authorities braced for the possibility that a damaged chemical tank in Southern California could leak or explode as an evacuation order continued into the Memorial Day weekend for 40,000 residents with no timeline on when they can return.

No injuries were reported after the pressurized tank overheated Thursday and began venting vapors at a company site in Garden Grove, according to the Orange County Fire Authority. Garden Grove is about 40 miles (60 kilometers) south of downtown Los Angeles. Several shelters remained open Saturday, including at three high schools.

But officials said the valves on the tank are broken or “gummed up,” so that prevented crews from removing the chemical or relieving the pressure on the tank, said Craig Covey, Orange County Fire Authority division chief.

Firefighters’ first hope is to find a way to cool off the chemical inside the tank so it won’t leak or explode. If that’s not possible, Purdue University engineering professor Andrew Whelton said it would be best if the tank sprang a leak so the chemical could be mostly contained. An explosion that could spread the chemical over a broad area and send shrapnel from the tank flying would be the worst-case scenario.