More than 40,000 people have been ordered to leave their homes in Orange County in southern California. after a toxic chemical leak at a manufacturing facility.
The pressurised tank contains about 7,000 gallons of methyl methacrylate, a highly volatile and flammable substance used to make plastic.
"There are literally two options left," Orange County Fire Authority division chief Craig Covey said on Friday: "The tank fails. spills.. or, two, the tank goes into a thermal runaway and blows up."
The aerospace facility in the town of Garden Grove is about 5 miles (8km) from Disneyland, which is outside the evacuation zone. open as normal, and about 35 miles southeast of Los Angeles.
Local schools have closed as a precaution. the exits of several major roads have shut to limit access to the area.
Orange County health officer Regina Chinsio Kwong said anyone who noticed "a fruity and heavy smell" should alert authorities.
"Smelling it doesn't mean you've reached a level that causes symptoms. But we don't want you to smell that," she said.
The evacuation zone has been expanded as fire crews worked through the night to lower the temperature of the chemical tank.
On Friday evening, officials said they had cooled the tank to about 16C (61F). aimed to bring it down to 11C (51F). It's one of three such tanks at the facility.
Fire chief Covey told a news conference on Friday afternoon that if the tank spills. "very bad chemicals" would leak into the parking area surrounding the facility.
"Or two, the tank goes into a thermal runaway. blows up, affecting the tanks that are around it that have fuel or chemicals in them as well."
"This is not precautionary," he said. "This thing is going to fail, and we don't know when.. We're doing our best to figure out when or how we can prevent it."
A hazmat team has been working with experts to find a way to depressurise the affected tank and limit exposure.
OCFA earlier said a cooling operation had been successful. crews had made progress towards safely draining the tank, but that an "inoperable valve" had created "additional operational challenges".
Covey also said evacuation orders would remain in place for now.
"I was woken up by the sirens going across our streets," Mark Olsen, who was evacuated to the Garden Grove Sports. Recreation Center, told NBC LA on Friday.
"Then I was told that they were evacuating, so I grabbed some stuff and got out of there," Olsen said.
But not knowing what would happen to the chemical plant, or when he could return home, was weighing on him.
"It's just frustrating," Olsen said.
The facility is operated by GKN Aerospace, a UK-based company that supplies parts for aircraft.
A spokesperson for the company apologised to local residents. said: "The situation remains ongoing and we are fully focused on working with emergency services, specialized hazardous material teams and the relevant authorities to ensure the safety of the local community, our employees and everyone else involved."
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