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By Nina Giraldo, Taylor Romine, Emma Tucker, Alaa Elassar, Holly Yan, CNN
(CNN) — Officials racing to prevent an overheating chemical tank in California from exploding breathed a small sigh of relief Monday after confirming a crack in the structure had helped reduce pressure inside, eliminating the risk of a catastrophic blast and allowing tens of thousands of evacuated residents to return home.
But with a toxic leak or smaller explosion still possible, officials are asking 16,000 residents closest to the site in Orange County not to return home yet, as they continue to try to cool the tank’s internal temperature.
Authorities are expected to meet Tuesday to assess the situation after officials expanded their focus overnight on cooling efforts since finding pressure in the tank at GKN Aerospace in Garden Grove had been alleviated, preventing a disaster known as a BLEVE, or boiling liquid expanding vapor explosion.
Until evacuation orders were partially lifted Monday evening, about 50,000 residents were asked to flee their homes as the temperature and pressure escalated inside the tank containing 7,000 gallons of a chemical called methyl methacrylate, or MMA, which is used to make plastics and can cause myriad health problems upon exposure.
A deluge of cool water has doused the overheated tank for days, helping to bring down the temperature, officials said in an update Monday night. Runoff water from the site – millions of gallons – is being tested for any hazardous chemicals and so far, none have been detected, officials said.
To help with the cooling process, crews are removing insulation from the tank’s outer shell, Orange County Fire Authority Interim Chief TJ McGovern told CNN Monday night, adding authorities will be monitoring temperature data throughout the night.
While the threat of a catastrophic explosion has been eliminated, “we still have threats that we’re mitigating right now,” said McGovern earlier, including the “potential for an explosion that’s not as bad as a BLEVE” but still a risk to public safety, he said.
Meanwhile, a presidential emergency declaration has been signed for the situation, bolstering federal assistance, California Gov. Gavin Newsom announced Monday.
A volatile situation
The tank contains MMA, a chemical that can cause skin and eye irritation as well as respiratory problems.
“There is still no chemical leak, as verified by continuous atmospheric monitoring,” the Orange County Fire Authority said Monday.
Still, some residents have reported symptoms of exposure. And some schools have closed and switched to online learning as a precaution.
The tank started showing signs of trouble Thursday, when heat and pressure started building up.
When MMA overheats, it starts a chemical reaction that is difficult to control inside a large tank, said Elias Picazo, a chemistry professor at the University of Southern California.
Authorities are trying to keep the tank as cool as possible to keep