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The threat of a failing chemical tank in Orange County exploding has been eliminated following an overnight operation, California fire officials say.

Division Chief Craig Covey said on Monday that the tank's temperature has decreased, which led to the "positive news".

The tank, which contains thousands of gallons of methyl methacrylate - a highly volatile and flammable substance used to make plastic - has been at risk of either a major spillage or an explosion in recent days.

There is currently no active leak, but crews have been working since Sunday to assess the risk. A leak remains a possible outcome, officials have said, as an evacuation order for the area remains in place.

The damaged chemical tank in southern California cracked over the weekend, and officials were hopeful pressure that had built up inside the tank would be gradually released

The threa.t of a BLEVE, or Boiling Liquid Expanding Vapor Explosion, has been "eliminated" after an overnight operation, Orange County Fire Authority Interim Chief TJ McGovern said on Monday.

"We are happy to report that the threat of a BLEVE is now off the table," McGovern said. "That threat has been eliminated."

Covey, who spoke alongside McGovern, said in a video shared on social media that crews confirmed a crack in the tank, which led to a pressure release.

The tank's internal temperature has decreased from 100F (38C) to 93F (34C).

The safety of the firefighters on site prevented officials from offering more updates on the tank's internal temperature over the weekend, Covey said.

"We were not doing tank temperature checks during the day while the sun was on it in the most extreme conditions for that tank to go the wrong direction," he said. "We were only doing tank temperatures at night."

No chemicals are currently leaking, officials have repeatedly said, and authorities have been working around the clock to ensure that it stays that way.

Emergency crews have been spraying the tank with water to try to stabilise its temperature, following some rapid heat increases since Thursday.

Some 50,000 people remain under evacuation orders as a precaution, and a state of emergency has been declared by California Governor Gavin Newsom.

The cities of Garden Grove, Stanton, Anaheim, Cypress, Westminster, and Buena Park are all affected by the evacuation orders.

The tank at GKN Aerospace Transparency Systems, an aerospace facility, is in the city of Garden Grove, about 35 miles (56km) south-east of Los Angeles. GKN Aerospace has apologised to local residents.

At an earlier stage, officials expressed hope that emergency teams could relieve the pressure issue by slowing the rate of the chemical reactions that were causing the substances to heat up inside the tank.

"Letting this thing just fail and blow up is unacceptable to us," Covey said.

Officials had also been developing "very aggressive plans" for a scenario in which the tank's integrity fails, Covey said at the time. They have been creating dykes and dams to contain any chemicals if the tank does spill - with the aim of preventing any substances from reaching storm drains or the ocean.

Methyl methacrylate is used to make resins and plastics. The chemical is irritating to the skin, eyes, and mucous membranes in humans, and can cause respiratory and neurological symptoms with exposure, according to the Environmental Protection Agency.