Threat of massive chemical tank explosion is 'eliminated', California officials say

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

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.

Response teams are still working to mitigate spill potential, and a "very small explosion concern", Orange County Fire Authority Interim Chief TJ McGovern said on Monday evening.

A new and reduced evacuation zone has been established, with approximately 16,000 people still unable to return home.

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 threat 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."

Though there is currently no active leak, officials cited the risk of another leak as a reason for reduced evacuation to remain in place.

"It's not over yet - we still have work to do," McGovern said when announcing the new evacuation zone, thanking residents for their patience.

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 were initially affected by precautionary evacuation orders, which included the cities of Garden Grove, Stanton, Anaheim, Cypress, Westminster, and Buena Park, and a state of emergency was declared by California Governor Gavin Newsom.

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.