Currently no active leak from failing chemical tank, California officials say

California fire officials say there is "currently no active leak" from a failing chemical tank in Orange County.

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. Crews have worked through the night from Sunday into Monday to assess the risk.

Tens of thousands of people are under evacuation orders as a precaution, and a state of emergency has been declared by Gavin Newsom, the state governor.

Officials are hopeful the pressure that has built up inside the tank is being gradually released by a crack that has formed - potentially avoiding a disaster.

"Continuous live atmospheric monitoring confirms there are no chemicals leaking," the Orange County Fire Authority (OCFA) officials wrote in a social media post on Sunday evening. They also denied any suggestion the tank had suffered more than one crack.

The cause of the incident is under investigation, and emergency crews have sprayed the tank with water to try to stabilise its temperature following some rapid heat increases since Thursday.

Explaining the purpose of Sunday's "all-night mission", the county's interim fire chief, TJ McGovern, said crews wanted to confirm the pressure in the tank had been released.

The hope was to ensure the "Bleve [Boiling Liquid Expanding Vapor Explosion] threat is eliminated", said McGovern. "The Bleve threat is the worst-case catastrophic event that we've been talking about. We are not there yet."

McGovern also appealed to locals to stay clear of the evacuation zone.

The aerospace facility is in the city of Garden Grove, about 35 miles (56km) south-east of Los Angeles. It is operated by GKN Aerospace, which 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," said Craig Covey, Orange County Fire Authority division chief.

Officials had also been developing "very aggressive plans" for a scenario in which the tank's integrity fails, Covey added. 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.

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

A number of schools have closed as a precaution, and the exits of several major roads have been shut to limit access to the area.

However, the nearby Disneyland theme park is outside the evacuation zone and has been operating as normal.

Speaking to local outlet NBC LA on Friday, local resident Mark Olsen said he was woken up by the sound of sirens.

"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," he said.

The facility is operated by GKN Aerospace, a UK-based company that supplies parts for aircraft.

In its latest statement, quoted by US media, the firm said: "We continue to monitor the condition of the affected material and are working round the clock to mitigate the risk of a leak. We are acutely aware of the uncertainty this incident is causing and sincerely apologize for the ongoing disruption to the local community."

Orange County health officer Dr Regina Chinsio-Kwong has said that anyone who notices "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.

Chinsio-Kwong said inhaling the vapour could cause respiratory issues, irritation to the eyes and dizziness. She said those outside the one-mile evacuation zone should be safe.