California fire officials sent a specialised team in the middle of the night on Saturday to investigate the facility.
“During that operation, our firefighters went in and they were able to visualise the tank,” TJ McGovern, Orange County’s interim fire chief said in an update on Sunday.
“What they found was a potential crack in the tank which could potentially be relieving some of the pressure in there,” he continued.
McGovern said experts are analysing findings from the data, and he expects “a lot more information coming today”.
The aerospace facility is about 5 miles (8km) from the Disneyland theme park, which is outside the evacuation zone and is open as normal. The facility is about 35 miles south-east of Los Angeles.
The cause of the failing tank is under investigation and emergency crews have been spraying it with water to try to stabilise the temperature. Officials said an “inoperable valve” had created “additional operational challenges”.
Covey said the damaged tank’s temperature had risen to 32C (90F) on Saturday and they had recorded it increasing in heat by about one degree Fahrenheit an hour.
On Friday evening, officials had used drones to measure the external heat levels and believed they had cooled the tank to about 16C (61F).
But when crews were able to get closer and read the tank’s gauge, they realised the internal temperature was much higher, Covey said.
He expressed hope that emergency teams could slow the rate of the chemical reactions that were causing the substances to heat up and build pressure inside the tank.
“Letting this thing just fail and blow up is unacceptable to us,” Covey said.
Officials were developing “very aggressive plans” for a scenario in which the tank’s integrity failed. They were also creating dykes and dams to contain any chemicals if the tank spilled, which Covey said would hopefully prevent any substances reaching storm drains or the ocean.
Local schools have closed as a precaution and the exits of several major roads have been shut to limit access to the area.
Orange County health officer Dr 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.
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.
