After six days of trying to avoid an overheating chemical tank erupting into a giant fireball or spilling thousands of gallons of toxic substances at an aerospace facility in Garden Grove, Orange County leaders announced Tuesday that the risk of catastrophic explosion had largely been eliminated.

Local authorities lifted a large section of the evacuation zone surrounding GKN Aerospace and allowed tens of thousands of residents to return.

Firefighters sprayed more than 9 million gallons of water onto a piping-hot tank of flammable methyl methacrylate (MMA), drastically bringing down the vessel’s temperature — but not before the high temperatures resulted in high pressure and a crack in the side of the tank, which acted as a relief valve.

Interim Orange County Fire Authority Chief TJ McGovern indicated in a Tuesday afternoon community meeting that evacuation zones might soon shrink further. He noted that crews had stopped spraying water onto the tank and were in the process of assessing whether the vessel’s temperature had stabilized.

“Once we know that temperature is stabilized, we will be taking the fire risk off the table,” he said. “If there’s no fire risk, our evacuation zones are going to shrink.”

McGovern said, about 5 p.m., that officials were “hoping that we’re going to have a very positive outcome very soon.” He asked the community for continued patience over the next few hours as crews worked to validate the initial data they were seeing regarding temperature stabilization.

The Orange County health officer and fire officials have insisted there have been no vapors or chemical leaks over the course of the six-day crisis. The U.S. Environmental Protection Agency said its air monitors surrounding the facility had not detected methyl methacrylate or other toxic airborne chemicals (known as volatile organic compounds).

But environmental experts remained skeptical that no toxic substances had been released. Andrew Whelton, a Purdue University professor who studies environmental disasters, said the ruptured chemical tank would have acted similar to a soda can with a hole punched in it.

“I find it hard to believe you can heat up a tank with a [chemical] like methyl methacrylate, see that it clearly cracked under pressure and think that nothing came out it,” Whelton said. “That defies logic.”

It’s possible, Whelton said, that spraying copious amounts of water on the tank had effectively suppressed much of the toxic vapors and the airborne risk.

Fire officials had previously said that the tank of MMA was experiencing thermal runaway, a chain reaction resulting in an uncontrollable spike in temperatures. They said the situation was likely to end in an explosion or chemical spill.

Whelton said an explosion is still possible.

To guard in the event of a spill, authorities set up sandbag barriers to block the chemical from storm drains that lead to the ocean.

The Orange County Fire Authority said it was also testing water that had been hosed onto the tank to ensure it didn’t contain elevated levels of contaminants.

Jane Williams, executive director of California Communities Against Toxics, warned that MMA is just one of the chemicals being stored on the site. She fears there is a danger from other chemicals.

The company in 2024 reported that, in addition to MMA, it had released thousands of pounds of flammable chemicals, including methyl ethyl ketone and methanol n-butyl alcohol, according to records from the South Coast Air Quality Management District.

GKN Aerospace had previously been cited for failing to disclose flammable chemicals at other facilities.

In 2007, the U.S. EPA alleged the company stored about 8,000 pounds of hydrofluoric acid and 34,000 pounds of nitric acid at a Kent, Wash., facility — but neglected to report these stockpiles to the appropriate government agencies.

A year earlier, the company settled with the EPA over allegations that it had improperly stored ignitable hazardous waste at a facility near San Diego.

“For me, this is not about MMA,” Williams said. “You have a company with a bunch of chemicals, and it lost containment, and it’s across from residences. I do not trust this company to disclose what else is on their site. I do not trust them with first responders. I do not trust them with my health.”