The risk of a “catastrophic” explosion at a chemical tank in a Los Angeles-area city was averted by Monday, but around 16,000 people remained under evacuation orders as some risks remained, officials said.

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The liquid in the 7,000-gallon tank of methyl methacrylate at the Garden Grove facility started to overheat Thursday following an issue with a valve in its refrigeration system, ushering in a tense several days as first responders and experts worked to avoid an explosion.

On Monday, officials said the biggest risk, of a potential “boiling liquid expanding vapor explosion,” or a BLEVE, had been averted.

“The most catastrophic and worst-case scenario was mitigated and resolved,” interim Orange County Fire Chief TJ McGovern said at a news conference Monday evening.

“It’s not over yet, and I want to re-emphasize that: It’s not over yet. We still have work to do,” he said.

McGovern said firefighters “still have to mitigate a fire and very small explosion concern, and also a spill potential.”

Evacuation orders in the city in northern Orange County, about 35 miles southeast of Los Angeles and 4 miles from Disneyland in Anaheim, were reduced from around 60,000 to 16,000 Monday.

The incident began Thursday at a GKN Aerospace facility, where Orange County fire officials reported a “vapor release.” Further investigation found that temperatures in a tank of methyl methacrylate, a toxic chemical used to manufacture resins and plastics, were rising, officials said.

The tank overheated after a valve that is part of a circulating refrigeration system, designed to keep the tank at 50 degrees, malfunctioned, Orange County Fire Division Chief Craig Covey said Monday.

“One of the valves in that system froze up, so it was no longer being circulated,” he said. “And due to that failure, the tank went into the heating-up process because it wasn’t continuing to be chilled.”

A BLEVE and a spill at the property could have been one of the worst chemical events in state history, Covey has said.

A temperature gauge read 100 degrees Fahrenheit, the highest the gauge would go, at one point. By Monday it had dropped to 93 degrees, Covey said.

Millions of gallons of water have been poured onto the tank to lower the temperature inside, Covey said, and deluge sprinklers also mitigate the risk of a fire.

On Monday, a crack in the tank released enough pressure without causing a leak, which allowed for more offensive measures, he said. That night, a crew was able to strip insulation and other materials off the sides of the tank so the water could cool the contents more quickly, Covey said.

Covey said GKN has been responsive. “They have absolutely been hip-to-hip with us down there” and hired a contractor to assist in the incident, he said.

There have been no injuries, Covey said.

With the progress, officials reduced the size of the evacuation zone, allowing the majority of the 60,000 people temporarily displaced to return home.

Evacuation orders for the remaining 16,000 would remain in effect through Monday night and be re-evaluated Tuesday, Covey said.

Officials said at the news conference that there has been no contamination from the incident or the response efforts.

Air monitoring with 20,000 instruments in the region has detected “no exceedances” throughout the incident, Chris Myers, the federal on-scene coordinator for the Environmental Protection Agency, said Monday evening.

Garden Grove Police Chief Amir El-Farra asked for patience from those who have been told to leave their homes and from the residents of the city. He urged people to obey the existing evacuation orders.

“Please understand that we are doing this for your safety,” he said.