SANTA ANA, Cailf. — Crews have confirmed that a crack inside a malfunctioning chemical tank at a Garden Grove aerospace plant is indeed reducing pressure inside the container, leading fire officials to say they have turned a corner in heading off the most catastrophic threat — a massive explosion.

Data collected from an “all-night mission” by fire crews determined that the threat of a highly destructive blast known as a “boiling liquid expanding vapor explosion,” or “BLEVE,” has been eliminated, interim Orange County Fire Authority Chief TJ McGovern announced in a Monday morning, May 25, video update.

“The crack is there,” Division Chief Craig Covey added. “We had identified it is there, and the tank has released it pressure.”

After days of often dire warnings by fire officials and experts, the Monday morning announcement was the most optimistic update yet by those working to head off a potential disaster. A boiling liquid expanding vapor explosion — which would have likely caused massive damage and released a toxic chemical plume into the air — occurs when a sealed, pressurized container holding a liquid chemical ruptures.

“The threat of a BLEVE is off the table,” McGovern said. “That threat has been eliminated.”

The temperature inside the tank has also gone down, from at least 100 degrees to 93 degrees, the chief said. That is a key metric: At a high enough temperature, the chemical process was at risk of accelerating out of control — which could lead to a massive explosion.

Addressing concerns from some in the community about the lack of regular updates on that internal tank temperature, Covey said that authorities had been allowing the tank to “settle” during the warmer hours and then sending crews in to check on it during the cooler overnight hours.

“We were not doing tank temperature checks during the day when the sun was on it in the most extreme conditions for that tank to go in the wrong direction,” the division chief said. “We were only doing tank temperature checks at night.”

All that means that the worst case scenario of mass damage to nearby homes and businesses, starkly illustrated in “blast zone” maps OCFA released days ago, is no longer one of the likely outcomes.

But it doesn’t mean the danger has fully passed.

A smaller explosion would still release chemicals and send debris from the plant flying. Or a leak could release the chemicals from tank. In either case, wind could disperse a chemical plume farther into the community.

That means the evacuation of tens of thousands of residents in Garden Grove and nearby communities continues. Those evacuations have lasted for four days, through the holiday weekend.

“We want to be clear that the evacuation zones are still in play,” McGovern said.

At some point — when the threat of any explosion or leak is eliminated — the evacuation zones will be strategically re-assessed, the chief said in a Monday morning interview following the video update. But there isn’t a time frame yet for when that will occur.

“When the evacuation zone decreases,” McGovern said, “repopulation occurs.”

When they first arrived at GKN Aerospace on Western Avenue on Thursday, fire crews were told by a project manager that there was nothing they could do: A tank filled with 7,000 gallons of methyl methacrylate, a highly flammable chemical, was going to fail, either spilling and sending the chemicals pouring into a parking lot or exploding with a fireball that would send a plume of chemicals into the wind.

“For a first responder to hear that there’s no options is unacceptable,” McGovern said in the interview. “We knew we had to come up with options, solutions. That’s what we do.”

While fire officials publicly warned of the likelihood of an explosion or leak, they also began troubleshooting with a group of experts, including specialists with ties to Los Angeles, San Bernardino and Long Beach.

Gummed up valves and concerns that drilling into the tank may spark a reaction with the volatile chemicals seemed to rule out obvious solutions. But a continued water deluge of the tank appeared to keep an explosion or leak at bay, giving the experts time to craft potential out-of-the-box solutions.

“We developed a think tank,” McGovern said. “These gentlemen got into a room and started developing options. That was a success. These are unprecedented issues.”

The current goal is to keep lowering the temperature of the tank, down to the 50-60 degrees, in order to stabilize the situation and put some of those solutions the experts have been crafting into play.

Crews and chemical experts — at their own personal risk — have carried out the overnight operations to get a first-hand assessment of the compromised tank. That led to the discovery — overnight on Saturday — of the crack at the top of the tank.

They don’t know if it is a new crack, or one that previously existed. Since it is at the top of the tank, the crack has not yet resulted in any chemicals leaking out.

The crews went back in on Sunday night, McGovern said, and confirmed the crack was releasing pressure.

Then, the chief said, they began removing some of the outside skin of the tank and the insulation around it. Those layers were keeping the temperature inside the tank high, despite the cooling water crews were spraying it down with.

Once the water was directly hitting the tank, the temperature started to fall.

“What we found was the internal temperature started dropping,” McGovern said.

The chemical also appears to be starting to solidify, McGovern said. But that process seems to be happening from the outside in, so the current density of the center is unclear.

“We need to see the density inside,” the chief said. “It solidifies into a plastic form. We don’t know yet if all the substance has solidified.

“That’s what we believe is going on. That’s what the readings are showing,” he added. “At the end of the day — and I don’t meant today, but at the end of the day — if we open that tank up, (there’s) a big chance that it’s a big plastic pill.”

Air quality experts, who have been tracking the atmosphere near the tank, have not detected any contaminants so far. But the chief said that has made it challenging at times to impress upon people the reason for the evacuations. Unlike the smoke and flames of a wildfire, the danger of a failing chemical tank isn’t as obvious to the naked eye.

“There’s nothing in the air,” the chief said.

Even with the internal temperature of the tank appearing to be trending down, authorities say it is still too risky to send crews in during the day. So, they will again wait until Monday night to directly approach the tank.

In the event of a new crack that would allow chemicals to leak out, crews have set up a system of sand barriers to divert the flow and prevent the chemicals from entering storm drains or river channels.

In an “earliest, best case scenario,” changes to the evacuation orders still wouldn’t come until at least Tuesday, if not later, OCFA Capt. Nick Garton said.

Gov. Gavin Newsom, meanwhile, declared a state of emergency on Saturday afternoon, opening the way for extra resources.

Newsom and other California leaders have also formally requested a federal emergency declaration to aid with the ongoing response at the aerospace company. That would include federal cost-sharing, for a response that has already run up a hefty price tag not only for the efforts to address the compromised tank but also to evacuate and shelter so many people.

The White House, in a statement released Monday morning, said the administration is “engaged and monitoring the situation in Garden Grove.”

“The U.S. EPA has integrated with the local Unified Command composed of state and local agencies, and has enabled air monitoring at 20 locations around the area,” the White House said. “FEMA has deployed a Liaison Officer to coordinate with officials and has also deployed a team to the State Emergency Operation Center to support incident contingency planning. FEMA has also activated the Interagency Modeling and Atmospheric Assessment Center to provide plume modeling of airborne hazards, informing incident leadership and contingency planning.”

The White House statement did not address the formal federal emergency declaration request.

While bringing the crisis to a close has been the main focus, the owners of GKN Aerospace have already come under heavy criticism for the apparent failure of any emergency systems involving the chemical tank.

The United Kingdom company — a worldwide leading manufacturer of cockpit windows, jet canopies and aviation widows for civil and military aircraft — has been operating in Garden Grove for decades, and employs more than 500 local workers. It previously paid a nearly $1 million fine to settle numerous environmental violations.

Orange County District Attorney Todd Spitzer has already announced that his office is investigating the current failure of the chemical tank. And at least half a dozen law firms pledged to file lawsuits seeking class-action status on behalf of residents and businesses impacted by the evacuations.

Most of the shelters that opened in the wake of the evacuations remained at capacity early Monday, with authorities indicating that only a new Los Alamitos High School shelter and Golden West College were not full.

(Staff writers Rachel Fobar and Kaitlyn Schallhorn contributed to this report.)

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