In the early morning hours of April 26, 2018, workers at the Husky Energy refinery in Superior were preparing for a planned shutdown to allow for routine maintenance throughout the facility.

Beginning at 5:40 a.m., crews started shutting down the fluid catalytic cracking unit where crude oil is distilled into gasoline and other products. For over four hours, a valve meant to keep air and flammable hydrocarbons in separate chambers within the cracking unit was closed.

Just after 10 a.m., an explosion ripped through the cracking unit injuring 36 refinery workers and launching shrapnel into a nearby asphalt tank. The blast was heard and felt throughout Superior and the initial fire it caused at the refinery was quickly controlled.

But the day's events were just beginning.

Two hours after the explosion, more than 15,000 barrels - 630,000 gallons - of hot asphalt from the punctured tank ignited, burning into the evening and sending thick black smoke into the air for hours.

Over concerns that a tank containing hydrogen fluoride - a highly toxic chemical - could rupture, much of Superior evacuated.

Months later, the U.S. Chemical Safety and Hazard Investigation Board would determine the valve in the cracking unit meant to keep air and flammable hydrocarbons separate was worn, allowing the two to create a mixture which would later ignite and explode.

Two federal agencies said Husky didn't consider that scenario.

Investigators ID cause, solution

In separate investigations, both the Chemical Safety Board and the Occupational Safety and Health Administration found that the process hazard analysis - a guide outlining possible industrial risks - did not consider the scenario that caused last year's explosion.

OSHA recommended the company update their process hazard analysis for the cracking unit to include the scenario that caused the explosion while the Chemical Safety Board's recommendations have not yet been released.

In a recent interview with the News Tribune, refinery manager Kollin Schade said the refinery had improved its process hazard analysis and other safety features of the cracking unit shutdown procedures.

"That's a time when things are changing - we're outside of our norm," Schade said of startups and shutdowns. "And that's really where the improved procedures and training that we're going to have going forward, and all the safety elements, layers of protection that we're to incorporate will prevent this accident from happening in the future."

The rebuilt cracking unit and much of the refinery won't be operational until at least 2020 or 2021, Husky officials said earlier this month.

Husky Energy Chief Operating Officer Rob Symonds said the company is working to ensure the new safety procedures aren't just written down in a manual but are actually retained by workers.

"We're going through each of those procedures to ensure that they're as clear as they need to be and the operators who have to do them are comfortable," Symonds said.

In an interview with the News Tribune earlier this month, Superior Mayor Jim Paine said he was disappointed Husky continues to use hydrogen fluoride, but was more worried about the company fixing the root cause of the explosion.

"My biggest concern remains explosions, and the biggest thing I pressed them on the other day was - are they going to be making the improvements that the (U.S. Chemical Safety and Hazard Investigation Board) has recommended to prevent explosions during shutdown again?" Paine said.

Now, Paine said, Husky needs to be held accountable to those safety standards.

"Safety remains our No. 1 priority. We need this to be safe within the city of Superior," Paine said. "Even the safety improvements they're making right now, we need to be continually evaluating and improving the safety of that refinery."

Hydrogen fluoride concerns linger

Nearly four hours into the evacuation, the News Tribune was first to question Husky and government officials about hydrogen fluoride. While Schade confirmed the chemical was at the facility, he did not directly answer questions about the chemical's risk during a 5 p.m. news conference as the fire still burned.

But a Superior Fire Department official confirmed to the News Tribune that evacuations were based on the "worst-case scenario" - a rupture of the hydrogen fluoride tank that could have caused a toxic cloud of gas for miles downwind.

While no hydrogen fluoride was released during the incident and the fire never reached the hydrogen fluoride - which was 150 feet away from the location of the explosion - shrapnel was flung 200 feet, the Chemical Safety Board said.

A News Tribune investigation found the chemical threatens a 25-mile radius surrounding the refinery under a worst-case release scenario. While the refinery can handle about 78,000 pounds of hydrogen fluoride, according to federal EPA records from 2012, it contained about 15,000 pounds of the chemical at the time of the fire.

In the last year, people throughout the region voiced concerns over the chemical, which can cause death from an irregular heartbeat or from fluid buildup in the lungs when inhaled at high levels or in combination with skin contact, and have called on Husky to look at alternatives like sulfuric acid.

Husky decided to keep using hydrogen fluoride after determining several alternatives weren't viable. Instead, the company vowed to improve safety around the hydrogen fluoride tank by adding a separate tank where the chemical could be dumped in a leak, a laser detection system and additional layers of water curtains, which could suppress the chemical in the event of a leak.

Ginger Juel, co-founder of the Twin Ports Action Alliance, a group opposed to the chemical's use, said those changes aren't good enough.

"A mitigation system can be impaired by flying shrapnel from explosions," Juel said earlier this month.

The Chemical Safety Board this week said concerns by Superior residents were heard loud and clear and in a letter this week to the Environmental Protection Agency, which oversees the chemical, the board asked the EPA to reconsider its 26-year old safety study on hydrogen fluoride and weigh the possibility of safer alternatives.

Kristen Kulinowski, the board's interim executive, said: "Refinery workers and surrounding community residents are rightly concerned about the adequacy of the risk management for the use of hazardous chemicals like (hydrogen fluoride)."