In 2011, there was a major accident at Xcel Energy’s Sherco power plant in Becker, Minnesota, which many described as a catastrophic failure that would likely set the company back hundreds of millions of dollars in repair costs.

Now, an analysis has been released that shows the failure was caused by corrosion and stress that struck the turbine’s blades, the Minneapolis Star Tribune reports. 

According to the media outlet, an intensive study found that cracks on the turbine’s blades were responsible for the accident, in which the components became detached from a spinning rotor inside the structure. This led to roughly $200 million in damages that are far from being completed. The utility has submitted an update on the repairs to state regulators that shows the facility likely won’t be operational again until at least September 30.

The report concluded that the accident was caused by “extensive cracking” in the components that connected the blades to the rest of of the structure. These cracks were likely the result of “stress corrosion,” the report noted, and that the accident could ultimately be traced back to a problem with “a function of the original design,” as opposed to the maintenance that was performed on the plant. The last time the blades were replaced was in 1999 after they had been working for 12 years. 

The media outlet quoted plant director Ron Brevig, who stated that “there is no way to know” exactly how many blades were involved and had succumbed to the corrosion before the accident occurred. However, this information is irrelevant, as it would only take one to cause such a problem, he added. The report found that when balance among the blades was compromised, the entire system was affected.

The repair process has put to work 150 engineers who are making improvements to the 900-megawatt turbine. 

“It has been huge,” said John Loubier, vice president of TurbinePros, just one of the many companies that is helping with the repair project. “I don’t think anyone expected the scope and duration of this project. It was a catastrophic event. There is no template for it.”

According to a previous article in the Tribune, the entire event lasted only about eight seconds, during which the bulk of the damage was done, while the ensuing fire also had a hand in affecting the plant’s energy generation. 

Companies can avoid these design failures by performing extensive tests before components are permanently installed in structures.