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A violent storm that tore through Iowa on Monday caused an emergency shutdown at a nuclear power plant near Cedar Rapids.

The storm packing hurricane-force winds tore across the Midwest, compounding troubles for a U.S. farm economy already battered by extreme weather, the U.S.-China trade war and most recently, the disruption caused to labor and consumption by the COVID-19 pandemic.

Grain silos were ripped apart, and Minnetonka-based Cargill and Archer Daniels Midland closed crop-processing plants in Cedar Rapids.

The Duane Arnold nuclear plant lost its connection to the electricity grid. At about 1 p.m., the plant in Palo, 11 miles northwest of Cedar Rapids, declared an “unusual event” — an indication of a safety threat, according to a report posted Tuesday by the U.S. Nuclear Regulatory Commission (NRC).

An unusual event is the lowest of four levels of emergency conditions under NRC regulations. While the Duane Arnold plant is not now producing electricity, it does have power to run its emergency systems.

“The plant is stable and is using a backup power source at this time,” Duane Arnold’s majority owner and operator, Florida-based NextEra Energy Resources, said in a statement.

The storms damaged the plant’s cooling towers, which are used in electricity production to cool steam after it exits the turbine, NextEra said. The cooling towers are not part of the safety systems used to cool the reactor and other critical components.

The loss of power at Duane Arnold automatically triggered an automatic reactor “scram,” or shutdown. Standby diesel generators kicked in, giving power for the reactor’s cooling systems, the NRC report said.

“All of the safety systems functioned as designed,” said Viktoria Mitlyng, an NRC spokeswoman.

David Lochbaum, former director of the Union of Concerned Scientists’ nuclear safety project, said that loss of off-site power at nuclear power plants — usually due to storms — happens about four to five times a year in the U.S.

Iowa’s 45-year-old Duane Arnold plant, which is a little smaller than Xcel Energy’s nuclear plant in Monticello, is due to shut down later this year.

In Cedar Rapids, the storm damaged a silo at Cargill Inc.’s oilseed-processing plant. Cargill has been without power at the facility for the past two days.

“We don’t know yet when operations will resume,” a Cargill spokeswoman said. “Thankfully, all employees at our Cedar Rapids locations are safe.”

Rival grain trader Archer Daniels Midland’s corn processing plant there also is offline and being inspected for damage, a company spokeswoman said. No one was injured, she said.

The storm damaged farmland, and there was widespread property damage in both cities and rural towns and businesses and left a half-million homes and businesses in Iowa without power.

It toppled grain bins in dozens of counties and tore into livestock farms in Iowa, the nation’s top hog and corn producer. Bin losses, ahead of this fall’s harvest, could leave some farmers scrambling to find storage for their crops, said agronomists.

The storm — classified as a “derecho” — started early Monday and caused a wider scope of damage than a tornado typically would, meteorologists said. By Monday evening, it was moving east to Michigan and Indiana, and at least 500,000 people were without power, according to media reports.

“This corridor of wind went through and flattened corn and crops,” said Andrew Ansorge, meteorologist with the National Weather Service in Des Moines.

Heartland Co-op, which has dozens of grain storage facilities across Iowa, said in a statement it had sustained serious damage at 21 locations.

“Several locations are rendered inoperable and we are making contingency plans for managing the fall harvest,” the company said.

Landus Cooperative, one of North America’s largest grain-storage companies, saw damage at three of its facilities — including conveyor equipment at its Bondurant, Iowa, location, Chief Executive Matt Carstens told Reuters.

About 30% of the cooperative’s 7,000 producers farm in the path of the storm, Carstens said.

The storm crossed where about 20% of Iowa’s corn is grown, Carstens said. “There’s no doubt we’re going to lose some of that,” he said.

Includes reporting by Star Tribune staff writer Kristen Leigh Painter and Reuters.