(Reuters) – Nebraska Public Power District (NPPD) on Friday declared an “unusual event” at its Cooper nuclear power station in Nebraska due to the possibility of flooding along the Missouri River following a powerful winter storm this week.

The plant continues to operate safely and “there is no threat to plant employees or to the public,” the utility said in a release.

The late winter storm, dubbed a “bomb cyclone” by meteorologists, left blizzards, floods and tornados in its wake after hitting the U.S. Mountain and Plains states this week, before pushing east into the Midwest and the Great Lakes Region early Friday.

NPPD said its workers have filled sandbags along the river levee and procured other materials and supplies for flood protection.

The biggest danger to a nuclear plant from flooding is the loss of power, which can make it difficult to cool the uranium fuel in the reactor core and the fuel stored in the spent fuel pool.

That is what caused the fuel in some reactor cores at the Fukushima Daiichi plant in Japan to partially melt down in 2011 after a giant earthquake and tsunami cut power to the plant.

Since Fukushima, all U.S. reactors have been upgraded with additional safety equipment, including portable pumps and generators to keep cooling water circulating through the reactor in case the plant loses offsite power.

NPPD said its procedures require it to declare an unusual event to the U.S. Nuclear Regulatory Commission when the Missouri River tops 899 feet above sea level. It reached 899.05 feet Friday morning, the company said.

Should the river rise to 900 feet above sea level, NPPD said plant workers will “barricade internal doorways as another layer of protection for facility equipment.”

If the river reaches 901.5 feet above sea level, NPPD said it would take the station offline as a protective measure.

The plant was built at 903 feet above sea level, which is 13 feet above natural grade, NPPD said.

The Cooper station is three miles (4.8 km) southeast of Brownville, Nebraska, near the Missouri River.

(Reporting by Scott DiSavino; Editing by David Gregorio and Richard Chang)