OMAHA, Neb. (AP) — A Nebraska nuclear power plant on the west bank of the Missouri River has declared an emergency because of the rising river.

The Omaha Public Power District, which operates the Fort Calhoun plant, says Monday that the river has reached a lever where the company is declaring a “notification of unusual event,” which is the lowest of four emergency levels that are standard in the U.S. nuclear industry.

The utility says it has notified the Nuclear Regulatory Commission and emergency management agencies in Nebraska and Iowa.

The Fort Calhoun plant, which is about 20 miles north of Omaha, has been shut down since early April for refueling. The OPPD says in a statement no release of radioactive material requiring offsite response or monitoring has occurred or is expected.