Federal officials are investigating whether sediment drawn from the Mississippi River blocked a sprinkler system designed to protect against fires at the nuclear generating station in Monticello, Minn.

Xcel Energy Inc., the Minneapolis-based owner of the power plant, recently discovered that the fire-suppression system failed a surveillance test. The system is in a building that is separate from the reactor structure.

The utility reported the finding on Sept. 2 to the Nuclear Regulatory Commission, which this week sent a three-member special inspection team to investigate the blockage and whether it could happen at other nuclear reactors.

"There is no immediate safety concern here," Cynthia Pederson, deputy regional administrator for the NRC, said in a statement Tuesday. "Nevertheless, we want to gain a better understanding of this issue, how and why it occurred, and its implications for Monticello and for other plants."

The problem is confined to an intake building, about the size of a small warehouse, that contains pumps to draw river water for emergency systems, officials said. Xcel has stationed a fire watch and has backup fire suppression equipment at the intake building while pipes are cleaned and the sprinkler system is restored.

NRC spokeswoman Viktoria Mitlyng said investigators will assess whether sediment caused the problem, as now suspected, and whether other safety systems at the Monticello reactor or other U.S. plants are at risk. Many nuclear power plants rely on water from rivers or pools during emergencies, she said.

Xcel also is investigating the cause of the blockage. The agency's report on the matter is expected in 45 days.

David Shaffer • 612-673-7090