NEWPORT, Mich. — DTE Energy’s Fermi 2 nuclear reactor has been operating at reduced power for at least a couple of days because of what’s been described as unexpected — though not serious — pump vibrations.

The plant never got back to full power since its May 5 restart after sitting idle for three weeks because of a transformer malfunction.

Nuclear plants typically take several days to achieve full power after any restart.

But the pump-vibration problem caused operators to hold the reactor at 90 percent power or less for a few days.

Then, on Wednesday night, the decision was made to manually insert enough control rods to bring the reactor down to 63 percent power, a level the utility felt was reasonable for pump repairs to be made, Stephen Tait, DTE spokesman said.

U.S. Nuclear Regulatory Commission online records show Fermi 2 was continuing to operate early Friday morning at 63 percent.

It is unclear how long it will take for repairs to be made.

The vibrations are not a threat to the public or plant employees, Mr. Tait said.

Automatic safety systems brought the reactor down for an unplanned shutdown on April 14 because of the transformer malfunction. Workers repaired the transformer and a valve on the reactor recirculation system.

The work was overseen by onsite NRC inspectors.

Prema Chandrathil, NRC spokesman, has said the affected transformer is offsite and provides power to several safety and non-safety systems. The safety-related systems were powered by emergency diesel generators that were automatically activated.

Sensors detected a loss of power to pumps and a subsequent reduction in feedwater flow.

The plant is in northern Monroe County, along western Lake Erie and about 30 miles north of Toledo.

Contact Tom Henry at thenry@theblade.com, 419-724-6079, or via Twitter @ecowriterohio.

First Published May 18, 2018, 8:15 p.m.