A magnitude 4.0 earthquake could be felt by workers at the Seabrook nuclear plant Tuesday, but they quickly confirmed it had little effect on the facility.

Seabrook nuclear plant. Souce: NRCSeabrook's 1,295 megawatt Westinghouse pressurized water reactor was down for refueling, and according to a report filed with the Nuclear Regulatory Commission, immediate walkdowns at the plant revealed no damage. An unusual event was declared at 7:20 p.m. after the shaking began. As the station's seismic monitor was out of service for a scheduled calibration, operators confirmed the quake by calling the Manchester Dispatch Office, the report said.

Later that evening NextEra Energy, which owns the plant near Portsmouth, N.H., issued a release stating, "There has been no impact to the plant from the earthquake and our outage activities have not been affected in any way."

Crews also inspected casks of spent fuel at the site of the decommissioned Maine Yankee plant, which shut down in 1996. A plant spokesman told the Bangor Daily News that the quake had no effect on the containers.

The earthquake's epicenter was reported to be 20 miles west of Portland, Maine.