A small explosion in a switchyard at the Sequoyah nuclear plant triggered an unusual event notification Sunday, but safety equipment was unaffected and both reactors continued to operate at full power.At about 8 a.m., security guards reported an explosion that damaged an insulator at the switchyard of the Tennessee Valley Authority plant near Chattanooga, Tenn., according to an event report filed with the Nuclear Regulatory Commission. No one was injured, and a second offsite power source was verified. A condenser pump tripped in unit 2; otherwise reactor operations were not affected.In a release, TVA said it ended the unusual event, the NRC’s lowest emergency declaration, a couple of hours later after isolating the fault from the rest of the switchyard. Crews cleaned up about 10 gallons of spilled oil used to cool electrical equipment.