The Pilgrim nuclear power plant in Plymouth is back online after repairs to the electrical system that runs pumps to supply cooling water to the reactor core. A spokeswoman for the Pilgrim Nuclear Power Station’s owners said the plant was reconnected to the regional power grid at 1:47 p.m. Monday. The system had been manually shut down Thursday morning. The company said plant technicians replaced a breaker and corrected a ground fault on a solenoid associated with the water cooling circuitry. They also performed other maintenance that cannot be done when the plant is operating. The Nuclear Regulatory Commission and plant operators said last week that there was no threat to public safety. Pilgrim’s owner, Entergy, said backup pumps automatically activated and plant systems performed as designed.