Three Mile Island Trips Due to Flow Imbalance in Coolant Pump

Exelon’s 852-MW Three Mile Island Generating Station near Harrisburg, Pa., on Thursday automatically tripped owing to a flux to flow imbalance of the "C" reactor coolant pump, a filing with the Nuclear Regulatory Commission (NRC) shows.

Exelon told the NRC that the electrical grid was stable and Unit 1 was being supplied by offsite power. "Preliminary evaluation indicates that all plant systems functioned normally following the reactor trip, except for automatic operation of turbine bypass valve control due to failure of the automatic control function to control precisely at setpoint," it said. "Three Mile Island remains stable in hot shutdown mode while conducting the post trip review."

Repairs were being made to the coolant pump, Exelon said on Friday, though it did not indicate when the reactor would resume operations. During shutdown, steam was released into the atmosphere, "creating a loud noise heard by nearby residents," the company said in a statement.

Three Mile Island Unit 1 began commercial operation in 1974, followed by commercial operation of Unit 2 in 1978. Both units were Babcock & Wilcox pressurized water reactors. In 1979, following the infamous partial meltdown of Unit 2—which did not result in deaths or injuries to plant workers—Unit 1, which had been offline for refueling was kept out of service until 1985. The reactor finally received a 20-year operating license renewal in 2009 that will now expire in 2034. 

Sources: POWERnews, NRC, Exelon

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