Both units of the Calvert Cliffs nuclear power plant in southern Maryland were offline for 10 days after water leaked through the plant’s roof on Feb. 18, causing a ground fault at Unit 1 that led to a series of events resulting in the automatic shutdown of both units.
David Fitz, senior corporate communications consultant for Constellation Energy Nuclear Group, told POWERnews on Monday that Unit 2 had returned to service on Sunday. Unit 1 had been nearing a scheduled refueling outage, so it remains offline, and refueling work has begun.
A team from the Nuclear Regulatory Commission (NRC) was at the plant last week conducting an investigation. They left on Friday and will document their findings and conclusions in a report to be issued within 45 days of the end of the inspection.
An NRC news release on Feb. 22 gave this explanation of the event: “one of two electrical distribution buses for the site was lost at about 8:24 a.m. A bus is an electrical connection between multiple electrical devices. This loss of power caused Unit 2’s reactor coolant pumps and reactor to automatically trip. One Unit 1 reactor coolant pump also tripped, resulting in a unit trip.
“The loss of the electrical bus also caused a loss of power to some safety systems. Nuclear power plants are designed with backup power sources, typically emergency diesel generators, to provide power to essential safety systems. At Unit 1, one of the emergency diesel generators (EDG) started as expected and supplied power to the affected safety bus. The Unit 2 EDG initially started, but then tripped. Workers tied-in another power source to return power to the Unit 2 bus and its associated redundant safety equipment.”
The NRC added that “All other safety systems responded as designed.”
Fitz told POWERnews that a roof leak caused water intrusion into a breaker cubicle. The water then caused a fault on the bus that feeds a reactor coolant pump. “The protective relaying for the bus should have isolated the Unit 2 reactor coolant pump from the Unit 1 bus. A relay in the protective relaying scheme failed. The relay is being examined for cause of failure.” He also noted that other similar relays have been tested satisfactorily and that they are non-safety related.
The loss of reactor coolant pumps and subsequent loss of reactor coolant flow caused the dual unit automatic shutdown, Fitz said.
When asked how frequently routine inspections of the roof are made, Fitz said, “We routinely inspected our roofs during our operator walk downs and management housekeeping tours. It was very small leak, not something you see on a normal tour of the area.”
As for the diesel generator that started but then tripped, Fitz explained: “One diesel, which is one of the several backup power supplies to an electrical bus not associated with the reactor coolant pumps, started but shut down shortly after it started. Operators were able to supply power from an alternative power source to the bus the emergency diesel provides backup power to. Each bus has several potential power supplies. The emergency diesel generator that did not run correctly has since been repaired and successfully tested.”
The plant on the western shore of Chesapeake Bay is owned by Constellation Energy Nuclear Group and consists of two pressurized water reactors with a total capacity of 1,750 MW. Unit 1 went into service in 1975 and was followed by Unit 2 in 1977. This event was the first time since 1987 that both reactors shut down simultaneously.
In March 2000, Calvert Cliffs became the first plant in the U.S. to earn 20-year extensions of its operating licenses from the NRC. In July 2007, UniStar (a joint venture by Constellation Energy and Electricité de France) submitted an application to the NRC for a combined license to build a third reactor, a 1,600-MW AREVA EPR, at the Calvert Cliffs site. That application remains under review.
Sources: NRC, Constellation Energy Nuclear Group, POWERnews, Baltimore Sun