Fire in a main transformer resulted in a reactor trip and an unusual event declaration at a reactor near Houston Tuesday.
The transformer at unit 2 of the South Texas Project failed at about 4:40 p.m., according to an event report filed with the Nuclear Regulatory Commission. Although the onsite fire brigade extinguished the resulting flames in the switchyard within 16 minutes, the incident took out power to busses supplying reactor systems, which the report indicated resulted in a partial loss of offsite power. The reactor tripped from full power, and two emergency diesel generators energized the affected circuits.
According to the report, unit 2 was cooling down under natural circulation because of a loss of power to reactor coolant pumps. Auxiliary feed water systems functioned as needed, and heat was removed using steam generator atmospheric relief valves. A pressurizer power-operated relief valve momentarily opened and reclosed. The reactor is stable, unit 1 was not affected and no injuries or radiological releases were reported. Operator STP Nuclear Operating Co. ended the unusual event, the lowest of the NRC's emergency declarations, at 7:47 p.m. the same day.
The utility operates two Westinghouse pressurized water reactors at the plant 90 miles southwest of Houston, according to the NRC. Both were first licensed in the late 1980s.