Unit 1 at Arizona's Palo Verde nuclear plant retroactively declared an unusual event Thursday stemming from a hydraulic line problem in November.
In a report filed with the Nuclear Regulatory Commission, the plant said a control room alarm Nov. 6 indicated low fluid pressure in the hydraulic equipment of a main steam system isolation valve.
"A plant operator was dispatched according to the alarm response procedure to perform a pre-charge check of the accumulator. A pre-charge check returns hydraulic fluid from the accumulator to a vented tank that serves as a fluid reservoir. During this operation, the fluid reservoir tank and supporting piping were damaged by the rapid expansion of nitrogen gas that had entered the hydraulic system by way of a leak across an accumulator piston O-ring," the Arizona Public Service Co. plant reported.
NRC records indicate the Combustion Engineering System 80 reactor operated at 60 percent power the following day as the inoperable MSIV was repaired, returning to full power by Nov. 10. The plant later determined the event that caused the damaged hydraulic equipment should have been classified as an explosion, which requires an unusual event declaration under NRC rules.