If a picture is worth a thousand words...
The Union of Concerned Scientists' Nuclear Safety Project Director, David Lochbaum, writes:
Last December, a worker discovered an open phase condition on a backup power source for the Unit 3 reactor at Oconee. The NRC dispatched a special team to the site to investigate. UCS described the near miss in commentary posted to our blog at http://allthingsnuclear.org/dlochbaum/special-inspection-oconee-unplugged
[Beyond Nuclear has also posted Lochbaum's previous analysis, with his permission, at the entry below.]
Since then, I found two pictures of the open phase condition at Oconee. This picture clearly shows the open phase condition [see photo, top left].
The open phase condition resulted from the broken connection flapping in the breeze in the center of the photograph. The wire is a seven strand cable (six aluminum conducting wires around a central steel wire).
This photograph shows the broken wires on the right and where they were supposed to be connected on the left [see photo, bottom left].
The seven strand cable is connected via four bolts, shown on the left. Oconee's owner theorized that movement of the cable back and forth over years broke all seven wires, similar to how bending a paper clip back and forth eventually breaks it.
This cable was one of three cables carrying the three-phase electrical current from the 230 kilovolt switchyard to a transformer that reduced its voltage level for use inside the plant.
Conveniently, workers found a similarly damaged cable on Oconee Unit 1 a week later. It was convenient in that if the severed wires had been found the same day that the problem on Unit 3 was found, at least one of the reactors would have had to be shut down. But by waiting to find the problem on Unit 1 until after the problem on Unit 3 was fixed, the reactors could continue operating. Very, very convenient. Suspiciously convenient.
The open phase condition problem that the seven NRC engineers seek to resolve is that existing monitoring of three-phase electrical systems does not always detect problems. For example, it's not known how long this broken cable floated free at Oconee before a worker noticed it. The systems that are supposed to continuously monitor the three phases have a design defect that essentially blinds them to certain single-phase failures. That's the problem the seven NRC engineers petitioned the NRC to fix.
The original photographs are in the NRC's ADAMS thing under ML16027A069.
Thanks,
Dave Lochbaum