An electrical grid disturbance on March 3, 1989, caused the Unit 3 reactor at the Palo Verde Nuclear Generating Station in Arizona to automatically shut down. Following the shut down, the operators were unable to open the atmospheric dump valves (ADVs) from either the main control room or the remote shutdown panel. The ADVs are on the metal pipes carrying steam from the steam generators to the turbine. After a shut down, the turbine no longer accepts the steam, so the ADVs allow the non-radioactive steam to be discharged to the atmosphere to protect the pipes from failure caused by excessively high pressure.An operator was dispatched to open the ADVs at their local control panel. The operator found the area totally darkened. The normal lighting for the area was de-energized following the shut down. The emergency lighting in the area was improperly positioned. The emergency lighting was not functioning at all in other portions of the building.
The NRC discovered that the quarterly periodic maintenance and testing of the emergency lighting—intended to demonstrate that the lights would come on and remained lit for at least 8 hours—had not been performed in over a year.
The NRC also found that when the tests were performed, workers had replaced batteries for the emergency lights and cleaned their battery terminals before performing the tests. So the tests did not really indicate whether the lights would be expected to work in a real emergency.
And in fact, when called upon in this event, the emergency lighting units did not function, probably because their batteries had not been replaced and their terminals had not been cleaned.
Plant workers investigated the lighting problems. They found that in some areas, there would not have been sufficient lighting to perform required tasks even if all of the emergency lighting had worked.
They installed additional lights. A belatedly bright idea.
Our Takeaway
Nuclear power safety is not a game. Replacing batteries and cleaning terminals right before a performance test is no performance test at all—it’s a sham. Luckily, the sham at Palo Verde impeded but did not prevent a safe outcome. Nuclear power safety needs to be taken seriously so that skill remains the primary safety barrier with luck in reserve.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.