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Two fires at the Palo Verde Nuclear Generating Station earlier this year have once again raised safety concerns at the nation's largest nuclear power plant, located 50 miles west of downtown Phoenix.
The Nuclear Regulatory Commission met with Palo Verde officials last week to discuss the fires, which broke out in Unit 2 on April 4. The agency will decide in the next three weeks whether to levy as much as $50,000 in fines against Arizona Public Service Company for apparently violating two federal regulations, NRC spokesman Mark Hammond says.
In May, an NRC inspector concluded that the fires--which broke out simultaneously in the Unit 2 control room and in a battery equipment room--could have adversely affected plant operators' ability "to achieve and maintain safe shutdown conditions."
At first, Palo Verde officials agreed with the NRC inspector. But Palo Verde officials revised their findings, and on June 11 told the NRC both fires occurred in areas where there was little potential to spread. Since the fires were "self-limiting," APS concluded, there was "no potential to adversely affect" the safe operation of the plant.
The NRC inspector disagreed with APS' conclusion, leading to the August 1 conference between the agency and APS.
"We view it [the fires] with some concern," Hammond says. "In this instance, it was fortunate that the fires were small, quickly extinguished and didn't lead to any greater problems."
The battery equipment room fire resulted from an electrical fault in a transformer that was not properly grounded. The fault caused a fire in the control room operating panel.
"The fire in the control room occurred because grounding protection that by design should have been located in the battery equipment room was actually located in the control room," an NRC statement says.
Inspections after the fires revealed that APS had failed to make sure the battery equipment room transformers were built as specified in design documents.
Palo Verde spokesman Jim McDonald says APS presented technical explanations to the NRC last week showing that the fires did not threaten the safe operation of the plant.
One reason there was no threat, McDonald says, is that Unit 2 was already shut down to reload fuel in the reactor. The reloading operation attracted nationwide attention when APS experienced difficulty in removing a stuck fuel-assembly package from the reactor core.
The stuck fuel rods were eventually removed from the reactor core without incident. But the extraction procedure required the NRC to issue a temporary permit to allow a special crane to be used to hoist the assembly from the reactor core.
McDonald says APS believes that even if Unit 2 had been running at the time of the fires, there would have been no threat to the safe operation of the plant. McDonald acknowledges that "for whatever reason," the transformers in the battery equipment room were not properly grounded.
McDonald says plant operators have made modifications to 12 circuits in each of the three nuclear generating units to prevent a recurrence of the electrical problems.
The fire and subsequent occurrences show how quickly events can spin out of control at a nuclear installation.
The control room fire was detected when an APS employee smelled smoke about 5 p.m. on April 4 in the back panel area of the Unit 2 control room. The fire knocked out some control room lights and disabled a large number of "fire detectors" in an auxiliary building.
The deactivated fire detectors triggered alarms, alerting control room operators that many of the detectors were no longer functioning.
But that created another problem. The fire in the battery equipment room went undetected for a short period because, even though the battery equipment room alarm was still functioning, it was masked on the control panel by the fire detector alarms.
"The trouble alarms masked the actual fire alarm" in the battery equipment room, an NRC report states.
Control room operators dispatched workers to inspect the auxiliary building for possible problems. A worker discovered smoke and fire in the battery equipment room.
The Palo Verde fire department immediately responded and extinguished all fires.