ATLANTA – Four generators that power emergency systems at nuclear plants have failed when needed since April, an unusual cluster that has attracted the attention of federal inspectors and could prompt the industry to re-examine its maintenance plans.
None of these failures has threatened the public. But the diesel generators serve the crucial function of supplying electricity to cooling systems that prevent a nuclear plant’s hot, radioactive fuel from overheating, melting and potentially releasing radiation into the environment. That worst-case scenario happened this year when the Fukushima Dai-ichi nuclear plant in Japan lost all backup power for its cooling systems after an earthquake and tsunami.
Three diesel generators failed after tornadoes ripped across Alabama and knocked out electric lines serving the Tennessee Valley Authority’s Browns Ferry nuclear plant in April. Two failed because of mechanical problems and one was unavailable because of planned maintenance.
Another generator failed at the North Anna plant in Virginia following an August earthquake. Generators have not worked when needed in at least a dozen other instances since 1997 because of mechanical failures or because they were offline for maintenance, according to an Associated Press review of reports compiled by the U.S. Nuclear Regulatory Commission.
“To me it’s not an alarming thing,” said Michael Golay, a Massachusetts Institute of Technology professor who studies risk at nuclear plants. “But if this trend were to continue, you’d certainly want to look into it.”
At a minimum, the failures have prompted NRC inspectors to increase their scrutiny at plants where the problems happened. Beyond that, industry officials and academics say the incidents could lead the NRC to formally warn nuclear plant operators about the recent failures and prompt utilities to re-evaluate what can disable a generator. Some think these experiences may factor into upcoming rules the NRC will issue in response to the crisis in Japan.
A single generator failure is not a calamity. All reactors have at least one backup generator and sometimes more. If the diesel generators fail, nuclear plants can run safety gear off batteries for hours or use steam-driven pumps to keep cooling water flowing.
But the loss of all emergency power – including the diesels – is a crisis. That happened on March 11 when an earthquake and tsunami disabled all the diesel generators at the Japanese plant. Three of its six reactors suffered meltdowns. The facility was rocked by explosions and released radiation requiring the evacuation of roughly 100,000 people.
In the U.S., an average of roughly one diesel generator has failed when needed each year since 1997. Government researchers who examined diesel generator failures in the U.S. from 1997 to 2003 calculated the average odds that a diesel generator would fail to work at some point during an eight-hour run were slightly greater than 2 or 3 percent.
The recent failures in the Southeast came in a tight cluster.
Tornadoes tearing across the region on April 27 broke electric transmission lines, causing a loss of grid energy at Browns Ferry in Alabama. One of the eight diesel generators serving the three reactors was undergoing maintenance. The remaining generators immediately started, supplying the plant with emergency power.
The following day, plant operators noticed a small hydraulic oil leak on one of those emergency generators, according to reports that the TVA filed with the NRC. The leak went from roughly a drop a minute to a steady spray. As the electricity from the generator fluctuated, plant staff shut it down. Two reactors briefly lost their cooling systems, although no damage occurred.
Another generator failed on May 2. TVA officials blame that malfunction on equipment that was not properly set.
NRC inspectors at the plant say they are waiting on more information before taking additional action.
A fourth failure happened when the largest earthquake to strike Virginia in 117 years rattled the North Anna Power Station. The reactor lost offsite power and its emergency diesel generators automatically started. Less than an hour later, plant operators shut down one generator because it was leaking coolant, said Gerald McCoy, an NRC branch chief who oversees federal inspectors at the plant.
“We are concerned with the fact that diesels are having issues, and that could very well be the subject of future inspections,” McCoy said.
Dominion Virginia Power says the problem was caused by an incorrectly installed gasket that eventually created the coolant leak, utility spokesman Richard Zuercher said.
Experts say no single factor appears to connect the four failures. Nathan Ives, a senior manager of advisory services at Ernst & Young, said the incidents could prompt the nuclear industry to re-examine the kinds of component failures that can disable a generator.
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