Backup failures at nuclear plants draw scrutiny
ATLANTA - Four generators for 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 reexamine its maintenance plans.
None of these failures, which occurred in Alabama and Virginia, 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.
A meltdown occurred this year in Japan when the Fukushima Daiichi nuclear plant lost all backup power for its cooling systems after an earthquake and tsunami.
In Alabama, three diesel generators failed after tornadoes ripped across the state in April and knocked out electric lines serving the Tennessee Valley Authority’s Browns Ferry nuclear plant. Two failed because of mechanical problems and one was unavailable because of planned maintenance.
Another generator failed at the North Anna plant in Virginia after 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 US Nuclear Regulatory Commission.
“To me it’s not an alarming thing,’’ said Michael Golay, a professor at MIT 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 reevaluate 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 roughly 100,000 people to evacuate.
In the United States, an average of roughly one diesel generator has failed when needed each year since 1997. Government researchers who examined diesel generator failures in the United States 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 percent or 3 percent, depending on which database was analyzed.
Even at low odds, a generator failure can turn serious when combined with other problems, notably human error.
A prominent example is the March 20, 1990, accident that cut off electricity for less than an hour at Plant Vogtle, roughly 25 miles southeast of Augusta, near the Georgia-South Carolina line. At the time, plant workers had just installed fresh nuclear fuel into the Unit 1 reactor. One of two lines supplying the reactor with power from the electrical grid was offline for maintenance. So was one of the reactor’s two diesel generators.
A delivery truck driver backed into a pole, knocking out the sole source of grid electricity to the Unit 1 reactor. The available diesel generator turned on, then quit. Plant workers restarted it, but it failed again. Workers bypassed parts of the diesel’s electrical controls, forcing it to run. Temperatures inside the reactor rose from 90 degrees to 136 degrees until power was restored, but the accident did not worsen. No radiation was released.
Nathan Ives, a senior manager of advisory services at Ernst & Young, said the incidents this year could prompt the nuclear industry to reexamine the kinds of component failures that can disable a generator. Reports show that TVA officials had not previously considered that a component blamed for one failure at the Browns Ferry plant could disable the entire generator.