The Associated Press’ Ray Henry has been reviewing NRC Event Reports and filed the following today:
Four generators that power emergency systems at U.S. 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.
The cluster involved incidents at two nuclear power plants:
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.
Four diesel generator failures in six months are significantly more than expected:
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, depending on which database was analyzed.
Despite the odds, generator failure can have serious consequences:
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 (40 kilometers) 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 poorly supervised delivery truck driver backed his truck 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 finally bypassed parts of the diesel’s electrical controls, forcing it to run. Temperatures inside the reactor rose from 90 degrees Fahrenheit (32 degrees Celsius) to 136 degrees F (58 degrees C) until power was restored, but the accident did not become more serious. No radiation was released.
Both Browns Ferry and North Anna showed how initial failure can quickly be compounded:
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.
North Anna also piled failure on failure:
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. The power company and NRC officials are still examining the incident.
The Browns Ferry and North Anna incidents may prompt new rules about generator reliability:
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. 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.