Four generators that power emergency systems at U.S. nuclear power plants have failed when needed since April, an unusual cluster, which has attracted the attention of federal inspectors and the industry was asked to re-examine the maintenance plans.
None of these errors threatened public. But the diesel generators, the critical function of delivering electricity to a power plant cooling systems is hot, radioactive fuel to prevent overheating, melting and possibly releasing radiation into the environment. The worst-case scenario happened this year when the Fukushima Dai-ichi Nuclear Power in Japan lost all backup power for cooling after an earthquake and tsunami.
Three diesel generators are not after tornadoes rip in Alabama and the knock out power lines serving the Tennessee Valley Authority Browns Ferry nuclear plant in the April. Two failed due to mechanical problems and was not available due to planned maintenance.
Another generator at the North Anna plant in Virginia after an earthquake in August. Generators did not work when needed in at least a dozen other cases since 1997 due to mechanical failure or because they were offline for maintenance, according to an Associated Press review of reports prepared by the U.S. Nuclear Regulatory Commission.
“For me it is not an alarming thing,” said Michael Golay, a professor at the Massachusetts Institute of Technology who studies the risk of nuclear installations. “But if this trend would continue, would you really want to watch.”
At a minimum, the NRC inspectors following their failure to increase research in factories where the problems occurred. Beyond that, industry officials and academics say the incidents could lead to the NRC for formal plant operators to warn the recent failures and fast tools to re-evaluate what could be a generator off. Some think that this experience may factor into future rules of the NRC will respond to the crisis in Japan.
A generator failure is not a disaster. All reactors have at least one backup generator and sometimes more. If the diesel generators fail, the nuclear safety equipment batteries run for hours or use steam-driven pumps to keep cooling water flows.
But the loss of all emergency generators including diesel _ _ is a crisis. That happened on March 11 an earthquake and tsunami disabled all diesel generators for the Japanese factory. Three of the six reactors suffered meltdowns. The facility was rocked by explosions and radiation released by the evacuation of about 100,000 people.
In the U.S., has an average of about a diesel generator fails when needed each year since 1997. The government researchers diesel generator failure investigation in the USA 1997-2003 averaged the probability that a diesel generator would not work at some point during an eight-hour run was slightly larger than 2 or 3 percent, depending on which database was analyzed.
Even with a low probability, can not run a generator in combination with other serious problems, including human error.
A prominent example is the March 20, 1990, accident that cut off electricity for less than one hour at Plant Vogtle, about 25 miles (40 kilometers) southeast of Augusta, near the Georgia-South Carolina line. At that time, plant workers have just installed new nuclear fuel in a reactor unit. One of the two lines supplying the reactor with a power grid was offline for maintenance. This was one of two diesel generators of the reactor.
A poor supervision truck driver backed his truck into a pole, knocking out the only source of electricity from the grid to a reactor unit. The available diesel generator switched on, then stop. Plant employees resumed, but failed again. Employees eventually bypassed parts of the diesel electric controls, making it perform. Temperature in the reactor increased from 90 degrees Fahrenheit (32 degrees Celsius) to 136 degrees F (58 degrees C) until power was restored, but the accident not serious. No radiation was released.
The recent failures in the southeast was in a tight cluster.
Tornadoes tear in the region on April 27 broke electric transmission lines, causing a loss of grid power at Browns Ferry in Alabama. One of the eight diesel generators to serve the three reactors were in service. The other generators began immediately, providing the plant with emergency power.
The next day, plant operators noticed a small hydraulic oil leak on one of those emergency generators, according to reports filed with the TVA to the NRC. The leak was a drop of about one minute at a steady spray. Because the electricity from the generator varied, plant personnel shut. Two reactors recently lost their cooling systems, although no damage occurred.
One generator failed on May 2 TVA officials blame that failure on equipment that was not properly set.
NRC inspectors at the plant say they are awaiting more information before taking further action.
A fourth failure occurred when the largest earthquake to strike in Virginia 117 years rattled the North Anna Power Station. The reactor lost offsite power and the emergency diesel generators started automatically. Less than one hour later, plant operators shut down a generator, because it was leaking coolant, said Gerald McCoy, an NRC branch chief that the federal inspectors monitoring the plant.
“We are concerned with the fact that diesels have problems, and it may well be the subject of future inspections,” said McCoy.
Dominion Virginia Power says the problem was caused by an improperly installed gasket that ultimately made of coolant leakage, utility spokesman Richard Zuercher said. The utility and NRC officials are still examining the incident.
Experts say there is no single factor seems to connect the four failures. Nathan Ives, a senior consulting manager at Ernst & Young, said the incident, the nuclear industry asked to re-examine the types of component failures that disable a generator. Reports show that TVA officials have not previously considered that a debt component of a failure of the Browns Ferry plant, the entire generator off.
Ives, a licensed senior reactor operator who advises utilities on maintenance, said that while he did not think that the clusters illustrated a larger problem, he believed they were worth testing.
“Everyone, including myself, always concerned about a diesel failure,” he said.
John Lane, a senior risk and reliability engineer in the NRC Office of Nuclear Regulatory Research, said he had contact with other scientists for an off-the-cuff discussion about failures. He said it remains unclear how large the numbers are.
He compared the situation with the toss of a coin. As you would expect to get heads half the time when dozens of countries, it is possible that there may be an increase in heads over a shorter period of throws.
“If you are a coin 10 times, you’re subjected to 6 or 7 or 8 cups to get,” said Lane. “And our feeling is that is essentially what it is. … That does not mean it’s not a fair coin.”
Failures are reduced considering that she once hovered above 10 percent in the early days of the nuclear energy industry, according to NRC reports. Members of the Nuclear Energy Institute, an industry lobby group, years ago, aimed at obtaining reliable speeds of up to 95 percent, said Alex Marion, NEI vice president of nuclear operations.
Marion said that every time diesel generators fail, officials planted the root problem and to determine whether a one-time fluke or a potentially larger problem that could happen in other generators. The results are shared by industry. Marion said the NRC methods exaggerate the risk of generator failure because once a problem is identified and fixed, it’s not recur.
“We are constantly learning and developing and evolving,” he said.
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