Diablo has fail-safe in case of emergency

A redundant system of generators and batteries stands at the ready, backing up Diablo Canyon’s cooling systems in the event of an emergency

dsneed@thetribunenews.comOctober 2, 2011 

Engineer Rudy Ortega stands in an austere equipment room at Diablo Canyon nuclear power plant. On either side of him are rows of 60 batteries that look like industrial versions of car batteries.

In a nearby room, engineer Larry Price checks one of the plant’s six powerful diesel generators. The hulking 18-cylinder engines are the same type of motors that power barges, locomotives and Coast Guard cutters.

These batteries and generators are critical components of the plant’s backup safety equipment. They could be all that prevents Diablo’s nightmare scenario — a core meltdown that could release radioactive materials into the environment, poisoning people and animals.

In the event of an extreme emergency, the batteries would maintain power to equipment that safely shuts down Diablo’s two reactors. The diesel generators would provide long-term power to vital emergency equipment, including cooling water pumps.

Although it has historically garnered little public attention compared to the reactors and spent fuel, emergency backup equipment like this has come under unprecedented scrutiny since the magnitude-9.0 earthquake and tsunami in Japan on March 11 that crippled the Fukushima Dai-ichi nuclear power plant, causing the worst nuclear accident since Chernobyl in 1986.

The tsunami that struck the Japanese plant inundated its backup emergency equipment, leaving safety equipment without power for days, causing meltdowns in three reactors.

At Diablo Canyon and other nuclear plants, the first line of defense in a blackout is the diesel generators. Once power is lost, compressed air automatically starts the engines in less than 10 seconds, said Price, whose job it is to keep the diesel generators ready for service.

Lubricating oil is kept at a steady 100 degrees so the engines are ready to take the load of the generators without warming up. Each unit has three generators that can supply enough power to maintain the reactors in shutdown mode.

The plant is required to keep enough fuel on hand to run the diesel engines for seven days. The fuel is stored in two 50,000-gallon, watertight underground tanks.

Price describes the generators as “a really reliable engine.” Diablo Canyon is one of nine nuclear plants in the nation that use this particular design.

The engines are tested regularly to make sure they are in proper working order. During each refueling outage, the generators for that unit receive a week of round-the-clock maintenance.

In the unlikely event that the diesel generators do not work, the next line of defense is the batteries. They provide 125 volts, which is enough power to shut the reactors down and provide emergency core cooling and other necessary safety measures for two hours if the diesel generators are damaged.

“We would have two hours to get one of the six diesel generators started,” said Ortega, whose job is to oversee maintenance of the batteries.

Improving safety for worst-case scenarios

In the aftermath of the Fukushima disaster, U.S. nuclear regulators and the American nuclear industry have been re-evaluating how well plants are prepared for what are called beyond-design-basis events like the one that struck Japan. These are low-likelihood, often multiple and interconnected accidents that have very big impacts.

The Nuclear Regulatory Commission says the nation’s 104 nuclear reactors are safe and are designed

to withstand the kind of ground shaking and tsu-namis nearby earthquake faults are likely to produce.

But an NRC task force has come up with 12 recommendations for increased emergency preparedness, using lessons learned from the Fukushima accident. These include improving the agency’s regulatory framework and updating information on earthquake risk.

The commission has moved ahead on some of the recommendations. Several of the commissioners said requiring all 12 recommendations would be too rushed. Some members of Congress are pressing the agency to fully implement all the recommendations by Oct. 21.

“The NRC cannot afford to be complacent here,” said Rep. Lois Capps, D-Santa Barbara, whose district includes coastal sections of San Luis Obispo County. “My constituents who live in the shadow of Diablo Canyon nuclear power plant cannot afford it, either.”

The San Luis Obispo Mothers for Peace joined 25 other anti-nuclear groups nationwide in petitioning the NRC to suspend all relicensing and rulemaking activity at nuclear plants until there is a complete response to the Fukushima accident. The agency denied the petition in September.

John Conway, PG&E’s chief nuclear officer, recently told the county Board of Supervisors that the crippled nuclear plant in Japan survived the initial earthquake well, only to be devastated by the tsunami, which inundated the plant’s backup safety equipment and left the station without power.

For this reason, much of the emphasis since March 11 has been on flood protection, he said. For example, watertight seals on cooling water pumps at the plant are being improved to better withstand flooding.

Conway said he expects an “onslaught” of additional safety requirements from both the NRC and within the industry as more lessons are learned from the accident in Japan.

Diablo Canyon’s best defense against flooding from a tsunami is the fact that the plant sits atop a bluff 85 feet above the ocean, above where a tsunami is likely to reach.

Additionally, Diablo Canyon’s batteries and generators are housed in rooms equipped with fire doors that can be lowered in an emergency to compartmentalize the equipment within. They are also designed to operate independently and can be cross-connected to provide additional redundancy. These kinds of redundant design features are known in the nuclear industry as defense in depth.

Theoretically, Diablo Canyon’s weakest link when it comes to the tsunami threat is its auxiliary saltwater pumps. The plant uses ocean water for cooling, and the saltwater pumps are located at the cooling water intake cove.

Ventilation for these pumps comes from large, inland-facing snorkels that extend 45 feet above the intake structure. These snorkels are the plant’s lowest elevation safety equipment, but are still above a likely tsunami height, plant operators say.

Critics of nuclear power point out that Diablo Canyon’s 85-foot height does not make it immune to tsunamis. Some historical evidence suggests tsunamis along the county coastline of up to 100 feet, Mothers for Peace spokeswoman Jane Swanson said.

“The ongoing disaster at Fukushima has shown that scientists are not able to predict with certainty the greatest possible size or consequences of an earthquake at any given site,” she said. “The location of the diesel generators is just one more reason why California should accelerate its development of sustainable energy and turn away from the hazards inherent in nuclear power.”

Why backups are vital

So what is it about nuclear power plants that make backup emergency equipment and redundancy so vital? A nuclear reactor is not like an automobile engine. You can’t just turn off the ignition and walk away.

Nuclear fuel continues to generate huge amounts of heat for weeks and months after the reactor has been shut down. For example, six months after the March 11 disaster, the temperature of the cores in the three stricken reactors in Japan is still close to the boiling point of water, Conway said.

Reactor cores that do not maintain the necessary cooling water can heat up to the point that they melt or catch fire, releasing radioactivity into the environment.

This means it is vital to maintain electrical power that keeps circulation pumps and other equipment working to cool the reactors. Losing that power, as happened in Japan, can be disastrous.

“That plant was clearly underdesigned for the tsunami threat,” Conway told county supervisors.

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