AVILA BEACH, San Luis Obispo County — California’s largest power plant churns out enough electricity for 1.7 million homes, yet pumps no greenhouse gases into the sky. Unlike the wind farms and solar plants spreading across the state, its output doesn’t vary hour by hour, day or night. It needs little land and less fuel.

But the Diablo Canyon nuclear plant, owned by Pacific Gas and Electric Co., is the last of its kind in the state. And in less than 10 years, it could close, ending nuclear power’s long history in California at the very moment that the state — determined to stop climate change — needs carbon-free electricity more than ever.

The first of its two operating licenses from the federal government expires in 2024, the second a year later. Federal regulators are weighing whether to renew those licenses and keep Diablo humming through 2045. PG&E, however, appears to be having second thoughts.

Once eager to extend Diablo’s licenses, company executives now say they aren’t sure. Since the deadly 2010 explosion of a PG&E natural gas pipeline beneath San Bruno, their focus has been on reforming the company and repairing its image, not relicensing Diablo.

And any extension will involve a fight. The plant sits within a maze of earthquake faults, all of them discovered after construction began in 1968. Seismic safety fears have dogged the nuclear industry in California for more than 50 years, forcing PG&E to abandon plans for one of its first reactors.

“We’ve got a lot on our plates, and we just don’t need to take on another big public issue right now,” said Tony Earley, PG&E Corp.’s CEO.

If Diablo closes, no nuclear plant will take its place. California law forbids building more until federal officials come up with a permanent way to deal with the waste. Thirty-nine years after the law passed, that still hasn’t happened. The state’s only other commercial reactors, at the San Onofre Nuclear Generating Station north of San Diego, closed for good in 2012 after a small leak of radioactive steam revealed defective equipment.

Many environmentalists have long dreamed of a nuclear-free California. Global warming hasn’t changed their minds. They don’t trust PG&E’s claim that Diablo can withstand the worst quake likely to strike the area in 10,000 years and call the plant an American Fukushima waiting to happen.

“It should be illegal,” said Linda Seeley, 71, a retired midwife who in the 1980s was arrested twice during mass demonstrations at Diablo’s gates. “They’re playing with fire, and the people who will get burned are the people who live here.”

Not long ago, nuclear power appeared poised for a renaissance, as utilities concerned about climate change planned new reactors across the country. The nuclear spring never came. Deterred by high costs, long construction schedules and public resistance, most power companies built gas-burning plants, wind farms and solar facilities instead.

Diablo represents nuclear power’s last stand in California. But the plant’s fate may not be decided by climate change, or seismology.

It may depend on dead fish.

In October of 1957, a small reactor near Pleasanton plugged into the grid and started feeding electricity to PG&E.

It was a first. Never before had private American companies funded and built a nuclear plant for civilian use. The Vallecitos Boiling Water Reactor couldn’t generate much power — just 5 megawatts, compared with Diablo’s 2,240. But its launch heralded big things, particularly for PG&E.

The utility, based in San Francisco, saw in nuclear power a way to feed California’s booming postwar economy. Less than a year after firing up Vallecitos, the company unveiled plans for a reactor near Eureka and a “steam-electric generating plant” at Bodega Bay, about 40 miles northwest of San Francisco. At first, PG&E didn’t publicly reveal that splitting atoms would generate the steam.

The proposal rankled Bodega Bay residents, who hated the idea of any power plant spoiling their scenic shore. Their skepticism only hardened after PG&E finally acknowledged in 1961 that the plant — rebranded the Bodega Bay Atomic Park — would use nuclear power.

But public opposition alone did not kill the project. Geology delivered the fatal blow.

In 1963, PG&E excavated the plant’s construction site — and dug straight into a fault line, a strand of the San Andreas. Work ended the following year.

Unable to build in Bodega Bay, PG&E set its sights on Nipomo Dunes, south of San Luis Obispo. Conservationists and state officials, however, wanted the picturesque sweep of coastal dunes preserved as a park. PG&E entered private negotiations with Sierra Club officials to find a less-objectionable site nearby.

They settled on a ravine bordered by steep hills, about 18 miles to the northwest — Diablo Canyon. The episode triggered a schism within the Sierra Club, many of whose members decided they wanted nothing to do with nuclear power. But by the end of 1968, construction had already begun.

The first fault turned up three years later.

Jearl Strickland’s daily commute takes him through the surfer hamlet of Avila Beach, past a guarded gate and over a twisting private road that hugs the rugged coast. Most of the grassy hills, oak groves and seaside bluffs he passes remain open space, untouched by developers. Diablo’s buffer zone.

The plant finally swings into view after several miles, commanding its own bluff 85 feet above a sheltered cove. Strickland started coming here before Diablo opened in 1985. Among his first assignments — solve a problem that had turned Diablo into a nationwide embarrassment.

In 1971, geologists working for Shell Oil spied an offshore fault running parallel to the coast, 3 miles from Diablo. With the plant largely built, discovery of the Hosgri Fault forced PG&E back to the drafting table, looking for new ways to fortify Diablo against earthquakes. The utility added braces and support struts for the plant’s equipment and concrete buttresses for its walls.

But in 1981, PG&E learned that many of the new supports had been installed backward, in a mirror image of their intended design. Strickland was one of the engineers tasked with straightening out the mess — essentially, fixing the fix. The process took three years and added $3 billion to the plant’s already ballooning cost. Estimated at $600 million before construction began, Diablo’s final price hit $5.8 billion, about $12.8 billion in today’s dollars.

Even after Diablo’s two reactors finally switched on, geologists kept finding fault lines nearby, the most recent — the Shoreline — identified in 2008. But after decades of analyzing the plant and its equipment, Strickland says he isn’t troubled. Diablo can survive any earthquake those faults are likely to produce, he says.

“I tell my friends and family that if there’s ever a major earthquake in this area, the place I want to be is Diablo,” said Strickland, now the plant’s director of technical services. “The last place I’d want to be is downtown San Luis Obispo, in a commercial building.”

Strickland, who speaks with the level gaze and matter-of-fact tone of a born engineer, has worked on other kinds of power plants, including hydroelectric dams and geothermal plants tapping the Earth’s natural heat. He’s convinced that California can’t meet its global warming goals without nuclear power. Solar and wind are fine but fickle sources of energy, he says. Diablo is steady.

“You need baseload power that’s going be dependable when the sun doesn’t shine and the wind doesn’t blow,” he says.

Strickland is among 1,500 people who stream into the plant each day, protected by an abundance of heavily armed guards. That kind of operation isn’t cheap. The annual cost of running Diablo — including operations, maintenance, capital projects and fuel — tops $600 million, according to documents PG&E filed with California officials in 2010. PG&E passes that expense on to its 5.4 million customers.

And yet, the plant produces so much electricity that it remains cost-effective, according to PG&E. The utility doesn’t reveal exact prices but says Diablo can generate electricity for roughly 5 to 6 cents per kilowatt-hour. In contrast, PG&E last year paid an average of 10.1 cents per kilowatt-hour to buy electricity from other suppliers, according to the company’s annual report to shareholders. More than 8 percent of all electricity generated within California’s borders comes from Diablo.

“It’s economical for our customers because of the volume,” said company spokesman Blair Jones. “Diablo is a large-volume, low-cost source of electricity.”

A San Luis Obispo resident, Strickland knows that some of his neighbors support the plant, while others emphatically don’t. He can list many Diablo opponents by name and says he’s given his phone number to several, to keep the lines of communication open.

“I’m not going to change their minds — I know that,” he said. “But I want them to have the facts.”

Before she even moved to California, Seeley had heard about Diablo Canyon. Not in a good way.

A partial meltdown at Pennsylvania’s Three Mile Island nuclear plant in 1979 had galvanized the antinuclear movement. So when Diablo’s backward earthquake retrofits came to light, the discovery made national news — and turned the plant into a fount of morbid jokes. Seeley and her then-husband took it into account when he was offered a job teaching electrical engineering at California Polytechnic State University in San Luis Obispo in 1982.

“I said to my husband, ‘It’s safe to move there, because they’ll never open that thing,” Seeley said. “I actually did not believe that they would ever open a nuclear power plant that was built backwards.”

That conviction fizzled after her family relocated to San Luis Obispo and she grasped PG&E’s determination to open the plant. Seeley was equally determined to block it. The 1986 documentary film “A Question of Power” shows her and hundreds of other activists trespassing en masse at the Diablo front gate in 1984, as ranks of law enforcement officers move in to arrest them. At one point, the camera settles on Seeley’s then-teenage daughter Julia, taken into custody with her fist raised in the air.

Stopping Diablo became a rallying cry for antinuclear activists who were convinced of the technology’s dangers, with or without earthquake faults nearby. They found an ally in Jerry Brown, California’s young governor, fast building a nationwide reputation for environmental causes. Addressing a crowd of more than 25,000 people packed into a field near San Luis Obispo, Brown in 1979 vowed to “pursue every avenue of appeal” if the U.S. Nuclear Regulatory Commission let Diablo open.

But by design, the decision to license nuclear plants rests largely with the federal government, not the states. A divided NRC granted Diablo’s operating licenses, despite Seeley, and despite Brown.

Diablo has now run for three decades without a meltdown or other major incident. Seeley isn’t reassured. Like many of the plant’s opponents, she saw in Japan’s 2011 Fukushima accident the incarnation of her fears. A tsunami triggered by a massive earthquake swamped the coastal Fukushima Dai-ichi nuclear plant, knocking out its backup power generators and cooling system. Three reactors melted down. Radioactive water from the plant still seeps into the sea.

Geologists estimate that the fault lines surrounding Diablo can produce major quakes, with magnitudes up to 7.3 for the Hosgri. That’s a far cry from the magnitude 9 earthquake that struck Fukushima Dai-ichi. But that plant lay 110 miles from the epicenter, whereas the Shoreline Fault comes within 650 yards of Diablo.

And while Diablo’s towering bluff would have blocked the 43-foot-high tsunami that doomed Fukushima, a 2003 consultant’s report for the Nuclear Regulatory Commission identified 11 scenarios in which quakes or underwater landslides could produce a tsunami tall enough to damage Diablo.

With each new fault discovery, PG&E has conducted more seismic studies and declared Diablo safe. But the company has repeatedly changed its methods for estimating the amount of shaking that the area’s faults can produce. Critics, including some trained in geophysics, have accused the utility of tinkering with the math to create an illusion of safety.

The San Bruno explosion only added to their suspicion of PG&E. Investigators blamed the blast, which killed eight people, on a lax attitude toward safety within the company. PG&E now faces a $1.6 billion penalty, as well as criminal obstruction-of-justice charges.

“This is the same company we’re trusting with over 2,200 metric tons of radioactive waste, sitting right there on the coastline,” said Seeley, who now lives in Los Osos, about 8 miles north of the plant. (PG&E says Diablo has 1,317 metric tons of waste.)

Nuclear energy, she insists, is a dead end. She wants the plant closed as soon as possible and replaced with renewable power.

“We have no gripes with the people who work there (at Diablo),” said Seeley, a spokeswoman for San Luis Obispo Mothers for Peace, an antinuclear group. “They work hard, and they keep us as safe as they can. It’s the corporate entity of PG&E we don’t trust.”

Fifty-five miles east of Diablo, the renewable future Seeley wants is taking shape on a quiet, dry plain.

The California Valley Solar Ranch lies on a stretch of highway so remote that, an hour before arriving, drivers pass signs warning them to check their gas. Winding through empty hills, the road finally spills onto the Carrizo Plain, a long valley coated in sparse, stubby grass.

From the highway, the Solar Ranch looks like some odd, metallic crop, row after row of flat silicon panels tilted to face the sun. Those panels, 750,000 in all, track east to west during the day, their movement almost imperceptible. Together, they can generate up to 250 megawatts of electricity, about 11 percent of Diablo’s capacity. PG&E buys the electricity.

“I’ve been in power generation all my life, and I chose to go into this — renewable power,” said Trevor Thor with SunPower Corp., the San Jose company that built the ranch. He steers his SUV slowly along the dirt roads crisscrossing the plant to avoid raising dust that could coat the panels. “It’s a no-brainer,” he said. “It’s an awesome technology. It’s clean. It’s dependable.”

The $1.6 billion ranch represents one front in California’s climate fight.

Since 2002, state officials have forced utility companies to buy ever-increasing amounts of renewable energy, slowly severing their dependence on fossil fuels. Solar plants are sprouting across the state in response. Under a law Brown signed in October, half of the state’s electricity must come from renewable sources by the end of 2030.

For all Thor’s enthusiasm, solar technology has limits. By tracking the sun’s arc across the sky, the Solar Ranch keeps its power output level throughout the day. But it generates nothing at night.

As a result, the ranch produces just 4 percent as much electricity over the course of a year as Diablo. The state would need many, many solar ranches to replace Diablo, plus some way of storing their electricity for use after dark.

Of course, solar isn’t the only form of renewable power.

Strong winds course over much of California at night, tapped by turbines spinning above hills and mountain passes. Their ability to supply power precisely when solar panels don’t means that the two energy sources can, to an extent, balance each other, with one kicking in when the other tapers off. But their output still varies by the hour and the day, dependent on the weather’s whims.

Therein lies Diablo’s value, supporters argue.

Diablo, like other nuclear plants of its generation, was designed to ramp up to full power and stay there, feeding a large and predictable amount of electricity onto the grid. It can be a foundation, providing the stable baseload power California needs while adding variable sources like solar and wind. Shut Diablo, and the state would have to rely more on natural gas to back up renewable power, raising emissions as a result.

“It would be really hypocritical to close that plant and see our carbon emissions go up,” said Jessica Lovering, a senior analyst at the Breakthrough Institute think tank in Oakland.

Solar and wind power cost more than electricity from Diablo, although the gap is narrowing fast. Last year, California utilities paid an average of 15.3 cents per kilowatt-hour for electricity from photovoltaic solar plants and 8.2 cents for wind energy, according to a state report. But solar developers are now offering power sales agreements for new plants at 5 cents per kilowatt-hour, a recent study from Lawrence Berkeley National Laboratory found.

To opponents, Diablo isn’t a foundation — it’s an obstacle.

By constantly placing a big block of power on the grid, Diablo makes it harder to incorporate solar and wind, essentially crowding them out. California, opponents say, needs flexible energy sources that can quickly ramp up and down to meet changes in electricity supply and demand. When they want to be really mean, foes call nuclear power a 20th century technology, ill-suited to the future.

“We don’t need a minimum amount of baseload power,” said Ralph Cavanagh, co-director of the energy program at the Natural Resources Defense Council. “The idea of baseload itself is obsolete.”

To both Diablo’s friends and enemies, few issues carry more weight than global warming. The regulators who will decide Diablo’s fate, however, have other concerns.

Since the industry’s birth, control over licensing nuclear plants has belonged to the federal government, not the states. That also applies to extending licenses.

The Nuclear Regulatory Commission makes the decision, following an evaluation process that takes years to complete. Commission staff study whether the plant’s equipment can safely run for another two decades, as well as the environmental impact of continued operations. Climate change and seismic concerns factor in, but not as primary considerations. The main focus remains the plant’s ability to continue generating electricity without mishap.

So far, every plant that completed the process has had its licenses renewed. Diablo, however, hasn’t followed the usual path to renewal. And for reasons specific to the plant, California officials could play a deciding role in its future.

PG&E first applied to renew its licenses in 2009, announcing the move with fanfare. Then the Fukushima meltdown hit. In April 2011, PG&E reported that it would put the renewal process on hold until another round of seismic studies, ordered by the state, wrapped up.

The hiatus lasted until this spring, when the Nuclear Regulatory Commission restarted its review. The decision to push forward came from the commission, rather than PG&E. In fact the utility, so enthusiastic about extending the plant’s operating life six years ago, now seems hesitant.

That may be because PG&E faces several state-level hurdles that other nuclear plant owners don’t. The biggest concerns fish.

Every day, Diablo’s cooling system sucks in 2.5 billion gallons of seawater. An estimated 1.5 billion fish eggs and larvae each year get swept along for the ride, churned, cooked and killed. The water then returns to the sea about 18.5 degrees warmer than it left.

Ever since the 1969 blowout of an offshore oil well near Santa Barbara, California has placed a high priority on protecting its shore. All of the state’s coastal power plants face orders to end the kind of “once-through cooling” employed at Diablo and switch to systems that kill fewer fish. For Diablo, that could mean building cooling towers — an expensive prospect. A report commissioned by PG&E found the effort could take up to 14 years and cost as much as $14 billion, in part because it would require carving a large chunk out of the nearby hills.

Environmentalists consider those eye-popping estimates a bluff. PG&E, they say, is overstating the costs in hopes that the California State Water Resources Control Board will give the company a waiver from the rules. But an independent estimate still put the cost of a new cooling system at $1.6 billion, no small sum.

While insisting it will respect whatever decision the board hands down, PG&E calls Diablo’s fish-butcher reputation an exaggeration. The number of eggs and larvae killed each year may sound immense, but it represents just 11 percent of all juvenile fish near the plant, according to the company. And even in pristine coastal environments untouched by man, most fish eggs don’t survive to adulthood.

Plus, some sea creatures seem to like the warm water, Jones said.

“The discharge cove itself has a very vibrant ecosystem full of fish, sharks, rays,” he said.

The water board isn’t the only state agency with a hand in Diablo’s future. PG&E also needs the California Coastal Commission to certify that relicensing Diablo would comply with all of the state’s environmental regulations. The Nuclear Regulatory Commission has made clear that it won’t renew the plant’s licenses without that certification.

And yet, PG&E has delayed moving forward at the state level, even as the federal government considers Diablo’s licenses. Earley, the CEO, said the utility has not decided whether to proceed.

“We’re doing the preliminary work we need to do,” he said. “I have not gone to the board of directors and said, ‘Full speed ahead.’”

Earley believes in nuclear power. Before arriving at PG&E in 2011, he used to chair the board of the Nuclear Energy Institute, the industry’s main lobbying group. But Earley was hired in the wake of San Bruno to reform PG&E. With nine years left on Diablo’s first operating license, he said the utility has more pressing issues to deal with first. He also understands the resistance nuclear plants face.

“Nuclear in California is a sensitive issue, so we’ve got to consider all options,” Earley said.

Should California’s water board require a pricey new cooling system, PG&E could try to pass those costs on to its customers. Doing so, however, would require the approval of another group of state regulators, the California Public Utilities Commission, which sets electricity rates. And the commission is in turmoil.

PG&E used to enjoy a close, collaborative relationship with the panel. The commission’s longtime president, Michael Peevey, called Diablo an important weapon in the state’s climate fight and seemed to favor keeping it open, saying in 2005, “We have a bird in the hand here, so to speak.”

The San Bruno explosion blew that relationship apart. California legislators pilloried the commission for being far too friendly with the companies it regulates, particularly PG&E. Under pressure, Peevey stepped down last year. His replacement, Michael Picker, won’t say whether he wants Diablo’s licenses renewed.

“I will say that California has not had great luck with nuclear power,” Picker said. “We have, in general, a wealth of choices, so no one plant is absolutely essential.”

David R. Baker is a San Francisco Chronicle staff writer. E-mail: dbaker@sfchronicle.com Twitter: @DavidBakerSF