A rare look inside California’s last nuclear power plant

The most striking view off one of San Luis Obispo County’s winding coastal roads is not the lashing ocean waves of the Pacific Ocean or cows plodding out from the shade of a California live oak tree.

It is two enormous concrete domes that come into focus along a final climb that began 7 miles back at Avila Beach. The land sinks away, and what looks like a small town emerges, showcased in a palette of grays, whites and terracotta.

This is Diablo Canyon, California’s last operating nuclear power plant.

Just years ago, the plant was slated to close, and employees worked to decommission it, until a 2022 about-face by Gov. Gavin Newsom led the state to extend its operations to 2030. Now lawmakers in Sacramento are talking about allowing it to operate even longer, potentially to 2045.

But local groups, some of whom have protested the plant since its construction, are banging the drum ever louder about their concerns for safety or a catastrophic meltdown, as well as the danger posed by spent nuclear waste at a site near several seismic fault lines.

Meanwhile, academics are furiously analyzing how much keeping Diablo Canyon open would cost and if it would support or hinder the state’s clean energy transition. And business groups are lining up in support.

So when PG&E offered press tours earlier this year, KQED accepted. The nuclear power plant has not garnered this much attention in years, but now, once again, all eyes are on Diablo Canyon. What does it look like inside?

Out on the water

PG&E’s Diablo Canyon Power Plant tour started on a boat in a protected marina just south of the reactors. This, and another cove just outside the breakwaters, are the site of a key piece of the plant’s cooling system — and a major concern for environmentalists, who argue it hoovers up and kills marine life and have called it “the most destructive facility” along California’s coast.

Dipping a hand in Diablo Cove, the water is lukewarm, not the frosty standard for the ocean in these parts.

That’s because Diablo Canyon draws 2 billion3-2.5 billion gallons of ocean water daily — enough to fill more than 3,000 Olympic-size swimming pools — into the plant to cool equipment, and discharges the water back into the ocean typically 16 to 17 degrees hotter.

The warmer water makes it feel as if a chunk of Southern California’s coast has been lobbed off and transferred north.

Out on the water, there was a hotbed of animal activity: a floating sea otter and chubby seals sunning themselves on rocks.

There were other species too — sea bass, stingrays, and California’s state fish, the garibaldi, which typically live farther south along California’s coast, but have moved here.

Diablo Canyon staff said the warm water leads to essentially no change to the environment. Because fishing and other activities are not allowed within 2,000 yards of the plant, it’s a “de facto marine sanctuary,” said Tom Jones, a senior director in charge of future planning for Diablo Canyon.

But the California Coastal Commission, the state agency tasked with protecting the coastline and its natural resources, reported in 2025 that the plant’s cooling system kills almost two billion larval fish annually, plus other organisms that aren’t measured.

While adult populations may be abundant in Diablo Cove, the commission wrote that adults often appear far from where they spawn, and their presence here may be the result of productive marine habitats nearby.

The commission also warned that removing eggs and larvae near Diablo Canyon leads to “a significant reduction” of species dozens of miles from the plant.

“These planktonic organisms,” wrote the commission, “constitute the base of the food web in California’s coastal waters.”

To the turbine deck

We donned hard hats and safety equipment and passed through heavy security to enter the “protected area,” which consists of spaces closer to the nuclear reactors.

We entered the turbine deck, an industrial building the size of two-and-a-half football fields. It was hot and loud on the deck, with a slight vibration underfoot.

The steam-driven turbine inside is an enormous semi-cylinder that looks like a horizontal steel pipe cut in half, and spins a generator to produce electricity.

The PG&E guide pointed out the window at a containment dome, where uranium atoms are split apart, releasing huge amounts of heat.

A cascade of effects follows: the heat warms water and creates steam, the steam travels through pipes to turn the turbine, the turbine connects to a generator, which makes electricity that’s then sent across the grid and delivered to about three million Californians.

Nuclear generates nearly 9% of the state’s energy supply, part of an energy mix that includes gas, hydroelectric, solar, wind, geothermal and even small amounts of coal.

While California’s demand for electricity has been flat for years, it’s now growing with the adoption of electric vehicles, people swapping gas appliances for electric ones, and data centers.

The debate to keep Diablo Canyon open is spurred, in part, by this uptick in demand. Maureen Zawalick, senior vice president and chief risk officer at PG&E, said stepping into the turbine deck reminds her of the end uses of all this power: “safety in hospitals, kidney dialysis, stop lights and everything else.”

California is walking its economy across a tightrope.

The state’s growth in the 20th century was built on a foundation of fossil fuels, but leaders see its future as being powered by the buildout of renewables like solar and wind, along with batteries to store excess power.

When heat waves strained California’s power grid and caused rolling blackouts in 2020, state lawmakers and Newsom voted to extend Diablo Canyon’s operation.

Now, as electricity bills continue to rise and demand is forecast to grow, proponents argue that keeping the plant open even longer can help California wobble across the precarious middle of the tightrope.

The simulator

We shed our safety gear and headed to the training building, with classrooms and an exact replica of the control room, called the simulator.

It was cool and quiet again as employees completed a training exercise, manipulating switches, lights and screens on a semicircle of vertical boards. Zawalick said the simulator’s seafoam green walls are meant to inspire calm, but its very existence is due to nuclear disasters that have occurred elsewhere in the U.S.

Simulators became a requirement for all nuclear power plants in 1979 after the Three Mile Island nuclear accident in Pennsylvania. The partial meltdown was the most serious nuclear accident in U.S. history and was caused by both human and equipment failure.

Practicing in a replica of Diablo Canyon’s actual control room is meant to train workers with the muscle memory to handle a variety of emergencies.

Employees spend 20% of their time in the Diablo Canyon simulator training for everything from planned refueling to routine maintenance to major emergencies.

Spent nuclear fuel

To finish the tour, we drove uphill and farther from the ocean to find dozens of hulking concrete cylinders that contain spent fuel, called “dry casks.”

The nuclear material is the concern of resident groups who fear an earthquake or terrorist attack could destabilize the storage and spew radioactive waste into the ocean or nearby communities. People living nearby are mailed annual emergency preparedness documents and have access to a free dose of potassium iodide, which protects the thyroid gland against radiation.

Linda Seeley has rallied against Diablo Canyon for decades as a member of the anti-nuclear nonprofit Mothers for Peace.

“As much as I would love it if nuclear waste were not toxic and lethal to a thousand generations in the future, that’s not the fact. The fact is that it is toxic,” she said.

Once fuel has been used inside the plant, radiation levels are dangerously high and have the potential to kill an exposed person in minutes.

The spent fuel spends 7 to 10 years next to the reactors in “wet storage,” a large pool of water treated with chemicals. The liquid absorbs heat and decays of the uranium, which has high levels of radiation.

The nuclear material is then packed into the double-lined, stainless steel and reinforced dry casks, roughly 20 feet tall. Each is bolted to a 7.5-foot-thick, steel-reinforced concrete pad designed to withstand earthquakes. The fuel requires special handling for tens of thousands of years.

Diablo Canyon is located roughly 3.5 miles from the Hosgri fault, which presents the main seismic risk to the plant. Another fault, the Shoreline, is closer to the plant, but smaller. Some seismologists are concerned that a quake along the faults could cause a meltdown.

The U.S. government is legally obligated to take ownership of all commercially spent nuclear fuel, but because the government has not yet built a permanent place to put it, the fuel is stored at the power plant.

Current solutions like Diablo Canyon’s dry storage casks, while they may be thorough, are only licensed until 2064 by the Nuclear Regulatory Commission.

Zawalick said PG&E is confident in the storage of Diablo Canyon’s spent fuel, though. She pointed out that nuclear power is “the only energy source that knows exactly where every ounce of our waste is.” The Nuclear Regulatory Commission and PG&E monitor the spent fuel on a daily and annual basis. “It’s secured, it’s inspected, it’s audited, it’s sampled. I’m a fan of all energy sources, but I don’t know where solar panels are sent when they’re done, and batteries, and all of that.”

Zawalick pointed to the powerful transmission lines carrying energy created here out to millions of Californians: to illuminate rooms for special and mundane occasions, preserve food in refrigerators, run air conditioners, and warm their shower water.

Order and safety come up frequently on the Diablo Canyon Power Plant tour: background checks, armed guards, seismic protective measures, reminders to hold on to handrails when on steps. The result is a calm and kempt environment, situated on a hillside overlooking the Pacific.

But underneath the serenity lie the inherent risks of nuclear power, especially when sited near seismic fault lines. Diablo Canyon has been the source of passionate debate as long as the idea of it has existed. And any effort to keep it operating longer will be no different.

And with that, the tour was over, and the guides returned to their work. A cow made its way slowly across the access road, with no idea of its contentious neighbor.