Even after the twin domes along I-5 are gone and the San Onofre nuclear plant is mostly a memory, fuel rods hot with radioactivity will remain behind in rows of tomb-like casks – perhaps for decades.

The shuttered plant is in the early stages of an estimated $4 billion decommissioning process, the dome’s reactors now empty of fuel and sealed.

Massive turbines that once pumped electricity through the Southern California power grid are silent. Giant steam pipes once too hot to touch are cold.

And a small crew walks the darkened halls, machine shops and empty yards, readying the plant for a decades-long dismantling.

“There’s something called ‘cold and dark,’” said Jim Madigan, Southern California Edison’s technical adviser to the plant’s chief nuclear officer, describing the ultimate goal of the decommissioning process. “The decommissioning project manager says now we’re kind of dim and cool.”

Madigan, a 33-year Edison employee accustomed to giving tours of the shut-down plant to public officials – but not to the public – begins a tour with a walk past towering white tents that once temporarily housed 640-ton steam generators.

The generators were switched out for brand-new ones in a $670 million upgrade between 2009 and early 2011. But the generators proved to be defective.

That kept both of the plant’s reactors shut down after January 2012. As costs and uncertainties mounted, Edison announced the permanent closure of the plant on June 7, 2013.

Now Edison is embroiled in a dispute over costs with the manufacturer of the generators, Mitsubishi Heavy Industries.

NUCLEAR RODS

Madigan passes along a perimeter fence topped with barbed wire overlooking the ocean. The narrow path below, sometimes used by beachgoers, is still watched closely by security guards in towers with green-tinted windows.

He walks through a gateway beneath a huge, sliding crane that once serviced the plant’s turbines, but is now “mothballed.” He stops to peer into a pool of water called a forebay, where fish pulled in along with ocean water for the plant’s cooling system swim calmly.

They’re awaiting a ride in a “fish elevator,” then a trip back to the ocean. An occasional savvy sea lion wanders in to catch the fish, Madigan says.

“When they’re ready to leave, they climb in the elevator,” he says. The elevator lifts fish out of the pool so they can be returned to the ocean.

The ocean-water cooling system is still operating, but no longer to cool the plant’s steam generators. Now it is used only to cool the two pools that house the plant’s spent nuclear fuel rods, which will stay in the pools for several years before being moved to dry cask storage.

The system once pulled in more than 1 billion gallons of ocean water per day. Now it’s down to about 25 million gallons.

Going into the plant through a sealed door, Madigan winds his way up a stairway to a slanted window that looks down on the power plant’s control room.

The room is full of panels, buttons, screens and banks of blinking lights and used to be bustling with reactor operators. But now two or three operators, with little to do, engage in quiet conversation.

Next is a trip into the bowels of the plant.

ONCE STEAMING HOT

Catwalks offer plunging views of a labyrinth of pillar-like pipes, some marked “MS” for main steam.

Steam from water heated by the reactors once coursed through the pipes to turn the power plant’s turbines and generate electricity. Madigan mentions that steam leaks can be fatal and points out a man was killed by such a leak at a power plant in Mojave.

“This is the turbine building,” Madigan says. “Everything here would be heat-hot. I wouldn’t be taking you through here if the plant were operating.”

Floor to ceiling, the room reaches about 70 feet.

“You can imagine the enormity of the thing, to take this back to ground zero,” he says.

Through another door, Madigan is out in the sunshine again, strolling past idled water purification systems and backup diesel generators.

All fuel has been removed from both of the plant’s reactors, and the areas inside the domes that house them – known as “containment” – are sealed.

“Nobody goes into containment anymore,” Madigan says.

He walks past the plant’s two spent fuel pools, also enclosed in buildings made of reinforced concrete.

Some activists worry that the pools could be damaged in an earthquake. Some of the spent fuel assemblies must cool there three to five years, when they can be moved into dry cask storage.

But Madigan says the spent fuel buildings are hardened against virtually any catastrophe. They are also elevated in the unlikely event of a flood or a tsunami that would first have to breach a 30-foot seawall that surrounds the plant.

HALLWAY DARK, SILENT

Ducking back inside the plant, Madigan treks down a long hallway to find racks of pocket-sized dosimeters that monitor radiation exposure.

In the plant’s heyday, the hallway was crowded with employees on their way to work, picking up their dosimeters and stepping in and out of narrow, human-size detectors to check clothing for radioactivity. Now, only a few people staff the gateways to controlled areas.

“The only reason those guys are still in the plant is because people need to monitor the spent fuel pools,” Madigan said.

Madigan allows he once routinely made rounds here but hasn’t been in the hall since March.

“I would walk around to make sure everything is safe, plus to get out and talk to people,” he said.

The power plant’s workforce now stands at about 500, down from the 1,500 who worked here when it was still operating.

“Those who remained, remained because they wanted to be part of the decommissioning,” Madigan said.

Many of the employees who were let go were helped into other jobs by Edison, though not all.

“In general, I would say the feeling here is, ‘Let’s move on with the project,’” Madigan said. “I’m sure there’s emotions centered around having to shut down the plant. I’m sure a year ago, emotions were high. Now we’re focused on the safe shutdown of the plant – to go from an operational focus to a project management focus.”

Madigan and other Edison officials will spend the next two years preparing decommissioning documents for approval by the Nuclear Regulatory Commission.

The rough timeline they’ve developed calls for dismantling buildings and structures over the next 12 years, including the domes.

State utility regulators are still working through how to replace the power San Onofre once generated, including the possible construction of new natural-gas fired power plants in Southern California.

The final stop on the tour is a broad section of property, home to about 50 boxy modules arranged in long rows. Each contains a shielded cask, each of those, in turn, hold 24 spent nuclear fuel assemblies. The assemblies are about 14 feet tall.

Edison must either pack more of the modules into this section or create a new dry cask storage area, possibly elsewhere on the property. The company’s experts are still deciding which; about 100 more modules are needed.

Efforts to create a national repository for high-level nuclear waste are stalled, and Madigan and other experts say there is little prospect of one being constructed for decades. The NRC has been directed by a court to evaluate the risks of long-term storage of nuclear plant sites around the nation.

The rows of casks will be the final legacy of the San Onofre plant after all the other structures are gone, and the land, now leased by Edison, is given back to the Navy.

“We plan to do it in 20 years or less,” he said.

Some of the material in the fuel assemblies, such as uranium, will remain radioactive for tens of thousands of years – plutonium, hundreds of thousands.

“This stuff will be radioactive for a long, long, long time,” Madigan said.

Contact the writer: 714-796-7865 or pbrennan@ocregister.com.