It’s hard to miss Diablo Canyon nuclear power plant when passing it by air or sea. One immediately sees the hulking containment domes that house and protect the plant’s two nuclear reactors rising above the squat, brown generator building.
Attention is soon drawn to another sight — a massive plume of whitewater cascading from the plant’s cooling water system. When operating at full power, Diablo Canyon uses 2.5 billion gallons of seawater a day to condense steam after it has passed through the two electrical generators.
On May 4, the state Water Resources Control Board adopted a new policy that declared these once-through cooling systems used at Diablo Canyon and 18 other coastal power plants in California to be antiquated. The board gave the utilities that own those plants deadlines for installing less environmentally damaging cooling systems.
Once-through cooling damages the environment because it kills adult and larval fish. The flood of warmer discharge water also alters the marine ecosystem around the plant.
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In the case of Diablo Canyon, owners Pacific Gas and Electric Co. have until the last day of 2024 to comply with the new policy. That date was intentionally set to coincide with the planned renewal of the plant’s two operating licenses currently under review by the Nuclear Regulatory Commission.
However, that’s where the clarity ends.
It won’t be known for another three years what type of alternative technology PG&E will be required to implement at Diablo Canyon. PG&E is not saying what technology it will recommend.
“At this stage, there is much more study that needs to be done,” said Cindy Pollard, a PG&E spokeswoman.
A range of cooling options is available. These include replacing once-through cooling with a closed-loop system that uses much less water, modifying the once-through cooling system so it is less damaging, or some combination of the two.
A third-party technical team with nuclear industry experience will be named to study these options. In three years, the team will recommend to the water board what technology best suits the plant.
The policy also gives PG&E the option to apply for a variance that will allow it to avoid fully complying with the new rules if the cost of doing so would be “wholly unreasonable.”
State water officials say the variance option is needed to ensure the continued operation of the state’s two nuclear plants — San Onofre in Southern California and Diablo Canyon. They supply 20 percent of the state’s electricity needs and play a crucial role in the state’s ambitious plans for reducing its greenhouse gas emissions.
“The policy has been developed to protect marine life and ensure electric grid reliability for Californians,” said Judie Panneton, a water board spokeswoman.
If Diablo Canyon is given a variance that results in continued damage to the ocean environment, PG&E would be required to offset that damage by funding habitat improvement work and other projects within the Central Coast’s newly established network of marine protected areas.
In spite of the uncertainty, one thing is clear: Leaving Diablo Canyon’s once-through cooling system unchanged is not an option, state water officials say.
“Under the new policy, a nuclear power plant will not be able to operate once-through cooling just by paying for a mitigation project,” Panneton said. “Mitigation may not be used by any plant as a substitute for meeting the best technology available.”
Environmentalists are critical of the nuclear plant variance. They want Diablo Canyon and San Onofre to be retrofitted with closed-loop systems that reuse a fixed amount of cooling water.
Closed-loop systems come in two basic forms. Cooling towers use evaporation to cool the water. So-called dry-cooling systems cool water in large radiators.
“Allowing exceptions for nuclear plants is inconsistent with the Clean Water Act,” said Linda Sheehan, with the California Coastkeeper Alliance. “Nuclear plants by far have the biggest impact on the environment because they use enormous amounts of seawater.”
PG&E officials say retrofitting the plant with such a closed-loop system would be prohibitively expensive because hilly areas around the plant must be leveled to accommodate the new cooling system. Getting permission from the state Coastal Commission to do that work would be difficult.
Estimates of how much it would cost to retrofit Diablo Canyon vary. PG&E says the work would take 17 months to complete and cost $4.5 billion. The state has a lower estimate of eight months and $1.6 billion.
State energy officials say a variety of options is available for modifying Diablo Canyon’s once-through system so that it is less damaging to the environment. The cost of these options would vary widely, but would be significantly less than retrofitting the plant.
Options include changing the intake location, installing variable-speed circulation pumps, seasonally limiting flows and installing additional screens or barriers in front of the intake.
PG&E officials say they appreciate the hard work the state water board did to craft the new policy, and they look forward to participating in the three-year study.
“PG&E supports an orderly transition away from once-through cooling,” Pollard said. “We hope that the new policy will achieve that goal.”
The state water board has three months to appoint the technical study teams for the nuclear power plants. The first preliminary reports from the teams are due in a year.
HOW PLANTS' SEAWATER COOLING HURTS MARINE LIFE
Water officials recently adopted a new policy phasing out once-through cooling at the state’s 19 coastal power plants because it is too damaging to ocean life.
The amount of damage varies from one power plant to the next, but all saltwater-cooled plants cause three types of ecological damage. They are:
Impingement. The suction generated at the intake structures of some plants is strong enough to suck in adult fish and marine mammals or pin them against grates or screens covering the intake. State water officials say 57 animals a year are trapped in the state’s coastal power plants. Diablo Canyon’s intake structure is large enough to almost completely avoid this problem. No marine mammals have been killed, and the plant kills from 700 to 800 pounds of fish and crabs a year, a fraction of the take of some other plants. Plants with severe impingement problems have bypasses that allow some fish to escape.
Entrainment. Every year, 19 billion fish larvae pass through the state’s once-through cooling systems. Most do not survive the trip because they are crushed in circulation pumps or are eaten by mussels and barnacles that line the cooling system’s outfall. Diablo Canyon entrains more than 1.5 billion larvae a year. Fish that inhabit rocky reefs are the most heavily impacted. Although most larvae die naturally, biologists consider the added mortality of once-through cooling to be significant.
Thermal effects. Cooling water is typically 20 degrees warmer when it is discharged from Diablo Canyon’s cooling system. This influx of warm water alters the ecosystem of the discharge cove. Kelp, other forms of algae and small fish, particularly in the shallow areas along the shore of the cove, are the most heavily impacted.