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Diablo Canyon: an asset California can’t afford to lose

Diablo Canyon from above. Credit: By Doc Searls from Santa Barbara, USA - 2007_04_24_sba-sfo-lhr_048.JPG, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=2417676

In the face of scorching temperatures in August, the California Independent System Operator (CAISO), which oversees the operation of the state’s electricity grid, warned that it might face a shortage of 4,400 megawatts (MW) of electricity. This had the potential to leave about 3.3 million customers in the dark and without air conditioning. Fortunately, blackouts ended up affecting far fewer customers but still resulted in power being cut off to more than 410,000 homes and businesses on August 14 and another 200,000 on August 15.

This same scenario almost repeated itself Labor Day weekend with record-breaking heat and supply failing to meet demand. Once again, conservation efforts helped to avert rolling blackouts with demand over the holiday weekend at levels below what was initially forecast.

With nearly one-third of its electricity coming from renewable sources in 2019, California serves as both a model and a lesson: a proving ground for the future of electricity generation and a place to uncover the challenges that utilities must solve as they integrate more renewables into their portfolios.

Decarbonization of the grid is an effort that should be applauded and the build-out of renewables in California is a significant accomplishment. As the state works toward a goal of having 60% of its electricity come from renewables by 2030 and 100% from zero-carbon sources by 2045, operators in the state must take a holistic approach that stabilizes the grid and offers maximum flexibility.

Over the past decade, the term “duck curve” has become common in the lexicon of professionals in the energy sector. The duck curve, named for its shape, is the net electrical demand vs. time, or what electricity supply needs to be furnished from resources other than wind and solar. CAISO first published this curve in 2013 when it began planning for the large addition of renewables and the changes that would be necessitated for grid management. In graphical form, the duck curve illustrates the challenge of the early evening loss of solar and a rapid transition to conventional generation.

For example, CAISO data on May 2, 2020, shows net demand sank from 15,888 MW at 6:00 a.m. to 9,276 MW at 8:00 a.m. It continued to fall to 5,720 MW at 1:45 p.m., at which point it rose rapidly to 22,264 MW at 8:30 p.m. This means that over the course of about a 14-hour period, 6,612 MW of conventional generation was taken offline in the morning as solar generation came online and 16,544 MW of additional conventional generation needed to be brought online quickly to meet increasing demand and decreasing solar output in the evening.

In 2016, CAISO published a paper, Flexible Resources Help Renewables, detailing the challenge that lay ahead and noted that “to reliably manage the green grid, the ISO needs flexible resources with the right operational characteristics in the right location.” This reinforces that the grid of the future needs to be built around a combination of carbon-free sources that works together to provide reliability and flexibility, especially as heat waves and other extreme weather events occur more frequently.

In the United States, the nuclear energy industry prides itself on setting the highest standard for operation and has continually achieved capacity factors north of 90%. It is lesser known that these plants can be remarkably flexible. Existing reactors can increase or reduce power at rates up to 5% per minute to meet grid demand, making this non-carbon-emitting technology a reliable and flexible partner for the increasing use of renewables on the grid. 

Nuclear power has proven the world over to possess the qualities that California needs in a future grid. Nuclear plants in France and Germany have operated for decades in a flexible manner, matching the output of the reactor to the needs of the grid. In the case of France’s nuclear plants, they daily modulate power between 30% and 100% while also providing frequency control as the grid requires. Whether operating at 100% on a scorching summer day, 30% on a breezy spring day or anywhere in between, nuclear energy is flexible, clean and reliable. It should also be the foundation on which a decarbonized grid is built.

However, despite these benefits, California is rapidly nearing the date when it will lose its valuable, carbon-free nuclear capacity. The state’s last two reactors, located at the Diablo Canyon Power Plant, are expected to shut down in 2022 and 2023. This follows the shutdown of the two reactors at the San Onofre Nuclear Generating Station in 2013.

While the 2,200 MW of carbon-free electricity from San Onofre would have helped to manage the strain the grid faced during the recent unrelenting heat, its absence offered a preview of the challenges that will worsen if the reactors at Diablo Canyon do, in fact, shut down.

Diablo Canyon produces about 8% of California’s power and approximately 15% of its carbon-free electricity. The plant generates 16.2 million megawatt-hours of electricity a year, which is enough to power 2.5 million homes. This is an asset that California cannot afford to lose as it seeks to manage power demand during heat waves while also working to meet zero-carbon energy targets.

California has an opportunity to bolster its grid, increase reliability and flexibility, and decrease carbon emissions all by continuing to keep Diablo Canyon operational. If the state follows its current trajectory and Diablo Canyon is closed, carbon emissions will increase and reliability will suffer. In addressing the recent blackouts, Governor Newsom was exactly right when he stated, “You shouldn’t be pleased with the moment that we’re in here in the state of California.”

The citizens of California should expect better and they can achieve it – with clean, reliable, flexible nuclear power.


Caleb Tomlin is an engineer for Framatome. His primary focus is the investigation and development of innovative solutions for the existing fleet of nuclear reactors as well as Gen IV reactors. These areas include integrated energy systems and flexible power operations.