Despite a massive wildfire, Diablo Canyon produces reliable power

California's grid was likely close to collapse on May 4, 2025 - see article end

At the end of the first part of this article, I was discussing the Gifford Fire. During the intervening week, the fire has grown considerably. The northwest tongue of the fire is now less than 14 miles from my home in San Luis Obispo. There have already been days of obvious smoke from the fire. I presently feel some irritation in the back of my throat from the fire and my eyes are red from the smoke. The Gifford Fire is by far the largest fire in California. The fire is 132,761.5 acres (207.44 square miles.) The fire is 41% contained. Part of the eastern edge of the Gifford Fire is the burn scar from the Madre Fire, which burned about 80,000 acres in eastern San Luis Obispo County in July, 2025. (Thus, this pair of fires have burned over 210,000 acres.) The Gifford Fire is burning through portions of the Garcia Wilderness within the Los Padres National Forest in San Luis Obispo County. This part of the Garcia Wilderness has not burned during recorded history, so the Gifford Fire is a unique and unusual event.

As a consequence of its gargantuan size, San Luis Obispo County will likely experience adverse fire effects for the next month or two. If there is high rainfall over the burn scars during the winter months, there could be serious mudslides like there were after the Thomas Fire burned the slopes above Montecito, California. The mudslides there on January 9, 2018 killed at least 23. The insurance claims exceeded $423 million dollars. Highway 101, a major highway through the California central coast was closed for some time after the mudslides. Repairs and rebuilding in Montecito are ongoing, over seven years later.

I live in San Luis Obispo, California, approximately between the "s" and "p" in the legend "San Luis Obispo." Note the 3 mile scale bar at the lower right-hand corner of the map.

The Gifford Fire is curtailing the output of Diablo Canyon

"Diablo Canyon reduces power as Gifford Fire rages," by Chloe Jones, August 6, 2025, front page of the print edition, San Luis Obispo Tribune. This article includes a color photograph of the smoke rising in a tall column from the fire.

....Diablo Canyon nuclear power plant has also reduced power on one of its units to 46% due to the fire and its proximity to transmission lines in San Luis Obispo County, PG&E spokesperson Carina Corral told The Tribune in an email....

The California Independent System Operator (CAISO) shows Diablo Canyon's output has remained steady at about 1,595 megawatts since it was curtailed as a consequence of the Gifford Fire. At 1:00 PM PDT on Sunday, August 3, 2025, CAISO showed Diablo Canyon was producing 2,274 MW before any curtailment. By Tuesday, August 5 at 1:00 PM PDT, Diablo Canyon was curtailed to 1,593 MW. The likely reason that Diablo Canyon remains on line is because the plant produces so much essential synchronous grid inertia (SGI) which stabilizes the California grid. Even while Diablo Canyon is presently curtailed, it is still producing more SGI and more power than any other California power plant.

The unique problem in California at mid-day because California has so much solar and wind generation

California has 21, 240 MW of solar and 8,375 MW of wind. In addition to not producing significant amounts of SGI, solar and wind are non-dispatchable (i.e. not under human control.) So, when the sun is overhead at solar noon, for most of the year, there is now too much solar and wind relative to the needs of the grid at mid-day. Thus, as noted in the beginning of part 1[1], California ratepayers must pay adjoining states to take California's excess power as supplied by the mid-day grid mix.

While California has many connections to adjoining states, those connections are configured to export the excess power to those states during times of excess solar and wind generation. The grid between adjoining states cannot be instantaneously reconfigured to import power during those intervals when California is exporting power. The cascading blackout that enveloped most of Spain and Portugal on April 28, 2025 required less than a minute for the Iberian Peninsula power grid to collapse. Thus, a unique new systemic problem at mid-day for the California power grid is imposed as a consequence of California building so much solar and wind generation.

The state must generate sufficient SGI to maintain grid stability solely within the state at mid-day when California is exporting power.

Think of the mid-day exports as a giant STOP sign for power imports to California. SGI can be considered to be the "heartbeat" of the electricity grid. DCPP is the biggest mid-day contributor. Combined-cycle natural gas turbine (CCGT) plants are the second largest contributor. Hydroelectricity produces a tiny amount of SGI at mid-day. Solar, wind, and batteries all produce negligible amounts of essential SGI. This leads to the concern CGNP identified in the next section.

California came perilously close to a mid-day cascading blackout on Sunday, May 4, 2025

The charts below show stacked bars only on days when California was exporting power at solar noon, preventing the import of SGI from out of state. The color code is green for Diablo Canyon's power. Red shows natural gas. The simplifying assumption is that at mid-day all of the natural gas power is from CCGT plants. When California is importing power, SGI is available from out-of-state generators.

The danger criterion CGNP used was a SGI-capable supply stack less than 2,000 MW at solar noon on export days, which occurred on May 3, 2025 and May 4, 2025 (note blue arrow) during the DCPP spring, 2025 routine refueling and service outage. A mere 1,643 MW of SGI-capable power was provided at solar noon on May 4, 2025, while power was being exported to adjacent states.

On April 11, 2025, the SGI-capable supply stack was less than 2,000 MW (1,862 MW). However, this date occurred before the April 28, 2025 Iberian Peninsula blackout. Thus, CAISO may not have been aware of the danger of inadequate SGI on export days.

During the August, 2025 DCPP curtailments caused by the Gifford Fire, CAISO has been much more conservative on days California was exporting power at solar noon. The curtailments started on August 4, 2025. The minimum SGI-capable supply stack was on August 4, 2025 at 5,132 MW, far in excess of the 1,643 MW value observed on May 4, 2025. The value for other export days have been higher. CGNP anticipates this dispatch pattern will continue on export days during the remainder of the days the Gifford Fire causes DCPP curtailments.

The 1:00 PM May 4, 2025 California grid mix

This is a simplified view of California's grid mix on May 4, 2025 at 1:00 PM.

The grid mix was 89% renewables at 22,180 MW. In addition, the state was exporting 4,509 MW of power and charging batteries at 4,979 MW. Just like solar and wind, these categories fail to contribute to California's SGI. Even though large hydro was 4% of the grid mix, the large generator turbines spin much slower than the generator turbines for nuclear and natural gas. SGI is proportional to the square of the rotational speed of a turbine. CGNP performed a detailed examination for the CPUC of the roughly million-pound rotors at the Helms Pumped Storage facility for the CPUC. The Helms rotors spin at only 360 RPM, 1/5 of the speed of DCPP at 1,800 RPM. To a first order approximation, each turbine-generator at DCPP produces twenty-five times as much SGI as each Helms generator. CGNP considers Helms to be representative of all California large hydro generators. Additional details regarding SGI are found in CGNP's March 4, 2024 article. [2]

Spain's Mid Day Grid Mix on April 28, 2025

Red Eléctrica (REE,) Spain's national grid operator is analogous to CAISO in California. Their website provides similar grid performance information.

Here is the Spanish generation by source at 12:30 PM on April 28, 2025 and some later dates.

REE's data has more categories. However, on April 28, 2025, Spain's wind plus solar PV totals 21,145 MW, slightly less than the 22,180 MW for California on May 4, 2025. Spain was exporting power, just like California. Spain had more nuclear at 3,387 MW versus California's 1,135 MW. Spain had more gas, coal, solar thermal, renewable thermal and cogeneration at 4,442 MW than California's 508 MW. But Spain's grid collapsed less than five minutes later!

These comparative statistics underscore the precarious power situation in California on May 4, 2025.

Just like CAISO's subsequent actions to insure adequate mid-day SGI, Spain has taken actions to insure there is adequate SGI, as is shown in the table above. Electricity Maps data shows that Spain typically runs all seven of its nuclear power reactors at mid-day.

Just like Spain, California has been having frequent power outages. There were oscillations in grid frequency in Spain on the morning of April 28, 2025 and on previous dates.

CGNP believes that a detailed inquiry of CAISO performance data will show oscillations in grid frequency during mid-day on the days with deficient SGI supply stacks highlighted above. CGNP calls for an investigation of this issue.

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[1] "Taxation Without Representation - Part 1 - California electricity prices are a private-sector tax on a necessary of life," August 7, 2025, GreenNUKE Substack. https://greennuke.substack.com/publish/post/170371078

[2] "Why is Grid Inertia Important? Without sufficient synchronous grid inertia, the grid becomes unstable and a blackout occurs," By Gene Nelson, Ph.D., March 4, 2024, GreenNUKE Substack. https://greennuke.substack.com/p/why-is-grid-inertia-important