The Diablo Canyon Power Plant is a nuclear power plant near Avila Beach in San Luis Obispo County, California. Following the permanent shutdown of the San Onofre Nuclear Generating Station in 2013, Diablo Canyon is now the only operational nuclear plant in California, as well as the state's largest single power station. It was the subject of controversy and protests during its construction, with nearly two thousand civil disobedience arrests in a two-week period in 1981.
The plant has two Westinghouse-designed 4-loop pressurized water reactors operated by Pacific Gas & Electric (PG&E). Together, the twin 1100 MWe reactors produce about 18,000 GW·h of electricity annually (8.6% of total California generation and 23% of carbon-free generation), supplying the electrical needs of more than 3 million people.Economic Benefits of Diablo Canyon Power Plant. , PG&E, June 2013. For 2026 forward, facility operations and maintenance costs are projected to be $0.03 per kWh generated, but when including new fees authorized by a recent law, that number rises to $0.08 per kWh. Intermittent sources like wind and solar cost $0.05 and $0.06 per kWh without battery storage, but $0.09 and $0.14 with battery storage.
Though it was built less than a mile from the Shoreline Fault line, which was not known to exist at the time of construction, and is located less than from the Hosgri fault, a 2016 Nuclear Regulatory Commission (NRC) probabilistic risk assessment of the plant, taking into account seismic risk, estimated the frequency of core damage at one instance per 7.6 million reactor years. The plant is located in NRC region IV.
In 2016, PG&E announced plans to close the two Diablo Canyon reactors in 2024 and 2025, stating that because California's energy regulations give renewables priority over nuclear, the plant would likely only run half-time, making it uneconomical. In 2020, experts at the California Independent System Operator (CAISO) warned that when the plant closes the state will reach a "critical inflection point", which will create a significant challenge to ensure reliability of the grid without resorting to more fossil fuel usage, and could jeopardize California's greenhouse gas reduction targets. Further warnings were made by the California Energy Commission and CAISO in 2021 that the state may have summer blackouts in future years as a result of Diablo's closure coinciding with the shutdown of four natural gas plants of 3.7GW total capacity, and the inability to rely on imported electricity during West-wide heat waves due to reduced hydroelectric capacity (from the decades-long drought) and the closure of coal plants. A 2021 report from researchers at MIT and Stanford states that keeping Diablo Canyon running until 2035 would reduce the state's carbon emissions from electricity generation by 11% every year, save the state a cumulative $2.6 billion (rising to $21 billion if kept open until 2045), and improve the reliability of the grid. Full decommissioning of the plant is estimated to take decades and cost nearly $4 billion.
2021-22 Senate Bill 846 (Dodd) extended the plant's operations through 2030.
Operation
Diablo Canyon Power Plant is on approximately of land located just west of Avila Beach, California. The power-producing portion of the plant occupies around . PG&E owns a total of of land at the site.
Unit One Unit One is a 1138 MWe pressurized water reactor supplied by Westinghouse. It went online on May 7, 1985, and is licensed to operate through November 2, 2024. In 2006, Unit One generated 9,944,983 MW·h of electricity, at a nominal capacity factor of 99.8 percent.
Unit Two Unit Two is a 1118 MWe pressurized water reactor supplied by Westinghouse. It went online on March 3, 1986, and is licensed to operate through August 20, 2025. In 2006, Unit Two generated 8,520,000 MW·h of electricity, at a capacity factor of 87.0 percent.
Cooling The plant's once-through cooling system (OTC) draws water from the Pacific Ocean to condense steam driving its turbines. Unlike evaporative cooling systems used at other plants, Diablo Canyon's OTC is designed so all water can be recycled, and to assure minimal impact on ocean ecosystems. Reactors can be throttled during heavy storm surges to prevent an excess of kelp from entering the cooling water intake, and power is limited during operation so that water returned to the ocean is no more than warmer than ambient temperature.
All thermal power stations in California using OTC systems for cooling employ various filtering capabilities to prevent larvae and other aquatic objects from being drawn into impacts with the grids on the intake tubes, known as entrainment.PG&E Submitted False Cost Data in Diablo Canyon Case, New EP Investigation Finds The Diablo Canyon facility was ranked 13th in estimated power station bio-fouling and egg larvae damage in the state of California in 2013; the less productive fossil gas power units 6 & 7 at Moss Landing Power Plant were ranked as having a far higher impact on fish larvae.Evaluation of Fine - mesh Intake Screen System for the Diablo Canyon Power Plant. pdf table A1-2 pg 13. In 2014, the California Water Board released a white paper detailing the costs to convert Diablo Canyon to use cooling towers instead of the once-through cooling cycle.Proposed Subcommittee Comments on Bechtel’s Assessment of Alternatives to Once-Through-Cooling for Diablo Canyon Power Plant pdf These upgrade cost estimates have been the subject of controversy and debate, with some arguing instead for construction of an artificial reef to better offset the environmental impact of diminished larvae spawning.
Cost of generated electricity The costs incurred by PG&E at the Diablo Canyon Power Plant include the costs of operating and maintaining the facility, and what the plant's management collects in fees, such as the fixed management fee and the volumetric performance fees, which it passes through to ratepayers.
In 2024, PG&E estimated that their base cost, consisting only of plant maintenance and operations, would have been $43.60 per MWh, but when those fees were included, the unit cost rose to $111.21 per MWh. PG&E representatives told the LA Times that these estimates were caused by temporary factors relating to the decision to extend the plant's operational period, and that from 2026 through to the planned end of operations in 2030, costs were expected to be lower, closer to $75.91 per MWh.
In 2025, PG&E filed projections with California's Public Utility Commission that it would generate 18,203 GWh of electricity during 2026, and it would incur operational costs of $887.1 million, and another $452.3 million in fees, totaling $1.34 billion in costs, for a projected unit cost of $73.58 per MWh.
Labor There are approximately 1,200 employees of Pacific Gas & Electric and 200 employees of subcontractors at the Diablo Canyon site.Public hearing minutes, Santa Barbara Board of Supervisors, Special NRC session of June 16, 2011 Several unions represent the workforce at Diablo, among them the International Brotherhood of Electrical Workers (IBEW) and the International Association of Machinists. The routine outages for maintenance, and the complex process of refueling, create more than 1,000 temporary jobs, according to PG&E.More than 1,000 temporary workers – a boost to the local economy – were brought in to work with PG&E employees to replace a portion of the reactor fuel and to perform maintenance and testing on plant system components that are inaccessible during regular plant operations. The staff uses generative AI to find and organize data from the large documentation.
History
Pacific Gas & Electric Company went through six years of hearings, referendums and litigation to have the Diablo Canyon plant approved. A principal concern about the plant is whether it can be sufficiently earthquake-proof; the site was deemed safe when construction began in 1968, but a seismic fault (the Hosgri fault) had been discovered several miles offshore by the time the plant was completed in 1973. This fault experienced a 7.1 magnitude quake 10 miles offshore on November 4, 1927, and thus is capable of generating forces equivalent to approximately of those felt in the 1906 San Francisco earthquake.
The company updated its plans and added structural supports designed to reinforce stability in case of earthquake. In September 1981, PG&E discovered that a single set of blueprints was used for these structural supports; workers were supposed to have reversed the plans when switching to the second reactor, but did not. Nonetheless, on March 19, 1982, the Nuclear Regulatory Commission decided not to review its 1978 decision approving the plant's safety, despite these and other design errors.
In response to concern that ground acceleration, or shaking, could cause spillage of submerged fuel rod assemblies which could ignite upon exposure to air, PG&E and NRC regulators insist that the foregoing scenario is anticipated and controlled for, and that there is no basis to anticipate spillage. The launch of additional seismic studies did not delay re-issuance of the operating licenses for the two onsite units.
The Nuclear Regulatory Commission's estimate of the risk each year of an earthquake intense enough to cause core damage to the reactor at Diablo Canyon was 1 in 23,810, according to an NRC study published in August 2010.
In 2009, PG&E applied to the Nuclear Regulatory Commission (NRC) for 20-year license renewals for both reactors.
In April 2011, in the wake of the Fukushima nuclear incident in Japan, PG&E asked the NRC not to issue license renewals until PG&E could complete new seismic studies, which were expected to take at least three years.
On June 24, 2013, at 9:20 PM PDT, Diablo Canyon experienced a loss of offsite power to the startup transformers of both units due to a failure on the 230 kV transmission system. At the time, none of the startup transformers were loaded as both units were online and their electrical systems were at the time being powered by the plant's turbine generators. However, the emergency diesel generators were started with no load during the outage as a precaution in case either unit tripped offline while offsite power was unavailable. The electrical output of the plant via the 500 kV transmission system was not interrupted, allowing both units to remain online during the outage.
Public participation and protest
Diablo Canyon was built and entered service in the midst of legal challenges and civil disobedience from the anti-nuclear protesters of the Abalone Alliance. Over a two-week period in 1981, 1,900 activists were arrested and sent to jail for protesting at Diablo Canyon Power Plant, including musician/activist Jackson Browne. It was the largest arrest total in the history of the U.S. anti-nuclear movement.
In the spring of 2011, State Senator Sam Blakeslee and US Representative Lois Capps both expressed concern for a renewed safety review. Speaking before the Senate Committee on Environment and Public Works, Representative Capps stated that she believed the "Nuclear Regulatory Commission should stay the license renewal process until the completion of independent, peer reviewed, advanced seismic studies of all faults in the area." The Alliance for Nuclear Responsibility began circulating a petition to similar effect, going further and calling for an outright halt to relicensing. An array of San Luis Obispo-based anti-nuclear groups including Mothers for Peace also called for closure of the plant.
Post-Fukushima developments