Diablo Canyon Power Plant

Diablo Canyon Power Plant
Diablo Canyon Power Plant
Country United States
Location San Luis Obispo County, California
Coordinates 35°12′39″N 120°51′22″W / 35.21083°N 120.85611°W / 35.21083; -120.85611Coordinates: 35°12′39″N 120°51′22″W / 35.21083°N 120.85611°W / 35.21083; -120.85611
Status Operational
Construction began Unit 1: April 23, 1968
Unit 2: December 9, 1970
Commission date Unit 1: May 7, 1985 (1985-05-07)
Unit 2: March 13, 1986 (1986-03-13)
Decommission date 2025 (planned)
Construction cost $11.556 billion (2007 USD)[1]
($13.2 billion in 2016 dollars[2])
Owner(s) PG&E Corporation
Operator(s) Pacific Gas and Electric Company
Nuclear power station
Reactor type PWR
Reactor supplier Westinghouse
Cooling source Pacific Ocean
Power generation
Units operational 1 × 1138 MW
1 × 1118 MW
Make and model WH 4-loop (DRYAMB)
Thermal capacity 2 × 3411 MWth
Nameplate capacity 2256 MW
Capacity factor 90.93% (2017)
87.25% (lifetime)
Annual net output 17,970 GWh (2017)
Diablo Canyon Power Plant

The Diablo Canyon Power Plant is an electricity-generating nuclear power plant near Avila Beach in San Luis Obispo County, California. After the permanent shutdown of the San Onofre Nuclear Generating Station in 2013, it is the only remaining nuclear plant operational in the state. The plant has two Westinghouse-designed 4-loop pressurized-water nuclear reactors operated by Pacific Gas & Electric (PG&E).

The facility is located on about 900 acres (360 ha) west of Avila Beach, California, of which about 12 acres form the power-producing portion of the plant. Together, the twin 1100 MWe reactors produce about 18,000 GW·h of electricity annually (about 8.6% of the electricity California uses), supplying the electrical needs of more than 3 million people.[3] 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 three miles from the Hosgri fault.[4][5][6][7][8][9]

The plant is located in Nuclear Regulatory Commission Region IV. In November 2009, PG&E applied to the Nuclear Regulatory Commission (NRC) for 20-year license renewals for both reactors.[10] In June 2016, PG&E announced that it plans to close the two Diablo Canyon reactors in 2024 and 2025.[11]

With the closing of the San Onofre plant in 2013, fossil gas became the primary source of electricity for the state.[12] However, the falling cost and increasing use of renewable energy coupled with grid energy storage systems and mandated state energy generation targets will make renewable energy the dominant source of energy in California in the coming decade. [13]


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.[14] In 2006, Unit One generated 9,944,983 MW·h of electricity, at a nominal capacity factor of 101.2 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.[14] In 2006, Unit Two generated 8,520,000 MW·h of electricity, at a capacity factor of 88.2 percent.


The plant draws cooling water from the Pacific Ocean, and during heavy storm surges the units can be throttled back to prevent an excess of kelp from entering the cooling water intake. The plant utilizes once-through cooling for the condenser, which is returned to the Pacific Ocean at a temperature that is regulated to be no more than 20 degrees Fahrenheit warmer than ambient ocean temperatures at the point of output, heating the water up to a one-quarter (1/4) mile radius from the discharge point of the plant to a slightly higher temperature.

All thermal power stations in California, including the fossil gas power stations that likewise use the cheaper once-through cooling (OTC) cycle and produce a similar warming impact upon the surrounding waters, employ various filtering capabilities to prevent larvae and other aquatic objects from being drawn into impacts with the grids on the intake tubes, a process known as entrainment.[15] 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.[16] In 2014, the California Water Board released a white paper detailing the costs to convert Diablo Canyon to utilize cooling towers instead of the once-through cooling cycle.[17] 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.[18]


Pacific Gas & Electric Company went through six years of hearings, referenda 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 1/16 of those felt in the 1906 San Francisco earthquake.[19]

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.[20] According to Charles Perrow, the result of the error was that "many parts were needlessly reinforced, while others, which should have been strengthened, were left untouched."[21] 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.[22]

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.[23] The launch of additional seismic studies did not delay re-issuance of the operating licenses for the two onsite units.[24]

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.[25][26] 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.[27][28]

In October 2008, Unit 2 was taken offline for approximately two days due to a rapid influx of jellyfish at the intake.[29] In February 2013, Unit 2 was shut down for refueling and upgrading.[30]

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 despite legal challenges and civil disobedience from the anti-nuclear protesters of the Abalone Alliance.[31] Over a two-week period in 1981, 1,900 activists were arrested and sent to jail for protesting at Diablo Canyon Power Plant. It was the largest arrest total in the history of the U.S. anti-nuclear movement.[31]

In spring of 2011, State Senator Sam Blakeslee and US Representative Lois Capps both expressed concern for a renewed safety review.[32] [33] 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,[34] 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.[35]

The Alliance for Nuclear Responsibility also works at the level of the California Public Utilities Commission and initiated a letter writing campaign to Governor Jerry Brown requesting he "instruct the CPUC to rescind the Certificate of Public Convenience and Necessity for...Diablo Canyon...and allow them to operate conditionally only under the agreement by the utilities to immediately begin to fully comply with completion of the state-directed AB 1632 [seismic] studies."[36]

Post-Fukushima developments

Due to international reactions to the Fukushima Daiichi nuclear disaster, concerns have continued over the ongoing operations of Diablo Canyon which, like the reactors at Fukushima, is in an area prone to earthquakes and tsunami. The elevation of the Fukushima site is approximately 20 feet above sea level, while Diablo Canyon sits on a bluff 85 feet above sea level. According to Victor Dricks, senior public affairs officer for NRC Region IV, the Commission conducted a nationwide review of nuclear power plants for their capacity to respond to earthquakes, power outages and other catastrophic events, and Diablo was found to have "a high level of preparedness and strong capability in terms of equipment and procedures to respond to severe events."[37]

On June 2, 2011, the NRC announced that it would delay the environmental part of the re-licensing application but that it had completed the safety portion.[38] A few days later, the Atomic Safety Licensing Board (ASLB) indicated that it would defer adjustment of the adjudicatory schedule of the four contentions brought by San Luis Obispo Mothers for Peace (SLOMFP), a community-based organization, accordingly. The ASLB made no findings regarding the merits of the contentions; both PG&E and SLOMFP claimed these developments as victories.[39] [40]

S. David Freeman, a former general manager of the Sacramento Municipal Utility District for four years, criticized the continued operation of Diablo Canyon, calling nuclear power the "most expensive and dangerous source of energy on Earth." According to Freeman, Diablo Canyon and the since-closed San Onofre nuclear plant are both "disasters waiting to happen: aging, unreliable reactors sitting near fault zones on the fragile Pacific Coast, with millions or hundreds of thousands of Californians living nearby."[41]

Save Diablo Canyon campaign

In January 2016, several authors of the An Ecomodernist Manifesto (including Robert Stone, David Keith, Stewart Brand, Michael Shellenberger, Mark Lynas) signed an open letter to California Governor Jerry Brown, Tony Earley, CEO of Pacific Gas & Electric, and California state officials, urging that the plant not be closed.[42] [43] [44] They argued that Diablo is an asset for California in achieving global warming goals since it does not emit greenhouse gases like a natural gas power plant, which cause global warming.[45]

Closure in 2024–2025

On June 21, 2016, PG&E announced a Joint Proposal with labor and environmental organizations to increase investment in energy efficiency, renewables and storage, while phasing out nuclear power. Specifically, the operating licenses for Diablo Canyon Units 1 and 2 would not be renewed when they expire on November 2, 2024, and August 26, 2025, respectively.[46] In February 2018, PG&E accepted the proposal and withdrew its application to the Nuclear Regulatory Commission for a licensing extension.[47] The California Public Utilities Commission had approved the shutdown the previous month, but reaffirmed its support for greenhouse gas-free energy sources, noting that the falling cost of renewable energy rendered Diablo Canyon’s nuclear power uneconomical. [48]


Overall, there are approximately 1,200 employees of Pacific Gas & Electric and 200 employees of subcontractors at the Diablo Canyon site.[49] Several unions represent the workforce at Diablo, among them the International Brotherhood of Electrical Workers (IBEW) and the International Association of Machinists. Control technicians and electricians who work for Crane Nuclear, one of many subcontractors, are represented by the IBEW Local 1245.[50] Local 1245 also represents PG&E meter readers, clerical workers, etc.[51] Other workers are represented by the Plumbers and Pipefitters.[52]

The routine outages for maintenance and the complex process of refueling create more than 1,000 temporary jobs, according to PG&E.[53]


Earthquake protection

Diablo Canyon was originally designed to withstand a 6.75 magnitude earthquake from four faults, including the nearby San Andreas and Hosgri faults,[54] but was later upgraded to withstand a 7.5 magnitude quake.[55] It has redundant seismic monitoring and a safety system designed to shut it down promptly in the event of significant ground motion.

Independent Safety Committee

The Diablo Canyon Independent Safety Committee (DCISC) was established as a part of a settlement agreement entered into in June 1988 between the Division of Ratepayer Advocates of the California Public Utilities Commission (CPUC), the Attorney General for the State of California, and Pacific Gas and Electric Company (PG&E). It consists of three members, one each appointed by the Governor, the Attorney General and the Chairperson of the California Energy Commission. They serve staggered three-year terms. The committee has no authority to direct PG&E personnel.

Emergency planning

The Nuclear Regulatory Commission defines two emergency planning zones around nuclear power plants: a plume exposure pathway zone with a radius of 10 miles (16 km), concerned primarily with exposure to, and inhalation of, airborne radioactive contamination, and an ingestion pathway zone of about 50 miles (80 km), concerned primarily with ingestion of food and liquid contaminated by radioactivity.[56]

The 2010 U.S. population within 10 miles (16 km) of Diablo Canyon was 26,123, an increase of 50.2 percent in a decade, according to an analysis of U.S. Census data for msnbc.com. The 2010 U.S. population within 50 miles (80 km) was 465,521, an increase of 22.4 percent since 2000. Cities within 50 miles include San Luis Obispo (12 miles to city center) and Paso Robles (31 miles to city center).[57]

Emergency sirens were installed when the plant initially went operational. Federal law requires an early warning system that radiates out 10 miles from any nuclear facility, but the county siren coverage goes farther, extending from Cayucos in the north down to upper Nipomo to the south. All businesses are required to have a siren information sticker in their business generally located within the restrooms. Schools, government offices, and any other public building will have a PAZ card (Protective Action Zone). These cards show the 12 zones of evacuation with zone one being the plant itself. The cards also show the direction of evacuation on the highways.

See also


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  53. 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.
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Further reading

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