Diablo Canyon Power Plant

Diablo Canyon Power Plant is a nuclear power plant located near Avila Beach in San Luis Obispo County on the central California coast. After San Onofre Nuclear Generating Station ceased operations in June 2013, Diablo Canyon became California's last nuclear power plant. The plant is operated by Pacific Gas & Electric (PG&E) and monitored by the Nuclear Regulatory Commission (NRC). In 2016, PG&E announced it was not seeking to relicense the Diablo Canyon Power Plant, and it will close in 2025 when its license expires. However, the subject of extending the use of the Diablo Canyon Power Plant became an area of debate. In August 2022, California lawmakers voted to lend $1.4 billion to PG&E to keep the plant running until 2030. This was followed by PG&E applying to the Energy Department for nuclear plant bailout subsidies. On November 1st, 2022, PG&E formally asked NRC to extend the Diablo Canyon Power Plant licenses until 2030. On November 21, 2022, the Department of Energy announced $1.1 billion in funding to continue running the Diablo Canyon Power Plant.

Diablo Canyon Nuclear Power Plant near San Luis Obispo, California, image taken March 31, 2015, by Michael Macor (San Francisco Chronicle).

First proposed in 1966, Diablo Canyon began operations in 1985 with a forty-year license from the NRC. The plant has a generating capacity of 2,323 megawatts and generated 16.5 million megawatt hours of power in 2020, enough for 1.6 million homes. Diablo Canyon's two reactors are the western grid's third-largest generators after Arizona’s Palo Verde nuclear plant and Washington’s Grand Coulee Dam. It is the largest single source of electricity in California, producing around 9 percent of the state's energy. Diablo Canyon Power Plant employs around 1,500 PG&E workers, making it the second largest employer in San Luis Obispo County.

In 1963, California's major public utility, PG&E, reported plans to treble its generating capacity over the next decade to meet California's projected population growth. The company estimated two-thirds of that capacity would need to be nuclear, building reactors at seven to ten new sites throughout the state. Water requirements for cooling meant most of these plants would have to be close to the ocean. PG&E had already received preliminary approval from California's Public Utilities Commission and the Atomic Energy Commission to construct a plant at Bodega Bay, fifty miles north of San Francisco. However, there were significant objections led by the Sierra Club, an environmental group based in San Francisco, which opposed plans based on the site's scenic value not safety concerns around nuclear power. PG&E would later withdraw its application for licensing in late 1964 after it was discovered the planned reactors would sit over the San Andreas Fault.

In February 1963, PG&E announced plans to construct five nuclear reactors at the Nipomo Dunes in southern San Luis Obispo County. PG&E purchased 1,121 acres of Nipomo Dunes, fifteen miles of shoreline stretching from Point Sal north to Pismo Beach in San Luis Obispo County. Officials of the county had previously zoned the dunes for heavy industry, with railroad tracks, a highway, an oil refinery, and a coking plant, already on the dune's western edge. Plans for a nuclear power plant on the dunes received pushback from the Sierra Club with PG&E entering into negotiations with the environmental group, to select an alternative site.

The utility company surveyed the coast of San Luis Obispo County, singling out ten sites, including Wild Cherry Canyon, that were large enough, accessible, remote, seismically safe, and near the ocean. By early 1965, rumors in the press stated the Sierra Club was going to agree to a compromise with either a plant moved back from the water (requiring conduits through the dunes) or support for another site. This caused dissent within the Sierra Club with Frederick Eissler, head of its Santa Lucia group in San Luis Obispo County, stating,

I am personally concerned about health, safety and public welfare factors and a number of experts are, too, I would judge. I have reason to believe that many more people would feel the same way if they had access to insurance and low level radiation data and a host of other facts... it would be unfortunate, and supererogatory for us to endorse the Wild Cherry Canyon site.

The owner of Wild Cherry Canyon, a cattle and land company, refused to sell. Instead offering another property, Diablo Canyon, an unzoned site in San Luis Obispo County that lay north of Nipomo Dunes, between Avila Beach and Point Buchen. The Sierra Club and Conservation Associates met with the chairman of PG&E's board and its president, reaching a verbal agreement whereby the utility company would open negotiations to acquire Diablo Canyon, freeing Nipomo to be purchased for park purposes, provided the Club would commit itself to the new reactor site. In 1965, the Diablo Canyon site became the alternative to Nipomo Dunes and PG&E began the process of constructing a two-unit reactor with the Atomic Energy Commission (precursor to the Nuclear Regulatory Commission) and the California Public Utility Commission (CPUC).

Aerial photo of Diablo Canyon before construction taken in the late 1960s.

Plans for the nuclear power plant at Diablo Canyon became public in 1966, sparking infighting among Sierra Club members. While club director David Brower opposed the project because it would industrialize a relatively undisturbed stretch of coastline, board president William Siri (who had previously worked on the Manhattan project) supported it arguing that building a single large nuclear plant on the coast would keep coal plants and hydroelectricity dams out of wildlands.

PG&E applied to the state Public Utilities Commission and Atomic Energy Commission for permits to build a $188.4 million plant capable of producing one million kilowatts of power in Diablo Canyon. Hearings before the Public Utilities Commission continued through 1967. Despite objections from the Scenic Shoreline Preservation Conference (a non-profit corporation founded by Fred Eissler), the commission did not find their evidence convincing and approved the utility's application in November 1967. The Sierra Club’s membership would eventually side with Siri endorsing the construction of the Diablo Canyon Power Plant. Brower would go on to leave the Sierra Club, founding "Friends of the Earth (FoE)" in 1969 to continue fighting the construction of nuclear power plants.

The construction permit for Diablo Canyon's unit 1 was formally approved in April 1968 with construction beginning in July 1968. Unit 2's permit was issued in December 1970, with construction starting in early 1971. Throughout the 1970s, Diablo Canyon was the subject of hearings, referenda, and litigation covering issues involving earthquake safety, security plans, and environmental quality. The plant also had a major redesign to increase safety features after the discovery of the Hosgri fault line.

During construction, Diablo Canyon was the target of numerous protests. These were amplified by the discovery of the Hosgri earthquake fault and a growing anti-nuclear movement after the Three Mile Island accident and the opening of The China Syndrome film on a nuclear plant coverup. Major protests took place in San Francisco as well as attempts to blockade the plant's gates and occupy its grounds.

In September 1981, the NRC issued a low power-operating license for Unit 1. This ruling giving PG&E the authority for low-level operations led to what the San Francisco Chronicle called "the largest anti-nuclear civil disobedience campaign in the nation’s history." After receiving permission from a local landowner, the Abalone Alliance, a group consisting of more than a dozen environmental organizations, set up a base camp on 65 acres near the plant. The protesters practiced fence-climbing and trained in both non-violent and non-cooperation techniques. In September 1981, thousands of demonstrators attempted to block the main gates of the plant site for almost two weeks, leading to 1,400 arrests.

Abalone Alliance protestors blocking the gates to the Diablo Canyon site, September 1981.

A week after the protests, PG&E officials revealed engineers had used the wrong blueprints while installing the plant’s earthquake safety improvements, delaying operations. PG&E voluntarily chose to postpone fuel loading after the discovery of design errors in the annulus region of the containment structure. As a result, the NRC suspended portions of the license on November 19, 1981, pending the completion of an Independent Design Verification Program. After completing redesigns and new construction activities, the NRC reinstated the fuel load portion of unit 1's low power-operating license in November 1983.

In 1984, after years of hearings, protests, blockades, interventions, court cases, retrofits, and reconstruction, the NRC granted PG&E full-power licenses for Unit 1 on August 2nd and Unit 2 on November 2nd. By mid-December 1984, the plant was operating at 50 percent capacity. Unit 1 began commercial operation on May 7, 1985, followed by Unit 2 on March 13, 1986.

Position of the Diablo Canyon Power Plant in California.

The Diablo Canyon Power Plant site consists of roughly 750 acres in San Luis Obispo County, California, next to the Pacific Ocean. The site is roughly equidistant from San Francisco and Los Angeles. The minimum distance from either reactor to the site boundary on land is half a mile. The low population zone for Diablo Canyon encompasses a 6-mile radius. This zone contains residents that have appropriate protective measures in the event of a serious accident. The population center distance, is approximately 10 miles, the distance to the nearest boundary of the city of San Luis Obispo.

The plant's site occupies a coastal terrace, approximately 1000 feet wide and ranging in elevation from 60 to 150 feet above sea level. The seaward edge of the terrace is a near-vertical cliff. Back from the terrace is an area of steep hillsides and deep canyons that are part of the San Luis Mountains. Access to the site is by a private road from Avila Beach, a distance of nearly 8 miles.

Seismological investigations to determine the potential for earthquakes in the site area formed the basis of the seismic design criteria as well as the evaluation of seismic design margins for the plant. Records show seismic activity within 20 miles of the Diablo Canyon site is low compared to other areas of California. Until PG&E's seismological investigation of the Hosgri fault zone (approximately 3 miles offshore), the most significant fault system near the site was considered to be the Nacimiento fault, roughly 20 miles away. The largest earthquake associated with this system had a Richter magnitude of 6 and an epicentral distance of roughly 44 miles away from the plant. The San Andreas fault passes 48 miles from the site.

The Diablo Canyon Power Plant facility includes the containment structures, turbine building, and auxiliary building, which houses the control room, the fuel handling areas, and the ventilation areas. The site has a machine shop, access control area, warehouse area, telecommunications system, and administrative offices. The auxiliary building and turbine building are common to both reactor units, along with the following:

• Raw water storage reservoir
• Fire pumps
• Fire water storage tank
• Diesel fuel oil storage tank
• Transfer pumps
• Auxiliary boiler
• Makeup water system
• Plant air system
• Lubricating oil storage system

Map of the Diablo Canyon Power Plant facilities.

Essential systems of the plant are designed to withstand, without loss of capability to protect the public, the forces resulting from normal operation and those that might be imposed by natural phenomena. Designs are based on records of the most severe natural phenomena recorded in the vicinity of the site, with margins to account for uncertainties in the historical data. The plant's units were designed to comply with the "General Design Criteria for Nuclear Power Plant Construction Permits," published in July 1967.

The NSSS consists of the PWR and auxiliary fluid systems. The NSSS was designed and furnished by Westinghouse Electric Corporation, supplying all components and systems. Westinghouse provided design criteria, outlines, assembly drawings, and system flow diagrams for PG&E to install, operate, and maintain their equipment. Westinghouse also performed a post-accident transient analysis of the NSSS containment system. This included determining the mass and energy releases as a function of time allowing PG&E to determine the design pressure, temperature, and containment volume requirements.

While the reactors, structures, and auxiliary equipment are substantially identical for both units, there are differences in the reactor's internal flow path, producing a lower coolant flow rate. As a result, reactor ratings on the original license applications were 3,338 MWt for unit 1 and 3,411 MWt for unit 2 (MWt refers to a thermal megawatt), corresponding to approximate net electrical outputs of 1,084 MWe and 1,106 MWe (MWe refers to an electric megawatt), respectively. During the design phase, the ultimate expected output of the reactors was 3,488 MWt for unit 1 and 3,568 MWt for unit 2.

Both reactors are Model W (4-loop) from Westinghouse. The reactor coolant system contains four steam generators, four reactor coolant pumps, and a pressurizer connected to the hot leg of a reactor coolant loop. The core has 193 fuel assemblies inside a reactor vessel with an internal diameter of 173 inches. These fuel assemblies are arranged in a 17 x 17 array.

Diagram of a four-loop Westinghouse PWR.

Each fuel assembly contains 264 fuel rods composed of uranium dioxide pellets enclosed in zirconium alloy tubes with welded end plugs. Fuel rods are pressurized with helium during fabrication to reduce stress and increase fatigue life. The reactor's control and shutdown functions are performed by the rod cluster control assemblies (RCCAs), stainless steel tubes containing a silver-indium-cadmium absorber. The RCCAs are positioned by drive mechanisms of the magnetic latch type. To compensate for long-term reactivity changes a soluble poison (boron) is introduced into the reactor coolant. While the power coefficient is negative at all times, at the start of a cycle (when boron concentration is higher) the moderator temperature coefficient can be slightly positive. The reactor vessel and internals support the fuel and RCCAs. Clad with stainless steel, the vessel is cylindrical with hemispherical heads. The pressurizer is a vertical cylindrical vessel with hemispherical heads, equipped with electrical heaters and spray nozzles for pressure control.

The steam generators consist of vertical U-tube type heat exchangers with Inconel tubes (high-temperature non-stainless steel tubes). Reactor coolant flows inside the tubes, steam is generated in the shell and flows to the turbine via the main steam lines. During full operations (100% power), integral moisture separating equipment reduces the moisture content of the steam at the exit of the steam generators to below 0.05%. During transient conditions (less than 100% power) the moisture content at the exit of the steam generators remains below 0.25%. The reactor coolant pumps (RCPs) are verticle, single-stage, centrifugal units with controlled leakage shaft seals.

Auxiliary systems charge the reactor coolant system (RCS) and add makeup water to:

• Purify reactor coolant water
• Provide chemicals for corrosion inhibition and reactor control
• Cool system components
• Remove residual heat when the reactor is shut down
• Cool the spent fuel storage pool
• Sample reactor coolant water
• Provide in case of emergency safety injection
• Vent and drain the RCS.

The reactors are refueled with equipment designed to handle spent fuel under water from when it leaves the reactor vessel until it is positioned in a cask for transport to the Diablo Canyon Independent Spent Fuel Storage Installation or for shipment from the site. Transferring spent fuel underwater provides an optically transparent radiation shield and a coolant to remove decay heat. Spent fuel is stored onsite in pools fitted with special racks to ensure that criticality cannot be achieved. The fuel handling system (FHS) also provides the capability to receive, handle, and store new fuel assemblies.

Radioactive waste treatment systems provide the necessary equipment to collect, process, monitor, and discharge radioactive liquid, gaseous, and solid wastes that are produced during reactor operation. Located in the auxiliary building, significant portions of the waste treatment equipment are common for Unit 1 and Unit 2.

The main function of the instrumentation and control system is to provide automatic protection against unsafe and improper reactor operation. This includes during steady state and transient power operation, providing initiating signals to mitigate the consequences of faulted conditions. Instrumentation and control system operations are monitored and controlled from the control room, located in the auxiliary building.

Electrical systems are responsible for generating and transmitting power to PG&E's high-voltage system, distributing power to the auxiliary loads, and providing control, protection, instrumentation, and annunciator power supplies for the units. Power is generated at 25-kV. Offsite AC power for the units' auxiliaries is available from two 230-kV transmission circuits and three 500-kV transmission circuits. Onsite AC auxiliary power is supplied by each unit's main generator and is also available from six diesel engine-driven generators (standby power supply). Each reactor unit has three dedicated diesel generators. The 230-kV line from the switchyard serves the standby startup transformers for both Unit 1 and Unit 2.

The turbines turn the steam generated by the NSSS into electricity via tandem-compound, four-element (one high-pressure and three identical double flow low pressure), 1800 rpm units. Between the high and low-pressure elements combination moisture separator reheaters are used to dry and superheat the steam. Auxiliary systems include the following:

• Deaerating surface condensers
• Steam jet air ejectors
• Motor-driven condensate pumps
• Motor-driven condensate booster pumps
• Turbine-driven main feedwater pumps
• Six stages of feedwater heating
• Full-flow condensate demineralizer system

The steam and power conversion system receives the heat generated by the RCS during normal power operation and follows an emergency shutdown of the turbine generator from the full load. Heat rejection under an emergency shutdown is accomplished by steam bypass to the condenser and pressure relief to the atmosphere.

Diablo Canyon Power Plant has a series of engineered safety features to protect the health and safety of the public. These include the following:

• A containment system consisting primarily of a steel-lined, reinforced concrete containment structure designed to prevent any significant release of radioactive materials resulting from an accident
• An Emergency Core Cooling System (ECCS) providing water to cool the core in the event of loss of reactor coolant water; the ECCS also supplies dissolved boron in the cooling water to provide a shutdown margin
• A containment spray and fan cooler system to limit the peak temperature and pressure inside the containment system if a loss of coolant occurs or the main steam line breaks
• A spray additive system adding sodium hydroxide, an iodine scrubbing solution, to the spray system water reducing the level of iodine and other fission products
• A mixed containment atmosphere and hydrogen monitoring equipment to prevent the buildup of gaseous hydrogen in the containment structure following a loss of coolant accident
• A fuel-handling building ventilation system reducing the amount of volatile radioactive material that could be released into the atmosphere in the event of a fuel-handling accident
• An auxiliary building ventilation system that provides a reduction in the level of radioactive material released in the event of leakage from the residual heat removal system recirculation loop following a loss of coolant accident.
• A control room ventilation system allowing continuous occupancy in the event of an accident
• An auxiliary feedwater system supplying water to the secondary side of the steam generators for reactor decay heat removal of the main system is unavailable

Unit 1 began commercial operations on May 7th, 1985, followed by unit 2 on March 13th, 1986. The International Atomic Energy Agency's (IAEA's) Power Reactor Information System (PRIS) maintains a database of nuclear power plants around the world including their energy production. PRIS data up until 2021 shows the lifetime output for Diablo Canyon unit 1 and unit 2 is 306.69 TWh (terawatt-hour and 298.81 TWh, respectively.

PRIS operating history data for both units 1 and 2 are shown in the tables below. For each year, the data includes the following:

• Reference unit power—the maximum electrical power that could be maintained over a prolonged period of operation
• Annual time online—the time spent online during the year
• Operation factor—the ratio of the time online to the total time over a given reference period
• Energy availability factor—the ratio of the energy produced to the energy that could have been produced given the available capacity
• Load factor—the ratio of the energy a reactor produced divided by the energy it would have produced at its reference power capacity, also known as the capacity factor

As of 2021, Diablo Canyon unit 1 had a cumulative energy availability factor of 88.4% and a cumulative load factor of 87.4%.

As of 2021, Diablo Canyon unit 1 had a cumulative energy availability factor of 88.3% and a cumulative load factor of 86.5%.

The radiological environmental monitoring program (REMP) for Diablo Canyon is designed to perform the following:

• Provide an early indication of the appearance or accumulation of radioactive material caused by the facility's operations
• Provide assurances to regulatory agencies and the public that the plant's environmental impact is known and below anticipated levels
• Provide monitoring capabilities for rapid assessment of risks to the general public in the event of an accidental release of radioactive material

The program monitors multiple possible pathways, including those below:

• Direct radiation
• Airborne radioactivity
• Waterborne pathways
• Marine biological, beach sand, and ocean sediment
• Food crops
• Milk
• Meat

Maps of Diablo Canyon's on-site and off-site REMP stations:

Map of Diablo Canyon Power Plant on-site REMP stations.

Map of Diablo Canyon Power Plant off-site stations.

PG&E submits an annual radiological environmental operating report for Diablo Canyon to assess the levels of radioactivity in the environment and verify the plant is operating within its design parameters. The 2019 report found the annual offsite radiological dose from plant operations received by the general public was less than one millirem (mrem), significantly smaller than the average annual radiation exposure of people living in the United States (20 mrem). Samples from offsite sampling stations continued to show no radiological contribution from the plant.

On June 21st, 2016, PG&E announced it would not renew the NRC licenses for the Diablo Canyon Power Plant, shutting down operations of the two reactors in 2024 (Unit 1) and 2025 (Unit 2) when their current licenses expire. PG&E's proposal, part of an agreement with environmental and labor groups, intends to help California reach its aggressive clean energy goals. In 2016, California's policies required utilities to include 50% renewables in the power they provide by 2030. Lieutenant Governor of California at the time, Gavin Newsom, started discussions on closing the plant as a member of the State Lands Commission. At the time of the announcement, executives estimated that decommissioning the plant would cost roughly $3.8 billion.

The closure of Diablo Canyon was approved by the California Public Utilities Commission in 2018. PG&E submitted a land use permit application for the decommissioning effort to San Luis Obispo county in March 2021. On October 27, 2021, the County accepted PG&E’s application, initiating the environmental review process pursuant to the California Environmental Quality Act.

The decision to close Diablo Canyon was met with criticism, including a 2021 report from researchers at Stanford University and the Massachusetts Institute of Technology. Their report claims that delaying the closure of the plant by a decade, until 2035, would reduce carbon emissions from state utilities by more than 10% from 2017 levels, reduce reliance on gas, and save $2.6 billion in power costs. If operations continue until 2045 and beyond could save up to $21 billion in power system costs and prevent a further 90,000 acres of land from becoming used for energy production. The report also suggests the following new applications for the Diablo Canyon Power Plant:

• A power source for desalination substantially augmenting fresh water supplies to the state
• A connected hydrogen plant that could produce clean hydrogen to meet the demand for zero-carbon fuels at 50% of the cost it would take to produce using solar or wind power
• Operating as a polygeneration facility with coordinated production of electricity, desalinated water, and clean hydrogen

On August 31, 2022, the California State Legislature passed Senate Bill 846 seeking to extend operations at Diablo Canyon Power Plant (DCPP) up to 2030.The bill lends $1.4 billion to PG&E to keep the plant running until 2030. After the legislation passed, PG&E applied to the Energy Department for nuclear plant bailout subsidies. On November 1s 2022, PG&E formally asked NRC to extend the Diablo Canyon Power Plant licenses until 2030. On November 21, 2022, the Department of Energy announced $1.1 billion in funding to continue running the Diablo Canyon Power Plant. The funding comes from a $6 billion Civil Nuclear Credit program made possible after the Infrastructure Law passed by Congress in 2021. Diablo Canyon is the first power plant to receive funding from the new program which plans to extend the operations of a number of nuclear reactors in the US. The Department of Energy grant is conditional with funding awarded incrementally over multiple based on costs accrued by the plant. To remain online, Diablo Canyon still needs to renew its license with the NRC.