The Diablo Canyon Power Plant is a nuclear power plant near Avila Beach in San Luis Obispo County, California. Since the permanent shutdown of the San Onofre Nuclear Generating Station in 2013, Diablo Canyon has been the only operational nuclear plant and largest single power station in California. 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 nuclear 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. The plant produces electricity for about 6 cents per kWh, less than the average cost of 10.1 cents per kWh that PG&E paid for electricity from other suppliers in 2014.
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 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 Nuclear Regulatory Commission Region IV.
In 2016, PG&E announced that it 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. (Nuclear plants are used for base load in order to spread their large fixed costs over as many kWh of generation as possible.) 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. In 2021 the California Energy Commission and CAISO warned 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.
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 back 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.