Diablo Canyon Nuclear Power Plant is an electricity-generating nuclear power plant at Avila Beach in San Luis Obispo County, California. It was built directly over a geological fault line, and is located near a second fault. The plant has two Westinghouse-designed 4-loop pressurized-water nuclear reactors operated by Pacific Gas & Electric. The facility is located on about 750 acres (300 ha) in Avila Beach, California. Together, the twin 1,100 MWe reactors produce about 18,000 GW·h of electricity annually, supplying the electrical needs of more than 2.2 million people, sent along the Path 15 500-kV lines that connect to this plant.

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

The Diablo Canyon Nuclear Power Plant draws cooling water from the Pacific Ocean, and during heavy storms both units are throttled back by 80 percent to prevent kelp from entering the cooling water intake. The cooling water is used once and not recirculated.

The plant is located in Nuclear Regulatory Commission Region IV.

Unit One

Unit One is a 1,122 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 101.2 percent.

Unit Two

Unit Two is a 1,118 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 88.2 percent.

Safety

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 started in 1968.

However, by the time of the plant's completion in 1973, a seismic fault, the Hosgri fault, had been discovered several miles offshore. This fault had a 7.1 magnitude quake 10 miles offshore on November 4, 1927, and thus was capable of generating forces equivalent to approximately 1/16 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. 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." 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, upon exposure to air, could ignite, PG&E and NRC regulators insist that the foregoing scenario is anticipated and controlled for, and that there is no basis to anticipate spillage. Additional seismic studies are in process, however completion of those studies is not a condition precedent to reissuance of the operating licenses for the two onsite units.

A PG&E request to extend the life of the plant by 20 years has been postponed from April 2011 pending the resolution of the nuclear emergencies in Japan.

DCISC

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 (PUC), the Attorney General for the State of California, and Pacific Gas and Electric Company(“PG&E”).

The DCISC 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.

Cooling intake event

Starting October 22, 2008, Unit 2 was taken offline for approximately two days due to a rapid influx of jellyfish at the intake.