Located on a picture-perfect bluff overlooking California’s Avila Beach and the Pacific Ocean lies the Diablo Canyon Nuclear Power Plant (DCPP). In service since 1985, DCPP provides more than 18,000 megawatts of electricity annually to the California grid, comprising roughly 10 percent of California’s energy portfolio and meeting the needs of more than three million Northern and Central Californians. Helping to create more than 4,500 jobs and creating an annual economic impact of more than $1.8 billion, DCPP is at the heart of California’s energy future. However, key to the future of the DCPP and to that of many nuclear power plants worldwide is how to handle the threat of nearby seismic activity.
When the Diablo Canyon area near San Louis Obisbo, Calif. was selected to host DCPP in 1965, Pacific Gas & Electric (PG&E) was unaware of the seismic makeup of the surrounding area. Geologists from Shell Oil discovered the Hosgri earthquake fault in 1969 during the design review process. The earthquake potential from the newly discovered fault located just 2.5 miles from the DCPP site exceeded the maximum seismic capacity of the construction plans outlined in the initial license application to the Nuclear Regulatory Commission (NRC). Throughout the 1970s and early 1980s, PG&E and the NRC hosted numerous joint-meetings and workshops, transparently modifying the approved construction plans to ensure DCPP was able to withstand a seismic event. In 1984, after 15 years of hearings, retrofits and reconstructions, the NRC granted DCPP final approval and the two reactors began operation in May of 1985 and March of 1986.
The 15-year period between the initial design and eventual operating approval was not a simple or easy process for PG&E. Some nine revisions to its costs and operating plans were submitted and the retrofit ended up increasing the cost of construction by some $4.4 billion. However, the end result was a nuclear power plant that exceeded the expected maximum earthquake velocity in the region and was granted a full operating license by the NRC.
The long process of building trust between the operator and the regulator has again been called into action in recent years due to the 2008 discovery of the Shoreline fault some 600 meters from DCPP and the PG&E’s 2009 application to extend DCPP’s operating license until 2045. The original operating license for the two reactors is set to expire in 2024 and 2025. Pursuant to the discovery of the Shoreline fault and in preparation for the license extension, the NRC asked PG&E to do a new seismic analysis to assess any newly apparent risks to the safety of the DCPP. In 2011, PG&E submitted a report that stated the earthquake hazard presented by the newly discovered fault fell within the existent earthquake resistant design specifications and required no additional action. The following year, NRC performed its own analysis and came to the same conclusion, namely that, according to a probabilistic risk assessment, DCPP’s existing earthquake retrofits (the ability to withstand up to an earthquake from the Hosgri fault measuring 7.5 on the Richter scale) were sufficient for protecting against earthquakes from the newly discovered Shoreline fault.
Despite its location in an active earthquake zone and only 600 meters from the nearest fault, DCPP continues to operate safely under the close watch of the federal regulators. Even as the relicensing process continues, additional seismic testing is ongoing due to both the federal reexamination of seismic risk in the wake of the Fukushima accident as well as a California law requiring additional vulnerability testing.
As DCPP moves toward its fourth decade of operation in a seismically active area, continual seismic testing and retrofitting help minimize the probability of an earthquake triggered nuclear accident. As Lloyd Cluff, PG&E’s Chief Seismologist recently stated, “there’s always uncertainty in everything we do in in this world when it comes to earthquakes… [but] we don’t see a concern about the uncertainty.” Thanks to a culture of trust and cooperation between the NRC and PG&E, DCPP balances between need for caution with the necessity of continued operations.