PG&E: Diablo nuclear plant safe from extreme natural events

Source:Pacific Gas and Electric Co.

New and extensive analyses performed at the direction of the Nuclear Regulatory Commission (NRC) re-confirm that Pacific Gas and Electric Company's (PG&E) Diablo Canyon Power Plant (Diablo Canyon) can safely withstand extreme natural events, including potential earthquakes, tsunamis and flooding.

The new analyses, known as hazard re-evaluations, will be submitted to the NRC this week for independent review. The agency called on all U.S. commercial nuclear power plants to perform such work following the events that occurred in 2011 at the Fukushima Daiichi plant in Japan.

"Safety is and always will be the top priority for PG&E and Diablo Canyon. That's why seismic, flooding and tsunami safety was at the forefront in the design of the facility. These updated findings are the culmination of years of study and analysis, and further confirm the safety of the plant's design," said Ed Halpin, PG&E's Senior Vice President and Chief Nuclear Officer.

The hazard re-evaluations used the latest regulatory guidance, scientific methods and models, site-specific information and independent expertise to re-evaluate the impacts that earthquakes, large waves and flooding could have on the Diablo Canyon facility.

The updated seismic assessment represents a more extensive evaluation of the seismic hazard than previously performed. Prior evaluations determined the ground shaking from an earthquake on a particular fault in the region, based on historical records and geological evidence, and then compared this information against structures, systems and components at the facility to ensure they could withstand seismic ground shaking.

Using the NRC's Senior Seismic Hazard Analysis Committee (SSHAC) process, independent seismic experts publically re-evaluated existing and new seismic information, including data acquired during the advanced seismic studies recently performed near Diablo Canyon, to re-evaluate how earthquakes could potentially impact the facility. This process included examining the probability of earthquakes occurring on individual and multiple geologic faults.

The result is a more thorough assessment of the seismic hazard, providing additional confirmation that the plant is seismically safe.

The NRC will review the re-evaluation in order to provide guidance on how and when PG&E should perform an additional risk evaluation that examines the probability of earthquakes potentially damaging plant safety equipment. The same evaluation is being asked of other U.S. commercial nuclear power facilities.

"The updated seismic source model for Diablo Canyon incorporates an extensive body of new onshore and offshore data and emerging new scientific concepts to characterize earthquake sources. For example, it is the first earthquake model developed for a nuclear site that allows for multi-fault linked ruptures to produce large maximum earthquakes. Through the extensive use of experts in the field of seismic geology and independent peer review, the model was developed to capture the full range of possibilities regarding the location, size and frequency of large magnitude earthquakes in the vicinity of Diablo Canyon. In my opinion, the model developed for Diablo Canyon will set the standard for how future earthquake source models are developed," said Bill Lettis, who serves as one the key technical leads for the SSHAC process.

PG&E's flooding and tsunami hazard update involved the use of the latest NRC guidance and methodologies to determine the maximum potential waves and rainfall that could impact Diablo Canyon.

The re-evaluation, utilizing independent expertise, determined that the plant's key safety systems and components continue to be safe from tsunamis, including those generated from underwater landslides and earthquakes.

The plant's design is also deemed appropriate to withstand expected storm flooding. In addition, measures were identified and implemented to address a rare, theoretical event of excessive rainfall and a quick build-up of water in some plant locations that greatly exceeds any known precipitation event recorded in the site's history. The NRC will review the re-evaluation to determine if additional actions might be required to further enhance safety.

"These important updates provide an in depth look at the seismic, tsunami and flooding hazards in our region, and most importantly, demonstrate the plant's design is safe. Our work in these areas, however, will never cease. Our commitment to safe operations and protecting public health and safety will continue to be reflected in our ongoing study and evaluation of the areas. Our customers expect no less," said PG&E's Halpin.

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