Seismic Safety at San Onofre

San Onofre's Independent Spent Fuel Storage Installation is designed to withstand the maximum credible earthquake for its location without releasing radioactive materials. The commonly known Richter scale is not used to determine earthquake building safety for any building. Instead, building safety relies on a more accurate value known as "peak ground acceleration," which is based on the anticipated ground movement at the site during the largest potential earthquake, estimated by geologists. Additionally, the proximity of the fault and soil conditions must also be considered. So it is not accurate to simply say that San Onofre was only built to withstand a 7.0 earthquake.

As approved by the U.S. Nuclear Regulatory Commission, San Onofre's ISFSI was built to withstand a peak ground acceleration of at least 1.5g (g refers to the force of gravity), which is more than twice the rating of Unit 2 and 3 when they were operational. For comparison, the current California Building Code design requires any buildings built in the vicinity of San Onofre to be designed to withstand an earthquake motion that has peak ground acceleration of 0.38g.

Understanding the Differences: Operating Reactors vs. Spent Fuel

A major earthquake and subsequent tsunami damaged the reactor cooling and back-up power systems at the Fukushima Daiichi Nuclear Power Station in Japan in 2011. With these systems out of operation, the fuel became overheated and the interaction between the hot fuel rods and the water resulted in the generation of hydrogen gas. The operators vented the gas from the reactor system to the surrounding secondary reactor building. The buildup of hydrogen gas from the venting led to explosions in two of the reactor buildings, damaging their structures and allowing radioactive materials to escape.

Canisters of spent nuclear fuel are far different than operating reactors. A spent fuel canister contains no water, only inert helium gas. The hottest fuel assembly at SONGS gives off as much heat as a hair dryer. Without the heat, or water to create steam, there is no motive force in a spent fuel canister to propel material out. The standard pressure inside a welded canister is minimal. Statements by some about 8.4 million people living within a 50-mile radius of SONGS potentially being affected by an incident involving spent fuel are not based in science or reality.

Comparison Of Seismic Risks

Fault Types

The earthquake fault system that generated the devastating earthquake near Honshu, Japan, in 2011, originated in a subduction zone. Tsunamis that can be produced by an earthquake in a subduction zone are projected to be larger than those resulting from earthquakes in a strike-slip fault system, such as the Newport-Inglewood/Rose Canyon faults near San Onofre.

Richter Scale vs. Ground Movement

Much attention has been focused on the Richter scale measurements of the Japan earthquake – a 9.0 – and the implications of such a quake in California. While the Richter scale is one common way to measure the magnitude of earthquakes at their epicenters, when assessing the seismic safety at nuclear facilities, "peak ground acceleration" at the facility's location is a more meaningful way to measure an earthquake's potential impact, especially when the epicenter is miles away. As approved by the U.S. Nuclear Regulatory Commission, San Onofre's ISFSI was built to withstand a peak ground acceleration of 1.5g (g refers to the force of gravity). The maximum ground acceleration experienced at Fukushima Daiichi was 0.561g in a horizontal direction.

Independent evaluations of earthquake ground shaking hazards in the vicinity of SONGS are provided by the California Geological Survey (CGS) and U.S. Geological Survey (USGS). The CGS Earthquake Shaking Potential Map for California (Branum et al., 2008) portrays the San Onofre area as a region of relatively low ground shaking potential, with the Big Sur coast being the only other part of coastal California having a comparably low shaking potential according to this assessment.

– California Coastal Commission staff report, June 2015

Emergency Response

Coordinated Response

SCE and nearby communities have worked together since 1982 to develop and continuously update a joint emergency response plan. This working group includes emergency response professionals from SCE, Orange County and San Diego County, the cities of San Clemente, Dana Point, San Juan Capistrano, Marine Corps Base Camp Pendleton and California State Parks. Members of this group meet frequently to ensure that coordinated plans are in place to protect public health and safety in the region surrounding the plant. The organizations plan, train and perform practice drills on all aspects of the emergency program, including scenarios such as earthquakes, terrorist attacks and loss of offsite power. Every two years the Nuclear Regulatory Commission evaluates the emergency program's preparedness through an integrated exercise.

Informing the Public

In the unlikely event of an emergency, SCE spokespersons and Corporate Communications will provide updates to the public to keep them informed of the plant's status in full cooperation with law enforcement agencies.