Seismic Safety at San Onofre
San Onofre 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. NRC, San Onofre was built to withstand a peak ground acceleration of at least 0.67g (g refers to the force of gravity). 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 Difference
A major earthquake and subsequent tsunami damaged the reactor cooling and back-up power systems at the Fukushima Daiichi Nuclear Power Station in Japan. 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.
Design Differences Between San Onofre And Fukushima Daiichi
Pressurized Water vs. Boiling Water
Fukushima Daiichi uses a boiling water design, while San Onofre has pressurized water reactors. One of the key advantages of a pressurized water reactor is that it has two cooling loops, a primary and secondary, separated by steam generators. The significance of the steam generators is that water and steam from the secondary system can be vented into the atmosphere to remove heat from the nuclear fuel without releasing radioactive steam/gases because the water being vented has not been in contact with the nuclear fuel rods.
Robust Containment and Tsunami Wall
San Onofre has a four- to eight-foot thick, post-tensioned, steel rebar-reinforced concrete containment that includes an internal steel liner. In addition, the San Onofre facility is protected by a tsunami wall that extends 30 feet above "mean lower low water," the common reference point used by geophysical professionals to talk about a structure or facility's height. The most severe tsunami for SONGS was calculated after extensive studies and assumes that it occurs at the same time as a high tide and storm surge, with wind driven waves.
Spent Fuel Storage
At San Onofre, the spent fuel storage pools are located in a separate building adjacent to the containment structure that encloses the reactor or primary system. The used fuel rods are stored much closer to ground level than they are at Fukushima Daiichi, making it easier to add water if necessary. San Onofre's spent fuel pools are structurally robust, with hardened, steel-reinforced concrete enclosures.
Comparison Of Seismic Risks
The earthquake fault system that generated the devastating earthquake near Honshu, Japan, 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 was built to withstand a peak ground acceleration of 0.67g (g refers to the force of gravity). The maximum ground acceleration experienced at Dai-Ichi was 0.561g in a horizontal direction.
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 spokepersons 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.