there was an ammonia leak, and as a precaution company employees were evacuated from the leak area, while the units continued normal operation. A mid-cycle inspection report spanning July 2011 to June 2012 revealed three incidents relating to human performance. An additional issue concerned a failure to develop procedures for a "cyber security analysis of electronic devices" that was later corrected. In 2012, coolant fluid was found in the oil system of a backup diesel generator, which would have caused the generator to fail if needed. An internal investigation found "evidence of potential tampering", making sabotage by staff a possible cause. Environmental risk and mitigation Southern California Edison stated after the Fukushima disaster in 2011 that the station was "built to withstand a 7.0 magnitude earthquake directly under the plant". Additionally, there was a 25-foot wall to protect the plant from a tsunami which could be potentially generated by the active fault 5 miles offshore. The closest tectonic fault line is the Christianitos fault, less than a mile away, which is considered inactive or "dead", but other active faults in the vicinity might pose some threat. The NRC's estimate of the yearly risk of an earthquake intense enough to cause core damage to the reactor at San Onofre was 1 in 58,824, according to a study it had published in August 2010. In June 2012 S. David Freeman, the former head of the Southern California Public Power Authority and "a longtime anti-nuclear voice", described San Onofre and Diablo Canyon as "disasters waiting to happen: aging, unreliable reactors sitting near earthquake fault zones on the fragile Pacific Coast, with millions of Californians living nearby". The San Onofre plant used seawater for cooling, like some other seaside facilities in Southern California, lacking the large cooling towers typically associated with nuclear generating stations. Limited available land next to SONGS would likely have required towers to be built on the opposite side of Interstate 5. More than 4,000 tons of spent nuclear fuel are stored at San Onofre. Surrounding population The NRC defines two emergency planning zones around nuclear power plants: 1) a plume exposure pathway zone with a radius of , concerned primarily with exposure to, and inhalation of, airborne radioactive contamination, and 2) an ingestion pathway zone of about , concerned primarily with ingestion of food and liquid contaminated by radioactivity. The average prevailing westerly wind direction at San Onofre blows inland