Since the Diablo Canyon nuclear power plant opened on a rocky stretch of California coast in 1985, researchers have discovered three nearby fault lines.
April 2015 – DIABLO CANYON, CALF. – Since the Diablo Canyon nuclear power plant opened on a rocky stretch of California coast in 1985, researchers have discovered three nearby fault lines capable of stronger quakes than the one that struck Napa last year. And yet the plant’s owner, Pacific Gas and Electric Co., insists that Diablo isn’t in greater danger than previously thought. If anything, it’s in less. PG&E has, at several times in Diablo’s complicated history, changed the way the company assesses the amount of shaking nearby faults can produce, as well as the plant’s ability to survive big quakes. To Diablo’s critics, PG&E keeps tweaking the math to make California’s last nuclear plant look safer than it really is. If PG&E’s seismic studies showed that nearby faults could produce more shaking than the plant was designed to handle, Diablo could be forced to close. “The company has been claiming that the plant is stronger and stronger as more faults have been discovered,” said former state Sen. Sam Blakeslee, who has a doctorate in geophysics and lives nearby. “The utility has been moving the goal posts.”
PG&E insists that years of seismic studies at the plant near San Luis Obispo have given the company a more accurate picture than before. The biggest neighboring fault line — the Hosgri, discovered 3 miles offshore while Diablo was under construction — can’t shake the plant nearly as much as initially thought, according to PG&E. And the methods PG&E has developed to assess seismic threats at Diablo are far more precise than the ones used when the plant was designed, the company says. “It is a gold standard of how to look at seismicity and the geology surrounding any infrastructure, not just nuclear power plants,” said Ed Halpin, PG&E senior vice president and chief nuclear officer. “In my opinion it should be held up and applauded.” Instead, PG&E’s methodology has ended up in court. Environmentalists pushing to close Diablo filed a lawsuit last year claiming the the Nuclear Regulatory Commission — the federal agency overseeing the nation’s nuclear plants — illegally let PG&E amend the seismic safety portion of Diablo’s operating license without public hearings.
The suit came after one of the commission’s former inspectors at Diablo argued that the plant was no longer operating within the terms of its license and should be shut down. His objections, and the commission’s handling of them, are now under investigation by the commission’s own internal watchdog office. When construction on the plant began, in 1968, PG&E considered the location free of active faults. But in 1971, geologists working for Shell Oil found the Hosgri just offshore. For a power plant, the most important gauge of an earthquake’s severity isn’t the magnitude — it’s the amount of ground motion an earthquake will create at the plant itself. Diablo had originally been designed to withstand a specific level of shaking — .4g, or .4 times the force of gravity. An earthquake on the Hosgri Fault, researchers decided, could produce more violent shaking than that, with a peak ground motion of .75g. By then, much of the plant had already been built. So PG&E had to retrofit Diablo before it ever opened. –Emergency Management