Diablo Canyon nuclear plant should survive 7.2 quake

July 22, 2011 2:07:10 PM PDT
Last week, California Watch, the Center for Investigative Reporting, Capitol Public Radio and Need to Know aired a report on seismic safety at the Diablo Canyon nuclear power plant operated by PG&E.

We didn't have a lot of time to go into it in the television show. So, I'd like to delve into it a little more now. A lot of the show focused on a debate simmering between the U.S. Geological Survey and PG&E about the size and connectivity of fault lines near the plant. Of particular issue is the relationship between the Shoreline Fault, a newly discovered fault right offshore from the plant, and the Hosgri Fault. The Hosgri is about 3 miles offshore. Both the USGS and PG&E think the Shoreline is capable of producing a magnitude 6.5 earthquake. However, a scientist at the USGS thinks the Shoreline Fault may connect with the more distant, but larger, Hosgri Fault. If this were the case, the power plant could be sitting just a few hundred yards from a fault with the potential to produce a magnitude 7.2 earthquake. PG&E thinks that scenario is unlikely. In fact, we found a PowerPoint presentation given by one of PG&E's consultants, Norman Abrahamson, indicating there was zero probability of the two faults connecting. His slide presentation [PDF] was given at a Nuclear Regulatory Commission workshop in September 2010. Yet, despite the utility's claim that such a scenario could not happen, Abrahamson included -- in that same PowerPoint presentation -- a graph showing the plant's ability to withstand such a combined rupture. If you look at the graph now, you will notice several lines. The red one, labeled the HE spectrum, represents the utility's 1977 estimate for likely ground motions at the nuclear power plant should a magnitude 7.5 earthquake erupt along the Hosgri Fault. The plant was built and retrofitted for this scenario. As long as no other lines exceed or go above that red line, the plant is likely to be safe. If they do, the plant is in danger. Now take a look at the gray line. That one represents PG&E's estimate for ground shaking at the plant should a magnitude 7.2 combined earthquake rupture right offshore. If you look at the right-hand side of the graph, you'll see that the gray line, in fact, goes above that red line. We thought that was alarming. However, we noted that the gray line didn't exceed the red line until after both were out of the shaded yellow area. What was going on here? I wrote to Abrahamson to get an explanation. He never wrote back. And PG&E declined to be interviewed about the plant or the geology and seismology around it. The utility did send us a statement, which you can read here. So, I called Jack Baker, a structural engineer at Stanford University, to walk me through it. The bottom line: As long as the gray line doesn't cross the red line in that shaded yellow region, things should be OK. And here's why: The bottom of the graph -- the X axis -- shows the frequency of ground motion. As Baker explained, think of the ground as water and the frequency as the interval between two waves. The lower the frequency, the longer the time between waves; the higher the frequency, the shorter the duration. So, if you look way to the left of the graph, where the frequency is low, those numbers would be like the change in tide -- a long slow wave that changes over six hours, as the tide shifts from low to high, or high to low. Such a wave is unlikely to rock a boat. But as the frequency picks up, things get a little choppier. And it's in that yellow region -- between 3 and 6.5 Hertz -- that the boat begins to really rock. If the boat was built with enough strength -- and also a bit of flexibility -- it should be able to withstand the rocking that those waves generate. And according to this graph, Diablo Canyon would be able to withstand the waves of the earth generated by a magnitude 7.2 earthquake right offshore. As you move right along the graph, at some point, the waves become so frequent, it's almost as if the water becomes flat again. And the rocking and motion of the ship stops. So, at these higher levels -- where the gray line exceeds the red line -- it's OK, because the rocking, or back-and-forth movement of the plant, is minimal. Baker says these models are based on lots of information, including recorded data of ground motion during other earthquakes, the type of soil or rock on which the plant sits, and the stiffness or flexibility of the structure. He said these models are updated constantly and represent the most up-to-date knowledge the seismology, geophysical and engineering worlds have to offer. So, if a magnitude 7.2 earthquake were to rupture right offshore -- even though PG&E says that will never happen -- the plant looks like it will be OK. But if a magnitude 7.7 earthquake ruptures right offshore -- as others wonder -- we'll have to find a graph that shows that scenario.

Story courtesy of our media partners at California Watch (A Project of the Center for Investigative Reporting)

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