In an attempt to douse concerns by environmental groups and neighbors in the wake of the December Sumatra tsunami disaster, San Francisco, Calif.-based Pacific Gas & Electric, Co. hired a team of seismic mapping experts in July at a cost of $500,00 to recalculate the height of possible waves near its two oceanfront nuclear plants.
Study will model threat to Diablo Canyon nuclear units.(Photo: UIC Corp.)
�In December, some questions were raised,� says David Lochbaum, nuclear safety engineer, U.S. Nuclear Regulatory Commission, about the validity of the 20-year high and low waves accounted for in the 1984 application to build the 2,150-MW pair of Diablo Canyon nuclear plants near San Luis Obispo. The last estimate put the highest possible waves at 36 ft above sea level. The plant is situated on a cliff 85 ft above sea level and spent fuel rods are being moved out of cooling ponds into dry cask storage containers 320 feet above sea level. Lochbaum is eager to see how computer monitoring predicts the structure and the most vulnerable part of the facility -- the cement intake system that brings ocean water into the plant for cooling -- would act during the draw back and crashing of a major event.
PG&E also owns the 1963, 63-MW boiling water reactor Humboldt Bay plant near Eureka, which closed in 1976 because of seismic concerns. The facility is 12 ft above sea level and the last study put possible waves at 42 ft above sea level. The nuclear fuel left in the storage pool is slated for above-ground storage 44 ft above sea level and eventually into a buried bunker with 3-ft-thick, steel-reinforced concrete walls.
Click to Enlarge
Model based on December 24, 2004, event shows relative threat to Thai area (in red), relative to proximity t1 known individual fault locations (in blue). California model will construct models for portions of the California coast. Photo: URS Corp.
�We have an ongoing commitment to study the developing state of earthquake and tsunami knowledge and apply new findings to Diablo Canyon, to make sure that it contains a sufficient margin of safety,� says Jeff Lewis, news department manager, PG&E. He stresses, �There is nothing that we have seen or learned from the Sumatra earthquake and tsunami that leads [us] to believe anything other than that the plant continues to have a sufficient margin of safety in its design.�
URS Corp. in Pasadena, led by Paul Somerville, principal engineering seismologist and senior seismologist Hong Kie Thio, working with the U.S. Geological Survey and the Tokyo Earthquake Research Institute in Tokyo, will do Probabilistic Seismic Hazard Analysis (PSHA) to quantify the threat. They will use seismographs to map all possible slip areas on the ocean floor using Greens Function to pre-calculate and store all possible fault elements on a model. From that, they will use linear wave equations to calculate how much the sea floor could move and extrapolate the size of waves any one of thousands of possible sizes, locations and combinations of events would generate. The pair did this for 2,000 possible events in the Indian Ocean and compared the results with the actual December event and found that it was �absolutely accurate.�
The reason this is such a great planning tool, says Somerville, is that in addition to outlining possible wave heights, it can show the likelihood in each area � 4-meter waves once every 200 years at in Thailand or 12-meter waves once every 1,000 years in Indonesia for instance � so that �rational� building mitigation measures can be made. PSHA can also show the wave velocity, inundation level and the �runup�, how water might act once it reaches shore.
The Madras Atomic Power Station at Kalpakkam, India, a pair of pressurized, heavy water reactors operated by the Nuclear Power Corp. of India Ltd survived the tsunami without damage. The utility company web site credits the design of a break wall and placing buildings above 23 ft for the plant�s ability to stay dry when wave heights of 16 ft hit the coast of Tirunelveli. The emergency shutdown spurred the International Atomic Energy Agency to hold a workshop (http://www.iaea.org/OurWork/ST/NE/NENP/NPES/Activity/lcm_current.html) on external flooding hazards at nuclear powerplant sites. The workshop will be August 29-September 2 in Kalparkkam, India. IAEA safety standards will be considered, warning systems evaluated and new modeling methods presented.
Compared to the Pacific, the Sumatra modeling was easy, according to Somerville, because there was really only one seduction zone to consider. �There are at least 10 times as many scenarios to consider because there are so many sources, including landslides,� says Thio.
Lewis estimates that the work will take five to six months to complete. �The results will tell us whether any changes are needed to make at Diablo Canyon to increase the margin of safety,� he says. Somerville observes that visits to facilities that survived waves as high as 38 meters were for the most part constructed of cement, featuring a rounded profile. Southern California Edison Co. and San Diego Gas & Electric own the two operating units at the San Onofre Nuclear Generating Station, a combined 2,150 MW plant that began commercial operation in 1983 and 1984. �We see no reason to reassess our facility as we are of the opinion that no new information has arisen to change our previous assumptions,� says Ray Golden, spokesman for Southern California Edison. The facility includes a 30-ft tall concrete reinforced steel wall buttressed by earth in order to survives what is assumed to be the worst case scenario: a 7 magnitude strike slip quake on the Newport-Inglewood fault that passes five miles offshore from the plant and could generate a possible 16-ft wave.