When the California Coastal Commission denied PG&E’s request to do high-energy seismic surveys off of Diablo Canyon nuclear power plant, it left the utility and seismic regulators in a quandary.
It denied seismologists the only effective tool available to explore earthquake faults six to nine miles into the Earth’s crust where the quakes occur. The commission ruled that PG&E had not demonstrated that the sonic blasting done in the high-energy surveying was the least environmentally damaging option to assess the earthquake danger facing the nuclear plant.
“It’s a conundrum,” said county Supervisor Bruce Gibson, who sits on a state panel that oversees the seismic testing. “We want the best science, but the damage to the environment has to be minimized.”
Even groups such as the San Luis Obispo Mothers for Peace, which have called for more information about the seismic danger facing the plant, are leery of high-energy surveys.
“Mothers for Peace urges extreme caution in the method used to obtain data about the faults in the 530 square nautical miles of the proposed testing area,” the group said in a position statement.
Opposition to the surveys centered on concern that the extremely loud blasts of sound would damage ocean resources, particularly marine mammals. Local fishing and diving activities would also be disrupted.
PG&E officials have not decided whether they will reapply to do high-energy surveys at the end of this year. In the meantime, the utility is busy analyzing a wealth of data it already has about the network of earthquake faults surrounding the plant.
“We are going to have to step back and re-evaluate,” said Ed Halpin, PG&E’s chief nuclear officer.
“There’s a lot of work we can still do evaluating the information we already have.”
Studying data already gathered
Gibson and other seismologists familiar with Diablo Canyon agree that mining the existing geologic
data is the best course of action in the short term for PG&E. That work could show that the high-energy seismic surveys are not necessary, or reduce their size and duration.
“They need to work up everything that is available and decide what they know and what they don’t know,” Gibson said.
One of the challenges facing PG&E is the fact that high-energy surveys are the only tool available to explore deep beneath the seafloor, experts say. The technology consists of using air guns to send 250-decibel blasts of sound into the seafloor and deploying long streamers of hydrophones to record and analyze the sound that bounces back.
“There isn’t anything that can replace the high-energy seismic surveys in the short term,” said Peggy Hellweg, a UC Berkeley seismologist who sits on the California Seismic Safety Commission. “PG&E could put additional seismographs on the seafloor, but you have to wait for an earthquake to happen, and then the data they produce is of a lower resolution than you get with high-energy seismic surveys.”
The Coastal Commission also investigated using a variety of alternate, less-
damaging technologies, such as passive seismic monitoring, electromagnetic surveys and deep-towed acoustic systems, and found them lacking.
“The commission has carried out an additional review of these technologies and also finds that none of them would be feasible alternatives to the proposed use of air guns,” the commission’s official findings concluded.
The Independent Peer Review Panel on which Gibson sits is also reviewing a suggestion by Gibson that an oil industry vessel towing a larger array of hydrophone streamers would be more effective than PG&E’s proposal. PG&E contends that larger streamers are not suitable for the study area, and the panel’s independent review has not been completed.
This leaves seismic experts to mine the seismic data already at hand. Since the 1970s, 19 seismic surveys have been undertaken around Diablo Canyon.
Most recently, PG&E has been spending $64 million conducting seismic investigations both onshore and offshore of the plant, of which the high-energy surveys were to be one part.
For example, PG&E has done extensive low-energy offshore seismic surveys that are intended to measure how active the faults offshore of the plant have been. It has also done onshore geologic mapping of the Irish Hills around the plant.
“There are lots of tools PG&E can use,” said Chris Wills, a geologist with the California Geologic Survey who sits on the Diablo Canyon Independent Peer Review Panel with Gibson. “Analyzing the slip rate of the offshore faults and the structure of the Irish Hills are the two most important.”
A team of PG&E seismologists is expected to complete that work later this year. Their findings will be a key component in whether PG&E decides to go before the Coastal Commission again later this year.
“I haven’t given our seismologists a deadline,” Halpin said. “I want them to take the time and do the job right.”
Key information at a critical time
The issue of high-energy seismic surveys comes at a crucial time for PG&E. The utility has applied to renew Diablo Canyon’s two reactor operating licenses, which expire in 2024 and 2025, for an additional 20 years.
Because the Central Coast is so seismically active, PG&E is the only nuclear utility in the nation that has a full-time seismic staff. The issue of seismic safety at the plant will be relevant for the foreseeable future because highly radioactive spent fuel will be stored onsite for decades to come, at least, because of the lack of a national nuclear waste storage repository.
The March 2011 Fukushima Dai-ichi nuclear disaster in Japan further focused the nation’s attention on the potential earthquake danger a plant such as Diablo Canyon faces. Although high-energy surveys are not required by the Nuclear Regulatory Commission to renew the licenses, PG&E agreed to put its renewal application on hold while it completes its $64 million earthquake mapping effort.
The main goal of the high-energy surveys is to determine whether the Hosgri fault and the recently discovered Shoreline fault are connected and could deliver an unexpectedly large earthquake to Diablo Canyon. In August, the three-person Diablo Canyon Independent Safety Committee sent a strongly worded letter to state and local officials describing the need for the high-energy work as urgent.
“If there is a surprise out there, we all want to know about it urgently; and if not — if in fact the new measurements confirm the interim conclusion that the Shoreline fault zone does not pose a seismic safety threat to the DCPP plant above that for which DCPP has already been designed — then knowing that is very important too,” stated the letter signed by Peter Lam, the safety committee’s chairman.