Environmentalists, fishermen and even the California Coastal Commission's staff have lined up against the project, fearing the high-decibel sounds would disturb sensitive marine mammals. In a report this month, the staff urged the panel to deny a permit to PG&E, citing "significant and unavoidable impacts to marine resources."
The staff estimated more than 7,000 sea mammals would be affected by the ear-piercing noise, including fin whales, blue whales, humpback whales, and harbor porpoises. The 12 commissioners, who take staff opinion into consideration, have the final say.
Charles Lester, the commission's executive director, said seismic hazards around the seaside plant need to be better understood, but "the case hasn't been made that this particular test is necessary in order to get those answers."
PG&E disagreed, saying that high-tech imaging is needed to understand the complex geology that other types of studies can't provide. The utility said similar research has been done around the world without long-term harm to animals.
The damage that strong shaking can cause to nuclear reactors came under the scrutiny after the 9.0-magnitude earthquake off Japan's coast that triggered tsunami waves, which swamped the Fukushima Dai-ichi plant in 2011. But even before the Fukushima disaster, state law mandated that utilities conduct extensive seismic studies of nuclear facilities.
Perched on an 85-foot bluff along California's scenic Central Coast, Diablo Canyon sits within three miles of two underwater earthquake faults, including one that was discovered in 2008.
PG&E's proposal is a four-pronged approach that includes the use of high-energy seismic imaging technology. Under the ratepayer-funded study, a research boat would tow 18 air guns that would fire the sonic pulses into the ocean every 10 to 20 seconds for several days. The utility had hoped to conduct the seismic survey between November and December to avoid peak breeding and migration seasons for sea mammals.
In August, a State Lands Commission environmental impact study determined there would be unavoidable consequences to marine life during the testing. But the panel ultimately decided the project's benefits outweighed the environmental risks.
PG&E faces a big test before the coastal commission, which received scores of letters opposing the offshore study. Conservation groups contend the company has not done enough to explore other less damaging options.
"If you're going to harm the coast, you've got to make sure that there's no alternative," said Michael Jasny, an attorney with Natural Resources Defense Council.
To minimize impact to sea life, PG&E proposed starting off with one air cannon at a low decibel before ramping up to full power. It also planned to have spotters on the vessel and in an aircraft to alert operators of marine mammals in the region.
Air guns will be silenced and work will cease if an animal strays too close.
The twin-reactor Diablo Canyon generates enough electricity to power more than 3 million homes in Central and Northern California. After the Fukushima disaster, the utility asked federal nuclear regulators to delay issuing extended operating permits until thorough seismic studies are completed.
The utility's permits expire in 2024 and 2025.