For the first time, a small jellyfish-like animal called a sea salp has forced the shutdown of Diablo Canyon nuclear power plant.
Plant operators and marine biologists say a convergence of natural factors caused the unlikely event.
Both biology and ocean physics have teamed up against Diablo Canyon, said Mark Moline, a marine biology professor at Cal Poly.
The event began Tuesday when southerly winds began blowing the salps into the plants cooling water intake cove. Plant operators noticed differences in water pressure at the intake structure, indicating the salps were beginning to clog the rolling screens in front of the intake, said Ed Halpin, PG&Es chief nuclear officer.
Ive been very pleased with how staff has reacted to this by putting safety first, he said.
Operators reduced the power output of the one reactor operating at the time, Unit 2, to 15 percent. The other reactor, Unit 1, had been shut down earlier in the week for regularly scheduled refueling.
On Wednesday, power output at Unit 2 was increased to 24 percent, but a fresh influx of salps overwhelmed the intake screens, which roll in a circular fashion that allow them to be cleaned.
Last night, the conditions got to the point where we had to shut it down, Halpin said. So well have to wait until the salp situation clears up.
Salps are small marine animals similar to jellyfish that are typically 2 to 3 inches long. They often link together and float in the water in long ropelike formations.
Salps can reproduce both sexually and asexually, and this gives them the ability to multiply quickly, a phenomenon called a bloom, Moline said.
By having this adaptive strategy, the numbers can ramp up quickly and you can have millions in a couple of days, he said.
Also, Californias geologically diverse coastline with its many currents is conducive to eddies forming that can trap and concentrate salp blooms. Salps are gelatinous, making them more likely to get caught in the rolling screens, Moline said.
Salps are most prolific from spring to late summer, he said. The eddies that cause salps to concentrate usually last about a week.
The only similar event in Diablo Canyons history took place in October 2008, when nearly 1,000 jellyfish floated into the cooling intake cove. One reactor was temporarily taken offline and the other reduced to half power.
With the temporary shutdown of Diablo Canyon, Californias two nuclear power plants are now offline. The San Onofre nuclear generating station in Orange County has been offline for three months following the discovery of leaks in the plants steam generators.
Diablo Canyons steam generators are designed differently, manufactured by a different company and are not experiencing the same problems, plant spokesman Tom Cuddy said.
Because of cool weather and ample reserves, the states power grid is not expected to experience any electricity shortages even with the nuclear plants off-line, according to the California Independent System Operator.