SAN LUIS OBISPO (AP) — An overwhelming number of jellyfish-like creatures clogging seawater intake screens forced operators on Thursday to shut down the Unit 2 reactor at the Diablo Canyon nuclear power plant, Pacific Gas & Electric officials said.
Both reactors at the central coast plant are now offline.
Unit 1 was shut down earlier this month for scheduled refueling. The twin-reactor San Luis Obispo County nuclear plant provides enough power for more than 3 million homes in Central and Northern California.
Grid managers at the California Independent System Operator say electricity shortages aren't expected because of ample reserves and cool weather.
The Unit 2 reactor was reduced to about 25 percent earlier this week because of salps entering the beach intake structure.
Salps are small barrel-shaped plankton tunicates similar to jellyfish. They can grow up to 4 inches long and often link together and float in the water in long ropelike formations.
Southerly winds began blowing salps into the plant's cooling water intake cove on Tuesday and plant operators noticed water pressure changes, indicating the creatures were beginning to clog the rolling screens in front of the intake.
Power was then drawn down.
"I've been very pleased with how staff has reacted to this by putting safety first," Pacific Gas & Electric Co. chief nuclear officer Ed Halpin said.
The Unit 2 reactor at the plant was shut down Thursday night because of the clogged screens, the utility's plant spokesman Tom Cuddy said Friday.
"We'll return to full power as soon as it's safe to do so and the salps migrate elsewhere. We hope to do that in the next couple of days," Cuddy said.
Salps can reproduce sexually and asexually, giving them the ability to multiply quickly in what is known as a bloom, said Mark Moline, a marine biology professor at California Polytechnic State University, San Luis Obispo.
"By having this adaptive strategy, the numbers can ramp up quickly and you can have millions in a couple of days," Moline told the San Luis Obispo County Tribune (http://bit.ly/IlkZYA ).
Because salps are gelatinous, they are likely to get caught in the nuclear plant's seawater screens, Moline said.
The varying currents along California's coastline are conducive to eddies forming that can trap and concentrate salp blooms. Salps are prolific from spring to late summer and the eddies that cause high concentrations of the creatures usually last about a week.
In October 2008, nearly 1,000 jellyfish floated into the cooling intake cove at Diablo Canyon and one reactor was temporarily taken offline and the other was reduced to half power.