Both of California's nuclear power plants are shut down, after a swarm of jellyfishlike creatures clogged the water intake pipes at the Diablo Canyon facility this week.

Pacific Gas & Electric announced that it shut down Unit 2 of its 2,300-megawatt plant due to a sharp increase in offshore concentrations of salp, a small, gelatinous animal similar in appearance to a jellyfish. The other unit has been down since April 23 for routine maintenance.

Company spokeswoman Kristin Inman said no reliability or grid issues had been reported. "We have other generation available if we need it, but those plants have not been started yet," she said.

She said plant operators would not ramp power back up until the salp are cleared. They had started cutting power Monday, but conditions did not warrant a full shutdown until last night.

"Now that we're doing a full shutdown, we're going to wait until it's safe enough to bring it back to [full] power," Inman said. She did not say when Unit 1 would be brought back online either, citing market-sensitive information. The last time Diablo Canyon was shut due to jellyfish was in 2008, when a swarm affected both units.

Although Diablo Canyon is on the coast of Central California, it supplies electricity to more than 3 million homes in Central and Northern California. The state's other nuclear plant, San Onofre, has been offline since the end of January due to unusual erosion of steam generating tubes in both its units (Greenwire, April 9).

This is the first time both the state's nuclear plants have been concurrently shut down in recent memory.

A nuclear expert said San Onofre is much more crucial to the state's electricity grid than Diablo Canyon, due to its location near high-demand San Diego County.

"The San Onofre problem is exacerbated because of the indispensability of the nuclear units in maintaining grid stability in Southern California," said John Geesman, a former California Energy Commissioner who now works with the group Alliance for Nuclear Responsibility. "In Northern California, you don't have that same problem, and there does appear to be an abundant supply of other power to buy if the Diablo outage were to be prolonged."