Unit 1 at Diablo Canyon nuclear power plant was taken down to 55 percent power Monday morning to allow workers to clean out the units cooling water tunnels.
The tunnels on each unit are cleaned about every 18 months, said Blair Jones, Pacific Gas and Electric Co. spokesman. The work is expected to take several days to complete.
The tunnels need to be cleaned out periodically because barnacles and other marine growth collect in them, reducing the units efficiency. The other unit at the plant is operating at full power.