Unit 1 at Diablo Canyon nuclear power plant has been shut down to perform work on the back-up cooling system, according to PG&E officials.
Unit 2 continues to operate at full power.
Unit 1 was taken offline after routine inspections detected a small buildup of boric acid on the Unit 1 Residual Heat Removal System. This system helps manage reactor coolant temperatures in the unlikely event of an emergency. Unit 1 remains in a safe condition and will be restored to service after repairs are complete, according to officials.