A spokeswoman for Pacific Gas & Electric said the company doesn't know why the fuse blew, and can't say when the reactor will be operating again.
PG&E spokeswoman Emily Christensen says a fan for cooling a transformer had stopped on June 30, which set off an alarm and prompted operators to shut down the reactor.
PG&E says the shutdown doesn't pose a safety hazard and the plant's other reactor is operating at full power.
Diablo Canyon's two reactors in California's Central Coast produce about 2,300 megawatts of electricity. PG&E says that's about 10 percent of all electricity generated in California.