PG&E reports an "unusual event" occurred overnight at Diablo Canyon Power Plant. A fire alarm sounded around 1:20 a.m. Tuesday in the plant's Unit 1 containment building. The on-site fire department investigated the incident and confirmed it was a false alarm and all conditions were normal by 3:06 a.m. According to PG&E there was no threat to public health or safety. If an unusual event is not resolved within 15 minutes, the Nuclear Regulatory Commission requires the plant to notify the public. An unusual event is the lowest of four emergency classifications. Investigators are researching the cause of the false alarm. )