"Unit 5 was undergoing pressure testing on the day the earthquake occurred. The unit lost all AC power due to flooding of seawater pumps and power panels. As a

"Unit 5 was undergoing pressure testing on the day the earthquake occurred. The unit lost all AC power due to flooding of seawater pumps and power panels. As a consequence, lighting and AC-powered instrumentation on the Unit 5 side of the common control room were inoperable; the Unit 5 side of the control room went dark after the batteries for emergency lighting were depleted" (emphasis mine) From: https://www.ncbi.nlm.nih.gov/books/NBK253938/ https://www.ncbi.nlm.nih.gov/books/NBK253938/ Which is quite a read in general, and clearly the diesel generators flooding was a major component of the disaster. But there was a lot that went wrong in addition to that, it's a real mess.