Following a magnitude 9.0 earthquake on the Richter scale, 3 of 6 boiling water reactors (BWRs) operating at the time automatically shut down, as designed. However, all 6 external electrical power supplies failed due to earthquake damage. Emergency diesel generators started up as designed. However, approximately 41 minutes later, the plant was hit by a 15 m tsunami which damaged the sea cooling water pumps for the main condensers and auxiliary cooling circuits (including the residual heat removal system). It also drowned the diesel generators and inundated the electrical switchgear and battery systems. All 3 reactor cores melted within 3 days. Fortunately, there were no in-core steam explosions, but 13 people were injured by hydrogen explosions which breached their respective nuclear containment buildings, releasing radioactive material to the environment. More than 100,000 people within 20 km of the site had to be evacuated and 2259 (mainly elderly) people died during the evacuation process. This accident was eventually declared a Level 7 (“severe accident”) on the International Nuclear Event Scale (INES).

Basic Cause:
Hydrogen explosions and release of radiation was overheating and extreme over-pressure of the reactor cores due to the total loss of offsite (earthquake) and onsite (tsunami) electrical power.