Unit 1 Reactor Building Destroyed by Hydrogen Explosion

At 15:36 JST on March 12, a hydrogen explosion blew apart the upper structure of Unit 1's reactor building. Hydrogen had accumulated after zirconium-cladding oxidation reactions in the overheated core at temperatures exceeding 1,200°C, indicating fuel damage had begun. The explosion injured four workers and caused additional damage to electrical systems. Prime Minister Kan had visited the plant earlier that morning in a controversial decision some argued delayed the venting decision. The evacuation zone was expanded to 20 km following the explosion.