Unit 4 Spent Fuel Pool Crisis — Feared to Be Boiling Dry

Unit 4's reactor had been defueled for maintenance prior to the earthquake, with 1,535 fuel assemblies stored in the spent fuel pool. When the Unit 4 reactor building exploded on March 15 (due to hydrogen migrating from Unit 3 via shared ventilation), the NRC and international experts feared the pool was on fire and losing water. US NRC Chairman Gregory Jaczko stated publicly on March 16 that the pool might be dry and recommended a 50-mile (80 km) evacuation for Americans in Japan—far exceeding the Japanese government's 20 km zone. Later investigation found the pool never actually boiled dry, but it remained a critical concern for weeks.