2. Course of the accident

2.1 Reactor status before the accident

On March 11, 2011, reactors no. 1, no. 2 and no. 3 were operating at full power; reactor no. 4 was shut down for maintenance, and its core had been unloaded from the vessel and stored in the deactivation pool; reactors no. 5 and no. 6 were also shut down for maintenance, but the corresponding cores were still in the vessels.

All the reactors' deactivation pools contain fuel assemblies, mainly spent fuel assemblies whose residual power must be evacuated. After a certain period of time, these spent fuel assemblies are transferred to the centralized pool located on site, near Reactor No. 4. At the time of the accident, this pool contained more than 6,000 assemblies requiring cooling.