the plant comprises six separate boiling water reactors originally designed by general electric (ge), and maintained by the tokyo electric power company (tepco). at the time of the quake, reactor 4 had been de-fueled while 5 and 6 were in the cold shutdown for planned maintenance. immediately after the earthquake, the remaining reactors 1-3 shut down automatically, and emergency generators came online to control electronics and coolant systems. however, the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. the flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through a nuclear reactor for several days in order to keep it from melting down after being shut down. as the pumps stopped, the reactors overheated due to the normal high radioactive decay heat produced in the first few days after nuclear reactor shutdown (smaller amounts of this heat normally continue to be released for years, but are not enough to cause fuel melting).