More details have emerged from a report to Japanese regulators on the chain of events at Fukushima Daiichi following the devastating March 11 earthquake and tsunami.
The report Tokyo Electric Power Co. submitted to the Nuclear and Industrial Safety Agency Monday showed that operators followed manual procedures during the station blackout, but those procedures included the temporary shutdown of emergency cooling equipment.
The report's timeline for the accident, as summarized in the Daily Yomiuri newspaper, showed that the plant lost external power immediately after the earthquake at 2:46 p.m., Japan time, but backup generators functioned and the reactors tripped automatically.
In the unit 1 reactor, cooling via the isolation condenser began 6 minutes later. Then at 3:03 p.m., plant workers stopped the isolation condenser's operation, per the reactor's manual, because the core temperature fell faster than 55 C per hour. Workers had restarted the isolation condenser by the time the tsunami reached the plant at 3:30 p.m.
The condenser then shut down again, this time automatically after the tsunami destroyed backup generators. The system, presumably operating on emergency batteries by that point, interpreted the loss of power as a pipe fracture and closed a series of valves that were reopened manually by plant workers to restart the condenser.
Units 2 and 3, which have General Electric's BWR-4 design as opposed to unit 1's BWR-3, were equipped with a reactor core isolation system instead of an isolation condenser. At these units, too, cooling from the emergency system stopped multiple times after power losses and was restarted by workers, per operation manuals. At unit 3, the reactor core isolation system stopped working at 11:36 a.m., March 12. An hour later, the emergency high pressure core flooding system was activated. At 2:42 a.m. March 13, that system was stopped for about 6.75 hours when coolant was switched to fresh water containing boric acid.
In unit 2 the reactor core isolation system stopped working in at 1:25 p.m., March 14.
TEPCO said earlier this week that it now believes fuel in all three reactors melted completely and has accumulated in the bottom of reactor pressure vessels.