A magnitude 9 earthquake near Sendai occurred at 14:46 JST (05:46 UTC) on Friday, March 11. The subsequent tsunami hit Fukushima Prefecture about an hour later. There has been very little information about what happened at Daini and Daiichi in the first hours after the earthquake and tsunami that hit the two plants.
I first blogged about the earthquake and nuclear power plants in Japan 16:58 ET March 11 (06:58 am Saturday March 12 JST) or about 16 hours after the earthquake. At that point it had been reported that after the earthquake stopped off-ste power at Daiichi, a backup generator also failed and the cooling system was unable to supply water to cool the Unit 1 reactor, although at least one backup cooling system was being used, and that the reactor core remained hot even after the shutdown. TEPCO was reported to be bringing in mobile generators to restore the power supply. Pressure inside the containment of Unit 1 was continuing to increase, so at this point TEPCO had lost the ability to control pressure in the Unit 1 reactor pressure vessel. Pressure inside the reactor had risen to 1.5 times the level considered normal and the government announced that vapour would be vented from the unit, to lower the pressure in an effort to protect it from "a possible meltdown."
March 12 TEPCO reported that the pressure suppresion function was lost at Daiini and it was preparing to vent vapour from the reactors, but that Units 1,2, and 4 retained off-site power.
But outside of that there has been very little information about what happened at Daini and Daiichi in the first hours after the magnitude 9 earthquake and the tsunami that hit the two plants. March 13 I blogged about what WNN inferred had happened. According to NHK, "the plant's operator, Tokyo Electric Power Company, has so far only disclosed data from the day after the quake."
Friday, April 08, NHK has obtained "unreleased data" that shows that 7 hours after the earthquake and tsunami the water level inside the Unit 1 reactor dropped to 45 cm above the top of the fuel assembly, or about one-tenth the normal level, and 11 hours after that the fuel rods become exposed. According to Professor Naoto Sekimura of the University of Tokyo, the loss of cooling functions at the Unit.1 reactor followed by the exposure of the fuel rods may have led to the first hydrogen explosion March 12.
Units 2 and 3
At Units 2 and 3 emergency generators kept the water levels in the reactors 4 meters above the top of the fuel assembly. It was much longer, between a day and a half to 3 days before fuel rods in these reactors were exposed.