I hardly paid any attention to Fukushima II (Daini) Nuclear Power Plant in the early days of the crisis, but I got curious reading the comment from the reader "Joe Neubarth" to the post on the eyewitness account of Fukushima II on the day of the earthquake:
"Core Damage comes from a loss of cooling. I know of no report of loss of cooling at that facility. Did they go dark when the one electrical transmission tower fell north of Fukushima during the earthquake? Did they then have a delay if the startup of Emergency Diesel Generators? If there was a delay or if they lost the EDG's totally THEN there might be a melt down, BUT I have not heard of this happening."
I didn't know either (or I totally forgot). Did they or didn't they? So I went to TEPCO's site and see what they say.
Fukushima II Nuke Plant has 4 reactors. When the earthquake hit on March 11, control rods were successfully inserted in all 4 reactors at 2:48PM. Then the tsunami hit, and at 6:33PM on March 11:
Occurrence of a Specific Incident Stipulated in Article 10 of the Act on Special Measures Concerning Nuclear Emergency Preparedness (loss of function to remove residual heat)
This happened on Reactors 1, 2, and 4, at the same time.
Then, the Residual Heat Removal System re-started one by one, on March 14:
Reactor 1: 1:24AM, March 14
Reactor 2: 7:13AM, March 14
Reactor 4: 3:42PM, March 14
So, the reactors weren't cooled for as long as 69 hours in case of Reactor 4, and 55 hours in case of Reactor 1. Is 55 hours long enough for the coolant (water) inside the Reactor Pressure Vessel to evaporate and for at least part of the fuel rods to be exposed and get damaged?
For all 4 reactors, the Reactor Coolant Filtering System came online between June 4 and July 17.
TEPCO also says the Suppression Chambers of Reactors 1, 2 and 4 suffered some event that caused the "loss of function to suppress pressure" in the morning of March 12. The function was restored on March 14 morning.
The above information is from TEPCO's status report on Fukushima II (Daini) Nuclear Power Plant as of August 21 (click to enlarge, or go to the link):