"Almost of all the fuel rods melted and dropped to the bottom of the pressure vessel by 6:50 am on March 12th." (NHK WORLD news)
NHK WORLD, whose outlandish report that there was a 300 millisievert/hr debris on a nearby mountainside is still uncorrected, says that TEPCO has disclosed the Reactor 1 meltdown occurred within 16 hours of the March 11 earthquake:
Tokyo Electric Power Company, the operator of the Fukushima Daiichi nuclear power plant, says most of the fuel rods in the No.1 reactor had dropped to the bottom of the pressure vessel within 16 hours of the earthquake on March 11th.
The utility revealed its study on the subject on Sunday.
TEPCO said it analyzed the data and calculated a timeline for the developments in the No. 1 reactor on the assumption that the reactor lost its cooling system as soon as it was hit by the tsunami.
The firm said that within about 3 hours after the reactor automatically shut down, the cooling water had evaporated to a level at the top of the rods.
In the next hour and a half, parts of the fuel rods are believed to have begun melting.
The temperature of the fuel rods is believed to have reached 2,800 degrees Celsius at this stage, and the meltdown advanced rapidly.
Almost of all the fuel rods melted and dropped to the bottom of the pressure vessel by 6:50 am on March 12th.
TEPCO said the temperature dropped after water was poured into the reactor starting at 5:50 am on the same day.
The firm says the melted rods created small holes on the bottom of the vessel, but that no major problems are developing there. It believes that the amount of radioactive substances that could spread from the reactor will be limited.
Sunday, May 15, 2011 23:29 +0900 (JST)
For people who can read the Wikipedia entry on "Corium", it's abundantly clear what happens once the water is no longer cooling the fuel rods:
During the Three Mile Island accident, slow partial meltdown of the reactor core occurred. About 19,000 kg of material melted and relocated during about 2 minutes, about 224 minutes after the reactor scram.
In the case of Chernobyl:
The first phase lasted only several seconds, with temperatures locally exceeding 2600 °C, when a zirconium-uranium-oxide melt formed from no more than 30% of the core.