The plant operator revealed its findings on Sunday.
TEPCO said it analyzed the data and calculated a timeline for developments in the No. 1 reactor on the assumption that it lost its cooling system as soon as the tsunami hit.
The firm said that about four and half hours after the quake the level of water in the pressure vessel fell below the top of fuel rods and that parts of them began melting.
The temperature of the rods is believed to have reached 2,800 degrees Celsius at this stage, and the meltdown advanced rapidly.
Almost all the 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 the same day.
The firm says the melted rods created small holes in the bottom of the vessel. It believes the amount of radioactive substances that could spread from the reactor will be limited.
NHK's correspondent says TEPCO should analyze the situations at the No. 2 and No. 3 reactors as soon as possible.