Tokyo Electric Power Co. said a "major part" of the fuel rods in reactor No. 2 may have melted and fallen to the bottom of the pressure vessel 101 hours after the earthquake and tsunami that crippled the plant.

The same thing happened within the first 60 hours at reactor No. 3, the company said, releasing a worst-case scenario analysis.

The fuel is believed to be sitting at the bottom of the pressure vessel in each reactor building.

Tokyo Electric also released a second possible scenario for reactors 2 and 3, one that estimates a full meltdown did not occur. In that scenario, the water inside the reactors stayed at a higher level. Tepco estimates the fuel rods may have also broken in this second scenario, but may not have completely melted.

Temperature data shows the two reactors have cooled sufficiently in the more than two months since the incident, Tokyo Electric said.

A March 11 earthquake and tsunami knocked out cooling systems at Fukushima Daiichi, causing the three operating reactors to overheat. That compounded a natural disaster by spewing vast quantities of radioactive material into the atmosphere.