Based on provisional analysis of data on the reactor, the utility concluded that the water level in the pressure vessel began to drop rapidly immediately after the tsunami, and the top of the fuel began to be exposed above the water around 6 p.m. Around 7:30 p.m., the fuel was fully exposed above the water surface and overheated for more than 10 hours. At about 9 p.m., the temperature in the reactor core rose to 2,800 degrees Celsius, the melting point for fuel. At approximately 7:50 p.m., the upper part of the fuel started melting, and at around 6:50 a.m. on March 12, a meltdown occurred.
On the reason why it took over two months after the earthquake to reveal the information, TEPCO said it had only been able to start obtaining detailed data on the temperature and pressure in the reactor for analysis in early May.
Junichiro Matsumoto, a senior TEPCO official, said, "Because there is similar damage to the fuel rods at the No. 2 and 3 reactors, the bottoms of their pressure vessels could also have been damaged." He said the utility would carry out similar analysis on the two reactors.
Hiroaki Koide, professor of nuclear safety engineering at Kyoto University, was critical of TEPCO.
"They could have assumed that when the loss of power made it impossible to cool down the reactor, it would soon lead to a meltdown of the core. TEPCO's persistent explanation that the damage to the fuel had been limited turned out to be wrong," he said.